And the stock tire is?
Anyway - whats the problem? You getting pretty good average mpg for X - nearly same as posted on sticker. Specially considering that you driving it out of recommended speed range.
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On a side note:
We got to Moab and back last weekend, with some back/off/road driving about - cut it to 374 miles total. It all was made on 16.5 gallons of petrol.
(got full tank before left Durango, and then had to add 8.4 gallons in Moab)
SWEEEET 8)
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“Yay! I’m gonna be sick!” –GIR