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Posted: 8/29/2015 4:55:44 PM EDT
GPS says I'm doing 51, speedometer says 55. Programmer was set for proper tire size calibration. Change calibration to a smaller tire and the GPS is just one mph slower than speedometer. The question is which do I go with? Match the speedo to the GPS or stick with correct tire size calibration. As is it would seem that is adding extra miles on the odometer. Trust GPS or programmer?
Link Posted: 8/29/2015 5:04:36 PM EDT
I think speedometers tend to read a little fast. Mine is 3-4mph fast at 65-70.
Link Posted: 8/29/2015 5:07:08 PM EDT
[Last Edit: 8/29/2015 5:20:46 PM EDT by Walkure]
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Originally Posted By c7aea15:
I think speedometers tend to read a little fast. Mine is 3-4mph fast at 65-70.
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Speedos are normally "optimistic", in part to err on the side of caution - that is, to avoid people being ticketed for minor speed infractions despite their speedometer apparently reading at or just under.

I believe that while the speedo may be calibrated to read high, the odometer may be set lower, to the expected actual condition rather than over-estimating.



ETA: I guess I should have said "often" or "tend to be" rather than "normally". As noted by others, some cars and trucks these days will be pretty close (but often still 1-2 MPH over GPS or so, maybe more), while motorcycles are far more "optimistic" (often by 5 MPH, or even more at certain speeds).
Link Posted: 8/29/2015 5:07:59 PM EDT
Link Posted: 8/29/2015 5:08:59 PM EDT
My cars and trucks are closer to my GPS than any of my bikes are.
Link Posted: 8/29/2015 5:10:01 PM EDT
How are you calculating your tire size, but the factory dimensions?


I'm gonna bet the squish of having some load on them makes them run smaller than the published dimensions
Link Posted: 8/29/2015 5:11:39 PM EDT
GPS
Link Posted: 8/29/2015 5:14:36 PM EDT
Link Posted: 8/29/2015 5:18:03 PM EDT
[Last Edit: 8/29/2015 5:19:39 PM EDT by ASUsax]
I'll go with the orbiting billion-dollar constellation of satellites for $1000, Alex.

GPS is going to be pretty damn close. Your car, on the other hand, is not going to be fast. That's by design, they don't want you exceeding the speed limit when you don't think you are.

The only vehicle that's ever been within a MPH or so is my Avatar, when it was sitting on 31's with the speedo gear for the stock 30's installed. That was pretty dead nuts.

It's since grown up, and reads faster than the GPS again, because it has the nominally correct Speedo Gear installed.

SOME newer vehicle can be calibrated, electronically, to be closer than older, mechanical Speedo vehicles. But 3-4 MPH higher than reality is pretty normal.
Link Posted: 8/29/2015 5:27:59 PM EDT
There are gps speedometers so that tells you what is more accurate.
Link Posted: 8/29/2015 5:28:08 PM EDT

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Originally Posted By IchWarrior:


How are you calculating your tire size, but the factory dimensions?





I'm gonna bet the squish of having some load on them makes them run smaller than the published dimensions
View Quote
I'd buy that. I moved down one inch in diameter to get the speedo closer to the GPS. Still I wonder if that adds extra miles on the odometer that I didn't really put on.

 
Link Posted: 8/29/2015 5:30:35 PM EDT
Maybe they even do it to pad the fuel economy numbers.

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Link Posted: 8/29/2015 5:30:59 PM EDT
The speedometer is much more accurate, assuming calibrated to tire size.  They do average for tire wear though so they can be off based on how new or old your tires are by as much as 2-3% either way.  The GPS is using several assumptions for you speed.  They also use averaging over time.  Couple that with a location fix that oftern uses averaging and may be jumping around wirh in 40 feet or so.

My older GPS in the trip tic section would often say things like my max speed during a route was 150mph.  Which was an indication of these errors.  My newer GPS doesn't, but it isn't becuase it's reception or the actual GPS chip are better or different.  It is becuase they have different software to hide these errors that areintrinsic in the system.
Link Posted: 8/29/2015 5:34:13 PM EDT
Trust the GPS over a speedometer.

