[ARCHIVED THREAD] - Tire size vs speedometer? (Page 1 of 2)
Posted: 7/13/2016 9:46:10 AM EDT
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Just pulled over by highway patrol. Said I was doing 78. Had cruise set to 70.
Just put 275 / 65 / 20's on it last month. Won't bigger tires make the speed show slower on the speedo since smaller tires turn faster? Got warning, didn't care about the .45 in the glovebox. Just didn't want me to touch it. |
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Just pulled over by highway patrol. Said I was doing 78. Had cruise set to 70. Just put 275 / 65 / 20's on it last month. Won't bigger tires make the speed show slower on the speedo since smaller tires turn faster? Got warning, didn't care about the .45 in the glovebox. Just didn't want me to touch it. Maths are hard. Get the phone app speedometer and gps check your speedo. |
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Just pulled over by highway patrol. Said I was doing 78. Had cruise set to 70. Just put 275 / 65 / 20's on it last month. Won't bigger tires make the speed show slower on the speedo since smaller tires turn faster? Got warning, didn't care about the .45 in the glovebox. Just didn't want me to touch it. LOL yes. The larger tires make your "speed show slower on the speedo", which is to say the speedo reads lower than your actual speed. GD math. I <3 this place. |
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Yeah, but 8mph faster at a displayed 70mph means MUCH bigger than stock tires. Quoted:
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Bigger tires will result in you going faster than what your speedo says. Yeah, but 8mph faster at a displayed 70mph means MUCH bigger than stock tires. He's got on dubs. |
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8 over thats a tough crowd. Here nothing happens untill at least 10 over more like 20 over at interstate speeds. Likewise in California. Exception is when the motorcycle cops are pulling a wolfpack. Pretty much anybody who drives past them gets cited for something. I got cited for 3 over once. Otherwise, it is mostly 20 over on the freeway. Also: I have seen 4 different speeds displayed at the same time with ~5mph difference between them (GPS on my phone, GPS on my in dash nav system, ECU monitoring computer and the speedometer). |
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Yeah, but 8mph faster at a displayed 70mph means MUCH bigger than stock tires. Quoted:
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Bigger tires will result in you going faster than what your speedo says. Yeah, but 8mph faster at a displayed 70mph means MUCH bigger than stock tires. I have 37's and I'm about 13mph off at freeway speeds. |
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Quoted: Yeah, but 8mph faster at a displayed 70mph means MUCH bigger than stock tires. Quoted: Quoted: Bigger tires will result in you going faster than what your speedo says. Yeah, but 8mph faster at a displayed 70mph means MUCH bigger than stock tires. You make the assumption that the stock speedometer/tire combo was "right" on. Yeah right. |
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You make the assumption that the stock speedometer/tire combo was "right" on. Yeah right. Quoted:
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Bigger tires will result in you going faster than what your speedo says. Yeah, but 8mph faster at a displayed 70mph means MUCH bigger than stock tires. You make the assumption that the stock speedometer/tire combo was "right" on. Yeah right. They usually read 2-3mph faster than actual speed from the factory. So OP went from going ~68mph at an indicated 70mph, to 78mph at an indicated 70mph, for a difference of 10mph, on 34" tires. His original tires must have been pretty small. |
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Just pulled over by highway patrol. Said I was doing 78. Had cruise set to 70. Just put 275 / 65 / 20's on it last month. Won't bigger tires make the speed show slower on the speedo since smaller tires turn faster? Got warning, didn't care about the .45 in the glovebox. Just didn't want me to touch it. So many revolutions to reach 70....so you car matched that many revolutions which turned out to be 78 for the larger tires.... |
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Quoted: http://i1376.photobucket.com/albums/ah32/308toyota/Screenshot_20160713-091335_zpsotwmxx7v.png unless you had really tiny original tires, I don't see it making much of a difference. Wall height change will have a far greater effect. |
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you are factoring tread width, not wall height. Wall height change will have a far greater effect. Quoted:
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http://i1376.photobucket.com/albums/ah32/308toyota/Screenshot_20160713-091335_zpsotwmxx7v.png unless you had really tiny original tires, I don't see it making much of a difference. The sizes shown there have different widths as well as heights. The second number (70) is the sidewall height as a percentage of the tread width, so 70% of 285mm is going to be greater than 70% of 265mm. |
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Just pulled over by highway patrol. Said I was doing 78. Had cruise set to 70. Just put 275 / 65 / 20's on it last month. Won't bigger tires make the speed show slower on the speedo since smaller tires turn faster? Got warning, didn't care about the .45 in the glovebox. Just didn't want me to touch it. That's correct. Which means you're going faster than your speedo registers if your tires are larger than stock. |
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you are factoring tread width, not wall height. Wall height change will have a far greater effect. Quoted:
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http://i1376.photobucket.com/albums/ah32/308toyota/Screenshot_20160713-091335_zpsotwmxx7v.png unless you had really tiny original tires, I don't see it making much of a difference. GPS says 74-75 with my Destination MTs... So there's some fudge factor. |
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Quoted: 8 over thats a tough crowd. Here nothing happens untill at least 10 over more like 20 over at interstate speeds.  I swear every LEO on 95/75/4 has an unwritten threshold somewhere in the 8-12mph over zone. Anything over and you risk a ticket. Anything under and you're getting passed by everyone except the snowbirds, tourists, or geriatrics. OP, as others have said, a larger tire will increase actual speed over indicated speed. Check actual speed with a gps/smartphone. You'd have to have a pretty big tire to through it off by much though. I went from 32" stock tires to 37"s and was off by maybe 7-8mph at highway speeds. |
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Turns out the app says I'm 7mph off. Can I adjust the speedo myself? 2013 F150. Quoted:
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Have your dealer re-calibrate your speedo. Turns out the app says I'm 7mph off. Can I adjust the speedo myself? 2013 F150. It's probably new enough for a computer. It only takes a few minutes and wasn't much. |
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I had that conversation back in the very early '80s with a Mississippi state trooper when I got stopped in my jacked up '67 Firebird. His words were "It's because of them big ass tires you have on the back of that thing. You're going to get your ass killed (meaning from the speed)". He was funny and I didn't even get a warning.
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Have your dealer re-calibrate your speedo. They wanted like $350 to do mine, I walked out 
It's pretty easy to figure without a website via a proportion. Probably not as accurate but it's pretty damn spot on for my jeep on 35's. ETA: (current/original)speedometer readout |
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They wanted like $350 to do mine, I walked out
It's pretty easy to figure without a website via a proportion. Probably not as accurate but it's pretty damn spot on for my jeep on 35's. ETA: (current/original)speedometer readout Quoted:
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Have your dealer re-calibrate your speedo. They wanted like $350 to do mine, I walked out
It's pretty easy to figure without a website via a proportion. Probably not as accurate but it's pretty damn spot on for my jeep on 35's. ETA: (current/original)speedometer readout Holy shit! My Jeep was like $40 at the dealer. |
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Let me clear something up here. Speedometers are never off by a certain number of miles per hour. They are off by a percentage.
If my speedometer is showing 70 mph, but I'm actually traveling at 77 mph, then my speedometer is off by 10%. It will vary by speed, this same speedometer would show me doing 10 mph if I was actually traveling 11 mph, and it would read 200 mph if I was actually going 220 mph. So it might be off by 1 mph or 20 mph depending on your speed. Start thinking of it as a percentage difference rather than a set speed value difference. A speedometer is never just "5 mph off." So, please do not think of it that way. I can't tell you how many people (IRL) I have heard say "Oh yeah my speedometer is off by 5 miles per hour." If you're parked it's going to show 0 mph no matter how far off it is. |