A query for message board gurus: How much does tire size affect odometer readings? I am trying to figure mpg, and I realized that my tire size has been changed pretty radically! When I bought my truck, it had a 235 spare and 265's on the rest. I now have 285's all around with one of the 265's as the spare. What size was the '89 F250 odometer orginally calibrated to run on? What does it take to recalibrate for the new tire size, which I intend on sticking with? Any help would be great--Thanks! :-)

Mtn. Scott
1989 F250 4X4
351w

 

Mtn Scott ,
I dont really think your speedometer / Odometer is off that much if you are running 285 tires . Ford speedometer gears are color coded and usually range from I think 18 to 22 teeth .I think each tooth adds 2.5 mph. Best way to check how much it is off is either use highway mileage markers as a comparison . Also some larger repair shops that deal with Rim & Tire upgrades will have a chassis dynometer that they can run your truck on and tell you exactly how far off the speedometer / Odometer are . I think in 89 , stock tire sizes ranged from 235 to 265 .
Paul

 

My '89 F250 4X4's door sticker says it came w/ LT215/85R16. I now have LT235/85R16. I know my speedo gear is a 20 tooth (black), I just replaced it. My speedo is off, but it's better then no speedo at all. The guy at the parts counter thought I should go to the 21 tooth gear, but of course, he didn't have any. Cost was just under $7 at the local dealer.
Roger Lane
Test Analyst
Sr. Automated Test Engineer
IBP, Inc.