cucmd1

Hey folks, I've been searching and can't find what I need so here we go:

What will 285/70/17 ATs on a stock 08 2dr X with 3.73/auto do to mileage? I don't plan on regearing anytime soon.

I figure that's step 1, and later I'm going to add a Black Diamond 3" lift when funds allow, or do I have it backwards and should go lift then tires?

I've really enjoyed seeing how frienldy and helpful the jeep community on this forum has been so far. Thanks,

Clay

ESPMike

Well I went tires then lift, and now Im not sure I'll ever do a lift because the tires made a big difference. I went with 285/75/16 with 4.10/auto, and I saw a decent hit on mileage. I was around 17, the tires saw me drop to 14-15 and bumper and winch finally settled it around 14.

Id definately go tires then lift, a 3" lift with the small rubber is pretty funny looking. And try not to think about gas mileage too much, if you have a Jeep already you dont really care that much in the first place. No matter what it sucks.

cucmd1

do you think the 3" lift is too much for 33's? Should I be looking at the OME 2" instead? I talked to the lift advisors at 4wd.com and they're the ones that steered me towards the 3" black diamond, and yes I told them I was thinking 33's.

ESPMike

Im running 33's with no lift and Ive never experience even a little rubbing. Id say 2" is fine, even a BB would do the trick. I know if I lift 2 or 3 inches Im going 35's when these wear down. If you want to go with 33's, go with tires first, then the lift. No need to lift it to fit the 33's.

cmh2001

I just installed a Teraflex 2.5 BB lift. Next week I'm installing BFG M/T 285/70R17's.

Here is a picture of my lift with BFG A/T 265/70R17's.

cucmd1

going with just tires first will definitely make my wife happier ($$$$$) and allow me to get some other odds and ends I need. Keep the info coming, its a big help.

Clay

AK4Dave

My 285/75/16 with no lift.....
Noticed a little less mpgs after installing the 33s but not too bad. Well.....I guess "not that bad" is a relative term. Even the loss of 1 mpg would be bad to some. I just haven't really checked it that close, as I'm so pissed all the time at the gas prices already, I don't even want to think about it anymore. I just love driving my jeep so I guess that has finally outweighed the mpgs now.
I do have an Airaid CAI and throttle body riser, along with the cruise control I installed awhile back. So they do help a bit.

TINMAN080

Hey Clay- look up tirereview.com ( calculators). They have all the formulas needed to compare tire sizes to gears, a pretty good site. It looks like the 285's are 34.63 diameter vs a stock X 255/75 tire diameter of 32.05. Anything over a 3/4'' difference will cause a noticable difference in performance. According to the formulas the 3.73 would be marginal for this diameter tire. Check out the charts.....

cucmd1

thanks, that's the kind of stuff I need to hear. Unfortunately with sharing the jeep with my wife I can't go as crazy as I might like with mods, especially with it being pretty much brand new.

One thing I've been confused about is the gearing issue. I always thought the taller gears gave crappy mileage (4.10, 4.88, etc) but am I correct that as the tire size increases the gears actually bring the mileage/power delivery back into line with what was originally intended by the manufacturer with rpms?(minus some mpg drop from more rolling resistance, air resistance, etc)

You guys about have me scared to change up tire size if I can't change gears. To top that off I was cruising on the interstate going about 75 today, it was really windy and it was shifting nonstop, and thats stock! I see where we can't afford to lose power, but man these stock tires suck and I'll only be able to talk the Mrs. into buying tires once in the next 50,000 miles or so. What to do?

Clay

Ryno

You are correct. Tire size and axle ratio go hand in hand. When you change your tire size you are effectively changing the overall gear ratio of your axles... That's why Jeep pairs 29" tires with 3.21 gears, and 32" tires with 3.73s or 4.10s, without a significant difference in fuel economy.

Given the same engine RPMs, lower gears will move your vehicle slower than higher gears.

If you took two identical Jeeps--except one with 4.10s and the other with 3.73s--and set them on cruise with the engine RPMs at 2000, the Jeep with 4.10s would be traveling slower than the Jeep with 3.73s. Less distance traveled at the same engine RPM = lower MPGs. Put bigger tires on the 4.10 Jeep, though, and it will be closer to the 3.73 Jeep's speed at the same RPM.

If rolling resistance and wind resistance stayed constant, you would probably see a slight increase in fuel economy when upsizing to 33" tires, because you are traveling more distance at lower RPMs (if the change is not enough to cause more downshifting). However, our world is not that ideal.

I'll be the one reassuring voice. 33s and 3.73s won't be much different than stock as far as power goes. Just go for it.