Link Posted: 8/29/2015 5:34:16 PM EDT
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Originally Posted By IchWarrior:
How are you calculating your tire size, but the factory dimensions?


I'm gonna bet the squish of having some load on them makes them run smaller than the published dimensions
View Quote

This.


GPS can be scary accurate. Trust the GPS if it has a good fix.
Link Posted: 8/29/2015 5:36:32 PM EDT
My BMW is at least 4mph fast, very common complaint, my old Ford truck is dead on when matched to GPS. I drive accordingly.
Link Posted: 8/29/2015 5:36:35 PM EDT
Police say my speedometer is exactly correct. Unfortunately I ignore it....
Link Posted: 8/29/2015 5:38:51 PM EDT
My truck speedo is matched to the gps at 35-45 mph, but when it goes to 60 it's 4 mph faster than the gps which when measured by mile marker is correct.
Link Posted: 8/29/2015 5:42:55 PM EDT
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Originally Posted By ajacobs:
The speedometer is much more accurate, assuming calibrated to tire size.  They do average for tire wear though so they can be off based on how new or old your tires are by as much as 2-3% either way.  The GPS is using several assumptions for you speed.  They also use averaging over time.  Couple that with a location fix that oftern uses averaging and may be jumping around wirh in 40 feet or so.

My older GPS in the trip tic section would often say things like my max speed during a route was 150mph.  Which was an indication of these errors.  My newer GPS doesn't, but it isn't becuase it's reception or the actual GPS chip are better or different.  It is becuase they have different software to hide these errors that areintrinsic in the system.
View Quote

I also wondered if it showed my speed as if I were on a level grade vs going up or down hills. Meaning if I was going East up a steep grade at 55 would the GPS only see my speed as a two dimensional measurement and not three dimensional. Sincerely doubt anyone will understand what I was just rying to get out of my head.

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Link Posted: 8/29/2015 5:45:54 PM EDT
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Originally Posted By shotar:
In my cruiser, with the proper tires, the gps, speedometer, and radar all match.
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But they go though a special calibration dont they?   My brothers cruiser had a little sticker on the bottom that said Certified Calibration.
Link Posted: 8/29/2015 5:46:08 PM EDT
The speedometer in my Javelin along with the other gauges were all spotty when it came to proper function. I installed a modern GPS speedometer. I hope it's correct. I haven't been ticketed since I put it it.
Link Posted: 8/29/2015 5:47:52 PM EDT
[Last Edit: 8/29/2015 5:49:30 PM EDT by Palm]
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Originally Posted By Waldo:

My truck and GPS are pretty close to each other. Within 1 mph.
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Me too and Texas DPS agrees with both.
Link Posted: 8/29/2015 5:49:03 PM EDT
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Originally Posted By sixnine:

I also wondered if it showed my speed as if I were on a level grade vs going up or down hills. Meaning if I was going East up a steep grade at 55 would the GPS only see my speed as a two dimensional measurement and not three dimensional. Sincerely doubt anyone will understand what I was just rying to get out of my head.

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Originally Posted By sixnine:
Originally Posted By ajacobs:
The speedometer is much more accurate, assuming calibrated to tire size.  They do average for tire wear though so they can be off based on how new or old your tires are by as much as 2-3% either way.  The GPS is using several assumptions for you speed.  They also use averaging over time.  Couple that with a location fix that oftern uses averaging and may be jumping around wirh in 40 feet or so.

My older GPS in the trip tic section would often say things like my max speed during a route was 150mph.  Which was an indication of these errors.  My newer GPS doesn't, but it isn't becuase it's reception or the actual GPS chip are better or different.  It is becuase they have different software to hide these errors that areintrinsic in the system.

