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Old 11-08-2019, 08:27 AM   #42
Dadhawk
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Join Date: Feb 2012
Drives: '13 FR-S (#3 of 1st 86)
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Quote:
Originally Posted by gravitylover View Post
My observation over the last 100k or so is that the speedo is spot on when my tires are new and gradually goes to about 2 over as they wear. My mpg also goes down as the tires wear. I figure the correlation is, because they're registering slightly higher miles traveled it manifests itself in speed and mpg being off by that percentage. I wouldn't mind finding the right tire size so that it's both over and under so it's more accurate.
I looked at mine, the 20 tanks of fuel before new tires, and the 20 tanks after it. Before was 6747 miles @ 33.41MPG, after was 7034 miles @ 32.72MPG.

It was the same tires, with the old tires having over 69,000 miles on them so they were just above the wear markers. Really though the change isn't significant and varies about as much as any randomly selected 20 measurements in my list. Time of year, driving variances could explain it as easily as tire wear.
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