Mpg

[RonJR]

So, I get using the calculations to account for larger tire diameter, but wouldn’t that only come into play if trying to compare mileage to the dash display? If you are calculating based on miles driven between fill ups, then it’s straight division, otherwise, you are calculating based on what it should be, rather than what it actually is, no?

You have it exactly backwards. The mileage calculated using the odometer vs the mileage display should track each other, because the mileage display uses the odometer for its calculation. The actual numbers won't be the same because of other errors in the onboard mileage calculator, but they'll go up and down together.

However, our goal is to find out how many miles (each = 5280 feet) we can go on a gallon of gas. That requires us to adjust our odometer reading when we change tire size, because the odometer is off by the ratio of actual tire diameter to the stock diameter. As AuSeeker says, the best way is to calibrate the odometer against a GPS so you have an accurate correction factor. You can also use the mile markers on an interstate highway. Just go 10 miles according the markers and see what your odometer reads. In your case it will probably read something like 9.2 or 9.3 miles. 10/9.3 = 1.075 which is your correction factor.

The odometer effectively counts numbers of wheel rotations (actually numbers of drive shaft rotations). It has no other knowledge of how far you've gone. If your wheel diameter changes, the odometer reading will have errors.

 

[mjotrainbrain]

On this topic, I certainly don't mind doing a conversion for calculating fuel economy (I calc by hand at the pump anyways, so what's one more number), but has anybody actually recalibrated their odometer to account for larger tires? I tend to like to have little things like that sorted out if at all possible.

 

[AuSeeker]

On this topic, I certainly don't mind doing a conversion for calculating fuel economy (I calc by hand at the pump anyways, so what's one more number), but has anybody actually recalibrated their odometer to account for larger tires? I tend to like to have little things like that sorted out if at all possible.

If you search online there are companies that can calibrate your odometer to correct it for tire size changes, gear changes, etc. as long as you can tell them the how much it's off, these same companies also usually repair defective odometers and or sell replacement odometers.

 

[Terminator03]

If you have different size tires than stock the only way to know the actual miles driven is using a GPS to track actual miles driven, the odometer is not going to be accurate if you have bigger or smaller tires, once you know the actual miles and gallons used then you can manually figure the MPG.

You have it exactly backwards. The mileage calculated using the odometer vs the mileage display should track each other, because the mileage display uses the odometer for its calculation. The actual numbers won't be the same because of other errors in the onboard mileage calculator, but they'll go up and down together.

However, our goal is to find out how many miles (each = 5280 feet) we can go on a gallon of gas. That requires us to adjust our odometer reading when we change tire size, because the odometer is off by the ratio of actual tire diameter to the stock diameter. As AuSeeker says, the best way is to calibrate the odometer against a GPS so you have an accurate correction factor. You can also use the mile markers on an interstate highway. Just go 10 miles according the markers and see what your odometer reads. In your case it will probably read something like 9.2 or 9.3 miles. 10/9.3 = 1.075 which is your correction factor.

The odometer effectively counts numbers of wheel rotations (actually numbers of drive shaft rotations). It has no other knowledge of how far you've gone. If your wheel diameter changes, the odometer reading will have errors.

Doh! Thanks for pointing out the error in my thinking! I realized that the speedo is now off by like 3mph at 60, or so, but for whatever reason spaced the fact that my trip odometer (which I use for hand calculating mileage) would be off as well. I’ll bet my calculus mileage will be better once using that correction factor!:tea: