Factory tire size - speedo reading?
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Record Breaker
Joined: Sep 2012
Posts: 2,438
Likes: 73
From: S. Florida
Factory tire size - speedo reading?
I changed out the stock Goodyear Wrangler tires that came on my truck not too long after I got it (9 months)... I didn't pay much attention to what size it came with, I just knew what I wanted to put on (BFG 265/75R16).
Many years later (when GPS nav devices had speed readings) I noticed a 5mph speed discrepancy at 70mpg (~3pmh at 60mph)... I put this down to tire size (what I put on must be bigger than what it came with).
However, when I recently looked at my build-sheet, my truck came with TXWP P265/75R16.
If it came with 265/75R16, shouldn't the PCM have had the count/rev correctly set for the speedo at the factory?
Many years later (when GPS nav devices had speed readings) I noticed a 5mph speed discrepancy at 70mpg (~3pmh at 60mph)... I put this down to tire size (what I put on must be bigger than what it came with).
However, when I recently looked at my build-sheet, my truck came with TXWP P265/75R16.
If it came with 265/75R16, shouldn't the PCM have had the count/rev correctly set for the speedo at the factory?
Administrator
Joined: Apr 2010
Posts: 87,475
Likes: 4,223
From: Clayton MI
You would think so....... Of course, a bit of speedometer error isn't unheard of..... On your 98, assuming you have the digital odometer..... you can reflash the PCM for 'pinion ratio', or some such, to correct your speedometer error. Would be interesting to know what the current value is, and then figure out what it SHOULD be.