Calculating mpg with dif size tires

[Cruiserhead05]

I know how to factor the change in tire size on the cruiser with a larger tire but had a question on a different vehicle.

Stock tire size is 205/50

New tire size is 225/40.

Can anyone give me a rough idea how to factor the change in size when calculating mpg?

Thanks

 

[CruisinGA]

rim size matters too...

tell me that and I will help you

 

[Cruiserhead05]

Rim is an 18", sorry for the delayed response.

 

[CruisinGA]

You travel, per rotation of the tire, 3.9% less than your speedometer thinks you are, because your new tires are smaller.

So, take whatever number your trip meter gives you, multiply by .039, subtract that number, and that is roughly your actual mileage. Divide by gallons consumed and you get your MPG.

I think this is correct. Someone tell me if it is not.

 

[Colorado Boy-74-FJ40]

Try this. It showed 3.78% difference.
http://pw1.netcom.com/~sgalaba/tiresize.htm

 

[Capt. Jim]

I use a hand-held GPS to determine what speed I am actually going vs. the speed indicated on the speedometer. Since the speedometer is directly linked to the odometer, I figure the difference in percentage and apply this to the actual milage traveled.

Example: On one of my trucks, when the speedometer reads 70 mph, my gps says I am going just under 77 mph. this is about a 10% difference. So I multiply the mileage on my odometer by 1.10 and calculate my gas mileage.

This also factors out any inaccuracy or mis-calibration that may exist with the speedometer.