Piginajeep

I wouldnt really say its the consensus. I would beat its even with 4.88's and 5.13's.


I love the 5.13's with the 6 speed.

Doc_D

My friend has an X with 3.21 gears and 32 inch tires. He gets about 225 mile to a tank. I get 300 miles to a tank with 4.10s and 32 inch tires. I suspect it's his driving style, but that won't keep me from bringing it up.

I'm just trying to say you cannot infinately keep reducing the final drive ratio and expect the fuel economy to keep increasing forever. I suspect a JK with a 1:1 final drive ratio would get HORRIBLE mileage.

3.21s and 32 inch tires puts your right around 2,000 rpm at 70 mph. You said you wanted to be below that. I suspect your fuel economy might actually go down if going lower.

As for low RPM being bad for your engine, I'm surprise someone with a background in airplanes wouldn't acknowledge that fact. Go talk to your pilot about piston aircraft, high prop pitch, low speed and high throttle opening (i.e low RPM / high load). It's a recipe for detonation (due to high cylinder pressures), overheating, and early bearing demise. Granted it would have to be pretty extreme like driving around at 3/4+ throttle in 6th gear doing 30 mph. But if your final drive was tall enough, that's the effect you could end up with on the highway.

sho_me

If you really want good gas mileage, by a Neon. Or even better, a turbo deisel VW. My bro's Jetta gets 50mpg no matter the load or driving style. The JK was tested in a wind tunnel, thats why the windshield is curved. Running below 2000 rpm of the freeway is pointless, you have no power and your foot will stay on the floor to keep it at speed, then all your gas is going out your tailpipe.

If that Sahara had 4.10's his gas mileage would improve. I have 35's with 4.10's now, I cruise the freeway in drive, od is too high to pull my Jeep. 5.13's are on order, be here in a month, can't wait to get them.

brent_f70

I have a 6 speed with 5.13's and 35's and it is perfect......no way I would go to 4.88's I have great power and can hit 90+ on the interstate.

JulietKilo

Our planes are props; however, they are TURBO props. We dont have any pilots that would know anything about them. Our engines are jet engines modified to use SHAFT HORSEPOWER to drive the props. I drive like grandma and see about 17 mpg in the city/highway and I use sixth gear more then I probably should but with a lifetime power train warranty I really dont care right now, when I can affford 35's I will take it from there on the decision for deeper gears.

But like someone else said if we wanted good gas mileage we would drive a honda, mazda, or some Japan made wonder.

huskerjk7

what was your deciding factor between the 5:13 and 4:88's. Have you driven both? More grunt with the 5:13's ? mpg?

Thanks
Husk

JulietKilo

Some driving impressions today while on my way to the base...

I noticed that 5th gear is the one to use for most city driving. Being 1:1 its the last gear you would want to use so you dont drop below 1K rpms. The engine then bogs down and starts to "click" as the other guy on here said it MAY be hard on bearings and such. 6th gear is only used if I can keep the rpms above 1K. Even with 35's and 6th gear I would still be at 2300 rpms at 70 mph. That is still good due to the engine being in optimum range. Of course if you change to deeper gears you will notice better gitty up and go but I think the trade off is not worth it right now while gas is so expensive.

The deciding factor between 4.10, 4.88, and 5.13 gears is...
your jeep a weekend toy? 5.13
Highway or city more often? 5.13 if in the city due to the need for acceleration
Daily driver? 4.10 or 4.88
37 or larger tire? 5.13
Off road 1 or 2 times a month or even less? 4.10
Etc.. etc..

Ghostship

Miles per Gallon is not simply a result of the rpm's an engine spins at. You have to account for work, and the energy the engine generates at the different rpm bands, and the increased work required to overcome the weight from larger tires and drag from higher lifts.

Think of the gears on a bicycle; too high and you peddle very fast while going no-where, too low and you can barely peddle and still go no-where.

JulietKilo

I concur... But I am talking about finding that sweet spot with the gears.


1K.........1.5K........2K.........2.5K.........3K. ........3.5K.........4+K..........$$$$$
-------------------------| sweet spot |-----------------------------------------------

I am trying to keep mine around 2k in the 6th gear. That is my point. Everything else is left up to driving style, work load, weight, tire size, etc...

Offroadme

i have 321 gears in the rear on my sahara im running 31's on a 17 inch wheel and i get like 280 miles on a tank.. i don't have the money to re gear the front and the back right now so i will have to wait but it works for me... when i do im going to 513