i went from a 225-75-15 to 32-11.5 it was averaging 15 with the 225 and now I gat 13 with the 32's. not much, but then again I did not redo my speed sensor wheel thingie.. maybe it remained the same.. O I would jump for joy if I can average 20mpg.
I passed several highway digital speed readers and it was always 5-7mph faster than my in dash speedo.
So, when my speedo says 60, I'm actually doing like 65-67 mph. Now, I calculated my gas and it was 15.1 miles per gallon with 33s and a 4 inch lift. Now, am I ACTUALLY getting worse or better mpg? (I assume no calibration was done hence the mph discrepancy.
10% error factor in the true speed from the radar set vs the speedo, try this, use the mile markers and see what your odometer says. Also try this site http://www.4lo.com/calc/gearspeedo.htm, just plug in your numbers, the gear is easy to change, it's in the transfer case where the wires go in on the drivers side.
Just as a side note, my 98XJ came with P215/75R15 GSR's, I never checked the speed or stuff. When I went to 235/75R15 Pirelli Scorpions I was curious and checked using the mile markers on the two interstates I used daily. Those mile markers were dead on with my trip odometer, in other words my STOCK setup was rolling on miles faster than it was actually doing. I also had a friend of mine who is a state trooper in NJ clock me, with the 235's it was dead on from 30-90mph. Jeep got out of their warranty early, I did not change the stock tires till about 50,000miles.