Speedometer accurate but odometer is not

[dustysprite]

Upsized from 255 75r 17 to 315 70r 17. On the drive home speedo was 60 actual was 67 ( google maps) as to be expected. I used a level and measuring tape to get the actual tire size and entered that into JScan. Test drove and was still off by 3 mph. Upsized in JScan and was still off by 2 mph. Upsized once more and was off by 1 mph but that’s how it was before I got the new tires, and I’ve checked off a few speed checkers on the highway and it is spot on to my speedo. However the odometer is still off. I checked off mileage markers on the highway and am getting .8 to the mile. Is there anyway to get this closer or should I just let it be?

Sponsored

 

[2nd 392]

Upsized from 255 75r 17 to 315 70r 17. On the drive home speedo was 60 actual was 67 ( google maps) as to be expected. I used a level and measuring tape to get the actual tire size and entered that into JScan. Test drove and was still off by 3 mph. Upsized in JScan and was still off by 2 mph. Upsized once more and was off by 1 mph but that’s how it was before I got the new tires, and I’ve checked off a few speed checkers on the highway and it is spot on to my speedo. However the odometer is still off. I checked off mileage markers on the highway and am getting .8 to the mile. Is there anyway to get this closer or should I just let it be?

Well, if you’re still under warranty let it be, it will be extended a little.

 

[WranglerMan]

I set mine as close as I could with JSCAN over the years and as long as it’s close 1-3 mph I don’t worry

 

[Terrymo]

To get the actual tire size I followed the advice of @roaniecowpony and got a 120” soft tape measure off Amazon and measured the circumference of the tire at about halfway between the tread edge and the center, then divided that number by 3.14. My speedometer and odometer seem accurate enough. I have other things to obsess over.

https://www.amazon.com/dp/B07H2W9DDW?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_2&th=1

 

[roaniecowpony]

Upsized from 255 75r 17 to 315 70r 17. On the drive home speedo was 60 actual was 67 ( google maps) as to be expected. I used a level and measuring tape to get the actual tire size and entered that into JScan. Test drove and was still off by 3 mph. Upsized in JScan and was still off by 2 mph. Upsized once more and was off by 1 mph but that’s how it was before I got the new tires, and I’ve checked off a few speed checkers on the highway and it is spot on to my speedo. However the odometer is still off. I checked off mileage markers on the highway and am getting .8 to the mile. Is there anyway to get this closer or should I just let it be?

The circumference is what advances the tire. That "squished" height method should be reasonably close, but the tread length per revolution can't be ignored. The truth is that there is some slippage in the tire under weight. This is more a tread "squirm" and it is part of the heat generated and the reason tires wear, even when driven only in a straight line. But, the circumference or some average of the high and low measurement of the tread crown, is going to be very close, when converted into the diameter tables of JSCAN. Me, I just used the published technical data for actual diameter and it was so close, I couldn't detect an error. That was with the tires when new. Now that they've worn about 3/8" of tread off, they are about 3/4" shorter, which should cause about a 2% error lower than actual.

 

[VKSheridan]

The way I see it, you’ll be able to drive from Seattle to Boston in just under 2,400 miles instead of the actual 3,023 but your (reported) miles per gallon is gonna suck!

Sponsored