First off, hello. I have been on a couple forums for motorcycling but this is my first for my jeep as well my first post on here. I used to have a yj and then had a bunch of tacomas and now I'm back with jeep after buying my 2011 base 2 door jk. Lovin it so far. Here's the problem:

It is an auto w 3.73 gears and since day one I was getting 18-19 mpg on the highway, sometimes as high as 19.8. I recently installed a set of 285/75R16's (33x11x16) on the jeep and my mileage only dropped by .5-1 mpg. No biggie. I drove it like that for a week with pretty consistently good mpg numbers. Last night I installed the AEV 2" spacer lift and accidentally hit the reset button for my mpg computer. Now it's only reading 15-15.5 on the highway. WTF is that all about? Do I just Need to give it more time to keep calculating or could I have actually done something to screw it up? I drive around 40 miles each way to work so mpg is pretty important to me. I can live with 17-18 but 15 is no bueno. Any advice would be appreciated. I have only logged around 50 miles since I reset the mpg computer btw. Thanks in advance for any responses.

 

Welcome to the community. The mpg reader on your dash is useually not the most accurate way to measure your tru mpg. Fill up your tank then drive it, when you need gas fill it up again, take the miles driven on the tank and divid it by the gallons you put in. that will give you the most accurate reading. Now on to your problem. Because you changed your tire size even though its only about and inch in hieght and and inch in width, its still a change. If you didnt have your computer re-calibrated it will not give you a good reading. So give it some more time, do the math MPG method and see what you get. If stock your computer was telling you, you were getting 18-19, you were probaly only getting 17.5-18.5 or so. Now that your increased tire size, even though its small. your probaly gonna get 17-17.5 but it should still be better on the highway then that, I'm on a stock rubicon and only get 17-18 on the highway on a good day, so dont feel too bad.

 

It is going to be impossible to keep up with the mileage from your computer. Now that you have changed tires, the speedometer needs to be re-calibrated. I bet is is running about 5mph slower than your actual speed, therefore not tallying every mile you actually drive. Your shift points will be slightly off as well until you re-calibrate.

 

That is a good point I knew the speedo would be off from other trucks I've lifted but did not think about the shift points since this is only my second auto vehicle; I grew up driving only manuals including my old yj. How can I fix that since there is no programmer available for the 2011's yet? Can the dealer do that or will it be a bad idea to bring it to them since it is lifted now?

Regarding g the other replies I got, thanks. I will definitely calculate the mileage and not just trust the computer, especially until I get the computer recalibrated for the larger tires.

I have the cheap direct bolt on MBRP aluminized exhaust getting delivered today along with a cai. I'm hoping that helps a little to regain some pep and mpg that was lost with the 33's.

 

Just to give you an idea of the computers mis-calculation... I was driving downhill on a couple mile stretch of freeway with approx 10-15% grade and reset the MPG at the top... within 30 seconds I was getting 185 mpg... not 18.5, 185. I think it was a tad off. LOL

 

My computer reads about 1-2 mpg low, but that's to a point of splitting hairs I think.

 

From what I've seen, mileage starts to drop kinda quick once heavier tires (or tires with more rolling resistance) are put on. I have seen about a 2mpg difference between stock Rubicon and stock Sahara wheels on my JK.

FWIW, I didn't see any decrease with my little 1.75" lift, so it may be just your tires and speedo/economy calibration at this point..