Mpg

[mattjeep]

Hey guys,
I'm getting ready to lift my KJ and I'm going to go with 265/75's, either 4 or 6 plys. I know that supposedly when you go with that size of a tire you will loose some mpg's. I was wondering how much do you think I will lose? On trip going about 75 I'll average 20-22 mpg's and go about 350-360 on one tank. One other thing, why would you lose mpgs, by going with bigger tires, doesn't the bigger tire size make your engine turn at lower rpms?

 

[tjkj2002]

Hey guys,
I'm getting ready to lift my KJ and I'm going to go with 265/75's, either 4 or 6 plys. I know that supposedly when you go with that size of a tire you will loose some mpg's. I was wondering how much do you think I will lose? On trip going about 75 I'll average 20-22 mpg's and go about 350-360 on one tank. One other thing, why would you lose mpgs, by going with bigger tires, doesn't the bigger tire size make your engine turn at lower rpms?

You won't see as bad of a drop being only 2wd,bigger tires are heavier,have more friction against the road surface.Also bigger tires without regearing is actually changing your final drive ratio and out of the engines "normal" powerband which equals less mpg's,and yes it makes the engine turn less RPM's but now the engine has to work even harder to maintain speed.You'll notice it alot going up hills and trying to accelerate from about any speed..You may drop to 17-18mpgs on the highway,maybe get 19mpg's if you are lucky.If you where 4wd you'd drop even lower.I only get about 11mpg's average and I have the 4.10 gears(lowest you can go in a 4wd KJ) but I also am 1500lbs heavier then stock.

 

[mattjeep]

Thanks for the info. Damn thats crazy. So i guess they only way to restore my mpgs and powerband is to get the 4.10 gears.

 

[JeepJeepster]

The moment of interia is also higher with bigger tires. In other words, the further out the weight is on a spinning object, the harder it is to turn.

 

[mattjeep]

That reminds me. I read something about getting your speedometer recalibrated for the bigger tires in here. They mentioned something about going to dealer to get it done. Do you know anything about that and how much it would cost.

 

[LibertyFever]

I understand that there are certain tire sizes available within the Liberty's engine computer but I've yet to find a list. If somebody has it maybe they can post it for us.

 

[mattjeep]

What do you mean theres a list of tires sizes in the computer? Do you mean the ecm will automatically recalibrate the speedo for you. Or do mean that the dealer has to plug the ecm in and choose from the list?

 

[JeepJeepster]

No, if the dealer knows what they are doing they can go into the computer and set it for certain size tires. I believe they set it by the revs per/min.

 

[fouros]

depends on the MY
MY04 and earlier you can have a pinion reset done by changing Revs per mile

05+ i think its set tyre sizes AFAIK

 

[mattjeep]

Oh ok, thanks guys for the info. Do you have an idea how much they would charge to go in to the computer?