I also wondered if it showed my speed as if I were on a level grade vs going up or down hills. Meaning if I was going East up a steep grade at 55 would the GPS only see my speed as a two dimensional measurement and not three dimensional. Sincerely doubt anyone will understand what I was just rying to get out of my head.

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I follow you completely. GPS is capable of delivering either velocity measurement (surface relative including elevation or just lat-lon relative). I'm not sure which they use in the software of commercial GPS navigation systems. If they are smart it would be surface relative (same as your speedometer).
Link Posted: 8/29/2015 5:50:40 PM EDT
Originally Posted By sixnine:
GPS says I'm doing 51, speedometer says 55. Programmer was set for proper tire size calibration. Change calibration to a smaller tire and the GPS is just one mph slower than speedometer. The question is which do I go with? Match the speedo to the GPS or stick with correct tire size calibration. As is it would seem that is adding extra miles on the odometer. Trust GPS or programmer?
View Quote


If both are calibrated, the spedo will be more accurate. Minor lag, plus accuracy being dependent on number of satellites.
Link Posted: 8/29/2015 5:52:24 PM EDT
My last 2 Ford 250 work trucks with factory tires read 2mph faster on the speedometer than I'm actually going.It doesn't matter if I'm going 35 or 65 the speedometer always reads faster than gps or the cops little portable radar machines read.
Link Posted: 8/29/2015 5:52:39 PM EDT
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Originally Posted By sixnine:

I also wondered if it showed my speed as if I were on a level grade vs going up or down hills. Meaning if I was going East up a steep grade at 55 would the GPS only see my speed as a two dimensional measurement and not three dimensional. Sincerely doubt anyone will understand what I was just rying to get out of my head.

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Originally Posted By sixnine:
Originally Posted By ajacobs:
The speedometer is much more accurate, assuming calibrated to tire size.  They do average for tire wear though so they can be off based on how new or old your tires are by as much as 2-3% either way.  The GPS is using several assumptions for you speed.  They also use averaging over time.  Couple that with a location fix that oftern uses averaging and may be jumping around wirh in 40 feet or so.

My older GPS in the trip tic section would often say things like my max speed during a route was 150mph.  Which was an indication of these errors.  My newer GPS doesn't, but it isn't becuase it's reception or the actual GPS chip are better or different.  It is becuase they have different software to hide these errors that areintrinsic in the system.

I also wondered if it showed my speed as if I were on a level grade vs going up or down hills. Meaning if I was going East up a steep grade at 55 would the GPS only see my speed as a two dimensional measurement and not three dimensional. Sincerely doubt anyone will understand what I was just rying to get out of my head.

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We understand vectors
I highly doubt the few feet elevation form the hill makes any real difference. Plus GPS can measure elevation so your slope could even be determined.
Link Posted: 8/29/2015 6:05:06 PM EDT
Most tires, esp. off road and all terrain tires can be pretty "off" from their advertised sizing. I know in the 33" range, some tires measured low 32s. BFG were the closest I believe to a true 33" I know my M/T Baja Radials were  32.8.

And dont forget the "squish" factor, air pressure and wear.

I change tire size and gear ratio so my speedo is wayyyyyyyyyyyyy off. but ive GPS'd it and im exactly 20mph off. So if I read 75, im actually going 55mph
Link Posted: 8/29/2015 6:18:53 PM EDT
[Last Edit: 8/29/2015 6:50:49 PM EDT by Star_Scream]
I'd go with GPS.


Not affected by tire size, tire wear, tire pressure/temp.


Plus I doubt they calibrate car speedos at the factory
Link Posted: 8/29/2015 6:21:41 PM EDT

I've noticed most cars I have owned have indicated anywhere from 1-4% over actual speed.  I've never had one read less than actual speed.  The German cars I've owned seem to come in on the lower end of that range, and the Japanese and American cars are on the upper end of that range.  










