Mileage decrease after some changes...?

I put some 245-75-16 MTR/Moabs on my 3" lifted XJ, and also put a full spare on my Surco roof rack.

Seems like my gas mileage went down in a noticeable way. Most of my commute is <45mph so I'm thinking it's mostly the heavier rolling stock not to mention the extra force needed to turn the bigger tires? I'm not thinking it's aerodynamics if I drive mostly in the city. What do you guys think?

that roof rack is a sail

dragging you down.. thus killing your gas

Big tires don't help either in city driving. All that stop and go stuff, it takes a lot of energy to start turning big heavy tires.

Did you recalibrate your speedo gear for the bigger tires, if not that might be effecting your calculation of mileage.

Even going 35-40 mph?

Hmm maybe I can source a used OEM tire rack for the back and compare then.

I have not recal'd the speedo yet, was not sure how to do that. I'm just eyeballing the gauge. I have been driving the XJ on stock tires for 3-4 months so I can tell when the mileage take a dive.

on my 98 with 4", 31's, roof rack, skids, and no front air dam (matter?), I get 13 mpg in the city, maybe alittle more highway. I was probably getting 17-18 mpg highway stock....so mine never really did as good as the common 20 mpg highway.

Suprisingly, my roof rack doesn't seem to effect those numbers.... maybe that fairing is helping more than I think?!

Recalibrate that speedo, it does more than correct your speedometer....

I went from stock tires to 32's without regearing yet, and I can tell you that my gas mileage went to hell. I'm currently getting 12 MPG on the highway, which is a far cry from what I was getting. I recalibrated my speedometer the same day that my lift was installed, so I know that my odometer is correct.

My 01 with 7-8 inches of lift, 35's and 4:88's got 10 MPG before I chopped & caged it.

its not a honda...im not sure of the tires you ran stock compared to now...but you would be suprised at how much more a 31" MT weighs compared to a 27" RT-S. there are a lot of things that come into factor with MPG: weather, your roof rack, wider tires...if you want your MPG and power back regear...mine went from about 16 to 12 when it got really cold around here, as soon as it warms up its fine.
I was running 10-20 city and 22-25 highway when i was stock...when i lifted it dropped to about where you are...nothing to worry about.

I have a link at home from another Jeep site, but basically you will account for the size of gears in the axles with the new size of tires installed. Now this is all on a chart, so just find the part number that corresponds to it. Call your nearest Jeep dealership and order. It took me about 3 days and 18 dollars to get the part. I think it took all of about 10 mins to install. I'd imagine that that same chart is floating in some archive on this site.
Like everything said above, one you start altering your ride so does the gas. A stock XJ gets anywhere from 18 to 21mpg on the highway, give or take some. With 6" lift, 4:56 w/ lockers, 32's. Gestimating I get 12 to 15 mpg's. It blows, but it's the price I pay.

I have a 5spd, 3.73 gears, 31's, 3" lift, ski rack, and full spare on the roof. Around town I get 12-14 mpg, and on the highway I can manage around 18 keeping it around 65 mph in 4th gear.