The following website calculates your approximate mph when you get bigger tires: http://www.offroaders.com/info/tech-...alculators.htm.

My question is I have a '99 2WD and went from stock 225s to 265s. From the calculator, if my speedometer shows 70mph, I'm really going 82mph. If my speedometer shows 80mph, I'm really going over 90. Can this be true?

Also, there wasn't a conversion for mpg on this site. Does anyone know how I can accurately calc mpg. I currently get 15 - 16 mpg with mostly street usage but can get close to 18 mpg if I drive mostly highway.

 

The tire diameter depends both on the width (the 225 or 265) as well as the aspect ratio (the 70, 75,etc.) part as well as the wheel diameter (the 15 or 16 part):

http://www.4crawler.com/4x4/4LoCalc.shtml#MetricTire

One you get the diameters of both tires then your speedo error is approx. equal to the percentage change in tire diameter. So if the new tires are 5% taller than the old tires, you are driving 5% faster than indicated. Same on the odometer, if the new tires are 5% taller, the actual miles driven will be 5% more than indicated on the odometer.

 

on my toyota, with stock gears and 33's, when im showing 15, im doing 20, 20 is 25, 26 is 30, 29 is 35, 34 is 40, 36 is 45, dont remember 50, 46 is 55, and 59 is 70, lol.

 

heres a little info i looked up from here for the mpg question:

I'm not sure if the gears change anything since I'm not sure where the odometer gets it's reading from. Assuming that it's at the wheel, then you could correct your mileage be figuring the difference in the circumference of the tires. For example, if your truck came stock with 31" tires that gives a circumference of 97.34" (Pi x diameter). A 33" tire has a circumference of 103.62". Divide 103.62 by 97.34 and you get 1.06. This is your correction value. If you multiply your odometer reading by 1.06 you will get closer to your actual mileage. So if your odometer reads 250 miles on a tank, you really went closer to 265 miles (250 x 1.06 = 265).