MPG calculation with non-stock tire size?

[AeroModder]

I'm not sure how to calculate miles driven with a non-stock tire size. I can't remember if you multiply the miles driven to the % difference in speedometer reading, or what.

Thanks in advance!

[NeilBlanchard]

Yes, you need to calibrate the odometer (by driving a marked distance on the highway, or measuring it with GPS, or measuring it in GoogleMaps) and then adjust the odometer reading by a percentage to correct it.

[AeroModder]

Alright. The new tires I'm getting will cause the speedo to be 2.3% slower, according to this: Wheel-Tire - WWW.RIMSNTIRES.COM size comparer&sw1=185&ar1=70&bd1=14&rw1=6.5&et1=45&sw2= 195&ar2=70&bd2=14&rw2=6.5&et2=45&text1=&text2=

Can I use that percent, or will the odometer reading be changed by a different percent so I have to go a set distance and compare?

[cfg83]

AeroModder -

I use tire calculators and then I cross-reference my odometer trip meter against a GPS :

My Continental Contact-Pro Tires

CarloSW2

[AeroModder]

Thanks for the info, but I don't have a GPS to go by, and can't afford one, especially after ordering the new tires. ^^;

But from reading that, it looks like the percentages for the speedo and odometer are the same.

So, if I'm not mistaken, the formula would be:

Distance reading x % correction = Distance error
Distance error + Distance reading = True distance

And in my case it would be about:

250x0.02=5
5+250=255

Is that right?

[cfg83]

AeroModder -

Yup, that's what I do.

My tires account for a +2% "bonus" when I fill-up. To keep track of what is going on, this is my typical "Details" section of my Fuel Log :

193217 miles on odometer. Tires are larger, so odometer is reading 2% less, hence +2% correction: 246.4 * 1.02 = 251.328

Maybe you can find a pal with a GPS that you can borrow. Hiker dudes usually invest in them. You usually only need about 20+ miles to get a good reading. I put the GPS on my dashboard to make sure it doesn't lose access to satellites, and shoot for long straight lines (i.e. freeways). I am sure it works fine on curvy roads, but I also think that will introduce a tad more error.

CarloSW2

[AeroModder]

Sweet. Thanks for the help! ^^