nangulator

I just did my 5.13s last week and already noticed better mpg, I went from 11.2 to 14.1 mpg, according to the instrument cluster.

JPop

Yep, worst case scenario is that you would lose a single mile per gallon. Then back out of that you are going to be up against the stall speed of the converter more on roll out, more kick downs at cruise speeds when passing or encountering grades, head and cross winds. Then as it's still a MAP based system, you will run more negative pressure in the manifold with 5.13s than 4.88s, leaving the throttle blade more closed which results in a nominal amount of fuel usage.

The 5% jump in gear ratios from 4.88s to 5.13s are very nominal. When you look at what robs efficiency from an automatic, it's all kick downs and unlocking the converter. A kick down and unlocking the converter increases the rpms by 50%, and if you can minimize the times it happens by 5% or more, you have regained most of the potential efficiency losses caused by nominally twisting the engine a little more.

Most of all you have a ready and waiting 4-5 additional hp at cruise speeds and there isn't any question as to which is going to perform better off road.

joneszj

Dude, what are you guys doing to get better MPGs???? I call shenanigans! I have 3.21s and 35s. I get between 13 and 15. I thought I was getting better than that, but if you actually calculate it yourself rather than rely on the computer in the dash, I think the results speak for themselves. I cant wait to regear.

Hoopa70

Im running an auto with 35's and 5:13's with the flashpaq. I am very lucky if i get 13. I usually get just under 12. Around town is better than highway also. hope that helps. I would be curious to see honest results of 4:88's.

chopper29

how bad was the jeep with stock gears. ill prob be in the same boat for awhile. just wondered if its tolerable withou gears or if its a must right away. thanks

JPop

I can routinely get over 20 hand calculated highway mpg rolling on heavy 35s, and in the city haven't ever hit less than 16mpg. Sure I made some changes to build efficiency, while not necessarily power, including headers, exhaust, Flashpaq and a CAI. With that said, much of this has to do with driving style and a knowledge of where your JK will perform best in the RPM range. Rolling under 1800 rpms doesn't buy you anything, and when you start hitting above 2500 the pendulum swings the other way. I also, skip gears when accelerating and generally coast to stops, keeping tabs on if the light is going to be red before I get there.

ckmehl

i drive like a grandpa. and im averaging 8 to 10 mpg's. 35's and 5" lift. still have not recalibrated. i'm looking at 5.13's as well as MBRP cat back relo.

JPop

No surprise there Planman. It takes more than twice the power to make a JK go 80mph as it does 60mph.

As for pushing that kind of speed, there isn't an appropriate gear set to get it done. You either need more power, turn OD off or get some D60s and find someone that has a gear set over 6:1.

joneszj

I keep the revs above the 2400 mark, and never use 6th gear because it wastes more fuel. I know how to drive my JK to get the best gas mileage, and if I keep it in 5th gear at 70 mph, I get the best MPG - But that is only about 16. If you are really getting 20+ and NEVER any less than 16, with "heavy 35s" you're engine must be different than mine. My only guess is that it is the flashpaq and headers because I also have a CAI and exhaust.

I know it could be the gears, too, but between 13-15 and 20+, that's a huge difference. Hell, I barely got 20 mpg before adding the lift and tires (with 17" wheels and GY SRAs). Maybe with the 16" little pizza cutter stock tires and wheels it's possible to get 20+, but I just don't see another way (except for the flashpaq or whatever). I know elevation has something to do with it as well, because my JK gets better mileage in Pennsylvania than in Michigan. I still just can't believe the 20+ claim and I think it's unrealistic for people to think that they can get that sort of mileage with "heavy 35s" and a lift. I'm not trying to get into an argument, and most of the MPG threads on this site have had similar results. I usually avoid them because it's such a subjective question.

I'm curious as well, what setting are you using on the flashpaq, the premium octane or regular, or something different?? I've been considering the superchips programmer for a while, but i just am not sure if it's worth the price.

JPop

Well, it's kind of hard to quantify, but you always want overhead. The 80mph jaunt is probably covered by 60hp at the wheels, but then add grades, wind and your altitude. It ain't a pretty picture. At 4,000 feet, 3 degree grades you're looking at around 100hp needed, so that's in the realm of 3,000 rpm. With 37s and an auto, that's 6.50s or 7.17s, in other words a lot of gear. OD off and you could make it with 4.56s or 4.88s.

Actually sounds like a good fit for a Supercharger to get some better air.