A friend on mine needed new tires for her 1994 Ranger 4X4 with extended cab. She asked me for help since I’m the “Mechanical Genius” in her eyes. I figured this would be an easy job and gain me serious brownie points, so I offered to help. The last owner had the car repainted and the #%^#&# body shop painted over the door sticker with the tire info. The last place she got tires sold her raised letter truck tires I though were over-kill, so I wanted to get the original equipment tires for the truck. I jumped on to TireRack.com and found that it took 215/75R15 tires. I ordered 4 and had them installed. Then I get a call from her saying the speedometer is way off now. When she’s doing about 60mph, it reads about 70. What did I do??? The tires I took off were 31X10.50R15. I’m not familiar with the numbering with the X in it. Is it possible that the last owner got bigger tires and had the speedometer re-calibrated? Did I buy the wrong size tires? Are the tires I put on the rims OK or are these special rims? Somebody please help save my reputation!!!
The stock tires are likely P235/75/R15s they are about 29 inches tall, the P215/75/R15’s are about 27.5 inches tall. A difference of about 1.5 inches or about 9.5% (my math may be off). The speedometer seems to be off by about 8.5% (again my math isn’t the best so correct me if I’m wrong). But that’s pretty close to the discrepancy between the tire sizes.
If I had to guess, the 215/75/R15’s are probably the stock tire size for the 2wd version of this truck. So the answer is, yes, you bought the wrong size tires.
FYI, the 31x10.5R15’s are about an inch and a half taller than the 235/75/R15’s that are probably the stock size for the 4x4 version of this truck. It’s a pretty common upgrade, and might have even been an optional size.
Refund the money you charged her for the tires. Tell her to buy whatever tires she wants and to return your tires. Sell your tires on craigslist and write off the loss as “educational expense” …
No good deed goes unpunished.
If the tires are of the correct load range, it might be less expensive to change the speedometer gear that probably runs the speedometer from the transmission in the '94. I am not sure that it uses one, but it is reasonably likely.
I think the speedometer fix is my best solution. Tirerack.com states the tires I bought were for the 4X4 version of the truck, so I think I’m OK with the ratings. There’s no way I’m going to eat the cost of 4 new tires. I’ll pay for the recalibration of the speedometer myself. I might still save my reputation yet! Thanks for your input everybody…