My speedo has not been recal'd yet on my 2dr Rubicon.

So given the difference from 32's to 35's... that assumes about a 9% difference. I clocked 219 miles on 14 gallons.

219/.91 = 240.6

240.6 / 14 = 17.2 mpg

Doesn't seem to horrible given that I was only getting around 18-19 before hand. Am I off here? (without going into the geometry of difference in circumference, etc.)

 

You need to recal you speedo to figure this out. If you went 219 miles and used 14 gallons of fuel I show 15.64 miles per gallon that is quite a change if you you was getting 18 -19.

 

Thats what I get with my 2dr X with auto and 35's. I checked the speedo to a GPS and found I was 9-10% off and did the same math.

I get around 16-17 in the witner and 18-19 in the summer. I have ran Mobil 1 since new but was told to run 5w30 instead of 5w20. I have always wondered if this causes the severe MPG loss in the winter. Because I don't start the vihicle up in cold weather and let it warm up which usually accounts for most of the poor winter MPG.