Link Posted: 8/29/2015 6:33:43 PM EDT
How about having someone pace you? That's what I did. My truck is 5 mph under at 65. When the speedo says 65 you're actually going 60. And 2-3 mph low at 35.
Link Posted: 8/29/2015 6:46:20 PM EDT
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Originally Posted By TheDirtyLarry:
How about having someone pace you? That's what I did. My truck is 5 mph under at 65. When the speedo says 65 you're actually going 60. And 2-3 mph low at 35.
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If their speedo is a little off (all are) and yours is a little off it's not going to be accurate.


GPS is the easy answer.
Link Posted: 8/29/2015 6:48:39 PM EDT
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Originally Posted By heavymetal762:


If both are calibrated, the spedo will be more accurate. Minor lag, plus accuracy being dependent on number of satellites.
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Originally Posted By heavymetal762:
Originally Posted By sixnine:
GPS says I'm doing 51, speedometer says 55. Programmer was set for proper tire size calibration. Change calibration to a smaller tire and the GPS is just one mph slower than speedometer. The question is which do I go with? Match the speedo to the GPS or stick with correct tire size calibration. As is it would seem that is adding extra miles on the odometer. Trust GPS or programmer?


If both are calibrated, the spedo will be more accurate. Minor lag, plus accuracy being dependent on number of satellites.


No.

Even if both were perfectly matched to each other in identical conditions at the same time, the moment friction modifies the tyre temps and the centripedal forces expand the tyre the speedo will already be off.

Link Posted: 8/29/2015 6:51:42 PM EDT
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Originally Posted By TheDirtyLarry:
How about having someone pace you? That's what I did. My truck is 5 mph under at 65. When the speedo says 65 you're actually going 60. And 2-3 mph low at 35.
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What if the "pace" vehicle is Off? its a viscous cycle. GPS all the way.......
Link Posted: 8/29/2015 7:12:56 PM EDT

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Originally Posted By die-tryin:
What if the "pace" vehicle is Off? its a viscous cycle. GPS all the way.......

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Originally Posted By die-tryin:



Originally Posted By TheDirtyLarry:

How about having someone pace you? That's what I did. My truck is 5 mph under at 65. When the speedo says 65 you're actually going 60. And 2-3 mph low at 35.




What if the "pace" vehicle is Off? its a viscous cycle. GPS all the way.......

Great googly moogly batman, getting all kinds of science in this hoe









I didn't think of that
Link Posted: 8/29/2015 7:17:47 PM EDT

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Originally Posted By Waldo:




My truck and GPS are pretty close to each other. Within 1 mph.
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This

 


Link Posted: 8/29/2015 7:41:24 PM EDT


My speedometer got so far off a couple of years ago that I bought a new GPS to use it for a speed indicator. $600 to replace the instrument guage vs. $150 for a good GPS...GPS won.






Link Posted: 8/29/2015 8:16:46 PM EDT
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Originally Posted By Star_Scream:
I'd go with GPS.


Not affected by tire size, tire wear, tire pressure/temp.


Plus I doubt they calibrate car speedos at the factory
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They do it to police cruisers.  Not sure if ford did it for the crown Vics but someone did.
Link Posted: 8/29/2015 8:23:13 PM EDT
My speedometer reads the same as my Gps which I did find surprising. I think the Gps would give a more accurate reading of speed of the two for the most part.
Link Posted: 8/29/2015 8:31:20 PM EDT
I tested my Escort radar detector gps, Garmin gps and phone gps all at once a while ago. If recall they were all within a half mph, seems like the phone was slowest to react to changes. I concluded that they were to be trusted over my speedos.

My KLR factory speedo is 10% high, odo is correct, I have a Trail Tech Striker(gage set) that allows you to adjust tire dia., I have adjusted it to match the Escort radar detector gps(9500ix).
Link Posted: 8/29/2015 8:36:38 PM EDT
GPS is more accurate, but has a bit of lag.

