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On my first set of tires the odo ran slow so it under reported actual mileage driven to the tune of about 0.7% on average, but my current and second set of tires is just the opposite -- the odo over reports mileage by about 0.8% on average. The percentage varies over the life of the tires so as the tread wears down the tendency is to move towards over reporting mileage. I suspect that over time the tire carcass expands a bit and this partially offsets the reduction in circumference due to tread wear.

My speedo has always appeared to report about 0.8mph less than actual. I've measured using GPS as well as time and distance on interstates that have precisely measured miles.


Brian
 

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On my first set of tires the odo ran slow so it under reported actual mileage driven to the tune of about 0.7% on average, but my current and second set of tires is just the opposite -- the odo over reports mileage by about 0.8% on average. The percentage varies over the life of the tires so as the tread wears down the tendency is to move towards over reporting mileage. I suspect that over time the tire carcass expands a bit and this partially offsets the reduction in circumference due to tread wear.

My speedo has always appeared to report about 0.8mph less than actual. I've measured using GPS as well as time and distance on interstates that have precisely measured miles.


Brian
So in 100 miles your odo was reporting 99.3 on old and 100.8 on new, how the heck do you get that precise of measurement with a GPS? or time and distance?
 

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So in 100 miles your odo was reporting 99.3 on old and 100.8 on new, how the heck do you get that precise of measurement with a GPS? or time and distance?
Don't let precision imply accuracy. I can say I'm 17.123329803542 years old. That's a very precise number, but not accurate.
 

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Oh, my bad, I should have been more precise and said "that accurate" not "that precise".

#Seconddeck
You were fine in your statement. I too wondered how he got to that level of precision in his measurements.

I've never heard of over/under reading based on tire wear / remaining tread depth. Mathematically it makes sense, but makes me wonder if in the noise level of reading or at least displaying on odometer / speedometer.
 

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Just for fun, yesterday, I used my phone's GPS to check the speed. I set the cruise at 100km/h. The GPS was reading 101.9km/h. I tested at 60km/h and the GPS was reading 62.1km/h.

I tested with another phone and actual GPS - same result. I have stock wheels (mag + tire)... my tires are at 4½/32.

I will be switching to winter tire in less than a month. I'll test again. Same size and everything but they are actually 0.5%/1% smaller.
 

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Old Phart
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So in 100 miles your odo was reporting 99.3 on old and 100.8 on new, how the heck do you get that precise of measurement with a GPS? or time and distance?
ROFL

Yeah, once you get into the single digit variations it's below the level of significant numbers.

Particularly for speed.
 

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Focus Preacher
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Are we really building a thread on a +/- 2 mph error?

You guys have so much free time in your hands [grinking]
 
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