Bigger Tires & Odometer - Check my math!

[Wesmertz]

Hey all so I am a month or so into running 32x11(ish) tires. Factory was 30x9.5(ish) or something like that. I have not had the odometer programmed and if my math is right I don’t think I am going to.

My understanding is this:

1- The larger tire completes revolutions over the same difference. In the case the difference is about 5% less rotations. Therefor my odometer should only be logging 95% of the actual miles driven.. so if I drove 100 miles the odometer would reflect 95 miles. The odometer is actually dead nuts on or within .1 miles over 8miles as I tested yesterday.

2- with the stock tires the average mpg screen was always 1.5-2 mpg higher than what I calculate. Now with larger tires the average mpg screen is (3 tanks in a row) +/- .1 mpg compared to hand calculation.
3- the speedo is off by 1-2 @sub 50 and grows up to 4mph at 70.

I’m confused as to how the odometer is on the money, suddenly the mpg deal has become accurate, yet my speed is off. Perhaps they aren’t getting info from the same sensors? (Not 100% sure how it all works). Just seems odd. However I’m stoked that I can trust my mpg screen as well as miles to empty.

also I figure if I don’t program it I will buy myself another 1800 miles on my warranty ?

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[RangerPNW]

Well thats only true if you're using a gps speedo instead, as 60 mph on a gps speedo would read like 55 on the dash. I think adjusting it in forscan is pretty simple, and worth the 30 minutes of time it takes to adjust, test, re-adjust and verify. I detailed my own experience doing it in the linked post below. Only annoyance was buying the license for forscan, but since I primarily only own Fords, i didn't mind paying for it.

https://www.ranger5g.com/forum/thre...can-speedometer-calibration.13651/post-282988

 

[D Fresh]

If your speeds are off that much, your odometer is not on the money.

You need to correct with ForScan or a tuner.

 

[got3fords]

Well thats only true if you're using a gps speedo instead, as 60 mph on a gps speedo would read like 55 on the dash.

Are you saying that a GPS is not accurate? Just wondering. My GPS and my Ranger speedo pretty much match all the time. And my recent 60 MPH ticket in a 45 zone was spot on with both.

 

[Joeiconic]

Im running 265/70’s and my Speedo is only about one mph off at 72mph according to Waze gps.

 

[RangerPNW]

Im running 265/70’s and my Speedo is only about one mph off at 72mph according to Waze gps.

Not all 265/70's are the exact same size, so your mileage may vary. Most 265/70/17 are listed around 31.6" or so, i know my duratracs are listed as 31.7, and some others are closer to 32. Even then those numbers aren't entirely accurate from my experience, i.e my speedo calibration experience. Used the numbers spec'd and it was way off...

 

[Cabose-1]

Use this. It may help. Does all the math for you

https://tiresize.com/comparison/

 

[Jason B]

Your GPS is correct, vehicle speedos are rarely correct or accurate.

 

[Blmpkn]

GPS speedos aren't spot on unless your driving on a perfectly flat road. If your driving up or down a hill, the Indicated speed will be off.. the steeper the hill, the slower it'll read.

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