[ARCHIVED THREAD] - GPS vs speedometer (Page 1 of 2)
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? |
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I think speedometers tend to read a little fast. Mine is 3-4mph fast at 65-70. 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). |
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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. |
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Quoted: 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 |
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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. |
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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. Posted Via AR15.Com Mobile |
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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. Posted Via AR15.Com Mobile Quoted:
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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. Posted Via AR15.Com Mobile 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). |
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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. |
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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. Posted Via AR15.Com Mobile Quoted:
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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. Posted Via AR15.Com Mobile 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. |
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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 |
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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. |
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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. 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. |
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If both are calibrated, the spedo will be more accurate. Minor lag, plus accuracy being dependent on number of satellites. Quoted:
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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. |
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Quoted: What if the "pace" vehicle is Off? its a viscous cycle. GPS all the way....... Quoted: Quoted: 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....... I didn't think of that |
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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). |
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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. Quoted:
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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 You went to smaller wheels to make your speedo match gps? For real? |
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You went to smaller wheels to make your speedo match gps? For real? Quoted:
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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 You went to smaller wheels to make your speedo match gps? For real? No, I set the programmer calibration to a smaller size. Posted Via AR15.Com Mobile |
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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. 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. Posted Via AR15.Com Mobile |
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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 . I would not put much faith in those being dead on. Temprature fluctuations might screw with the oscillator. |
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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. Posted Via AR15.Com Mobile |
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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. Posted Via AR15.Com Mobile Quoted:
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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. Posted Via AR15.Com Mobile Without being in a climate controlled environment they would most likely need calibration many times throughout the day. |