Adjusting for larger tire size
 

Hi folks

I have 31" tires while the stock tires are 28" diameter.

28" tires revolutions per mile: 742
31" tires revolutions per mile: 670

742:670 = 1.1074:1

So I multiply the odometer readings by 1.107 when I calculate my MPG.

This works perfectly as I have observed, via a gps-powered iOS app "Speed Box speedometer", that when I'm traveling 70 mph I'm actually going 77 mph. And at 65 mph it's actually over 70 mph. That adds up because 70 * 1.1 = 77 and 65 * 1.1 = 71.5

However! At lower speeds, like 55 mph and below, the car speedo and the speedo app read dead on. They're the same.

So should I still be multiplying the mileage by 1.107 if I never drive faster then 40 mph??

I do not understand why the speedometer and the app read the same 1 - 55 MPH, but not 56+.

It's not like the app and speedo both read 54 mph, yet the speedo reads 55 and the app reads 60. It's more gradual

... it's a multiplier.

Yes, multiply the odometer reading by however many percent larger the new tires are, even if you don't drive fast.