Link Posted: 8/29/2015 8:37:47 PM EDT
If you wanna get less sophisticated , use a Stop watch and mile markers on the highway.
Link Posted: 8/29/2015 8:39:50 PM EDT
GPS tells you how fast you were going during the sampling period.  with proper setup (tire size/inflation, no malfunctions in the device), a speedometer tells you how fast you are going.  

and your speedo is not subject to reception anomalies.
Link Posted: 8/29/2015 8:40:17 PM EDT
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Originally Posted By die-tryin:
If you wanna get less sophisticated , use a Stop watch and mile markers on the highway.
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best way to calibrate a speedometer.
Link Posted: 8/29/2015 8:41:48 PM EDT
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Originally Posted By sixnine:
I'd buy that. I moved down one inch in diameter to get the speedo closer to the GPS. Still I wonder if that adds extra miles on the odometer that I didn't really put on.  
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Originally Posted By sixnine:
Originally Posted By IchWarrior:
How are you calculating your tire size, but the factory dimensions?


I'm gonna bet the squish of having some load on them makes them run smaller than the published dimensions
I'd buy that. I moved down one inch in diameter to get the speedo closer to the GPS. Still I wonder if that adds extra miles on the odometer that I didn't really put on.  

You went to smaller wheels to make your speedo match gps?

For real?
Link Posted: 8/29/2015 9:04:42 PM EDT
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Originally Posted By Enlightenme556:

You went to smaller wheels to make your speedo match gps?

For real?
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Originally Posted By Enlightenme556:
Originally Posted By sixnine:
Originally Posted By IchWarrior:
How are you calculating your tire size, but the factory dimensions?


I'm gonna bet the squish of having some load on them makes them run smaller than the published dimensions
I'd buy that. I moved down one inch in diameter to get the speedo closer to the GPS. Still I wonder if that adds extra miles on the odometer that I didn't really put on.  

You went to smaller wheels to make your speedo match gps?

For real?

No, I set the programmer calibration to a smaller size.

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Link Posted: 8/29/2015 9:06:05 PM EDT
Using the roadside radar speed readers my car and motorcycle are spot on.
I would only go by GPS if it was a high dollar unit. You could ask your local LE agency to help you out .
Link Posted: 8/29/2015 9:07:47 PM EDT
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Originally Posted By sirensong:
GPS tells you how fast you were going during the sampling period.  with proper setup (tire size/inflation, no malfunctions in the device), a speedometer tells you how fast you are going.  

and your speedo is not subject to reception anomalies.
View Quote

I set the cruise control at 55 on the highway and watched it for five miles or so until I was convinced it was steady.

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Link Posted: 8/29/2015 9:08:20 PM EDT
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Originally Posted By Fairplay:
Using the roadside radar speed readers my car and motorcycle are spot on.I would only go by GPS if it was a high dollar unit. You could ask your local LE agency to help you out .
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I would not put much faith in those being dead on.   Temprature fluctuations might screw with the oscillator.
Link Posted: 8/29/2015 9:13:52 PM EDT
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Originally Posted By Fairplay:
Using the roadside radar speed readers my car and motorcycle are spot on.I would only go by GPS if it was a high dollar unit. You could ask your local LE agency to help you out .
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It's an older Magellan but a good one, besides roadside radars are kind of scarce around here and no telling if they've been calibrated themselves.

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Link Posted: 8/29/2015 9:15:42 PM EDT
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Originally Posted By sixnine:

It's an older Magellan but a good one, besides roadside radars are kind of scarce around here and no telling if they've been calibrated themselves.

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Originally Posted By sixnine:
Originally Posted By Fairplay:
Using the roadside radar speed readers my car and motorcycle are spot on.I would only go by GPS if it was a high dollar unit. You could ask your local LE agency to help you out .

It's an older Magellan but a good one, besides roadside radars are kind of scarce around here and no telling if they've been calibrated themselves.

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Without being in a climate controlled environment they would most likely need calibration many times throughout the day.
Link Posted: 8/29/2015 9:17:59 PM EDT
Even consumer gps should be accurate to less than 1/10 mph
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