this helps alot. i was pretty set on 5.13s but not sure on rpms down interstate and hiway since my wife drives this daily. and i wasnt sure if its even smart to keep that d30 up front anyways. i dont wheel too terribly hard so i thought it would be alright for now anywayz

 

If you are running 35" tires he 5.13s will make the JK much more driveable on the highway and in the city. If it is you dd, you will both appreciate these gears.

 

my main concern is how high are my rpms going to be at 60-65 or 75-80 with 5.13s? is it guna give me worse mileage?

 

The biggest thing that robs an automatic transmission JK of gas mileage is unlocking the converter and downshifting because the engine isn't making enough power. With taller gears and higher rpms the throttle blade position changes, which at cruise speed reduces the fuel per stroke, while leaving more overhead for handling hills or grades. You won't see anything more than nominal mileage differences until you get up over 2700 rpm.

 

i have noticed this considerably having the 5.13's with 33's( 35 soon I hope) I use less skinny pedal and the tranny downshifts less. Once the gears wear in I expect my mpg won't be much lower than stock

 

Check my first post that you skipped over.

There is a Drivetrain FAQ at the top of this section. It has an RPM chart that jpop spent months (could have been years!) putting together. Plug in your gears and tire size and you can see for yourself...

(use the 'actual' tire height, not the mfg's listed height)



Auto, 5.13's, mfg's tire height 33.4", 'actual' tire height on vehicle 32"

O/D on
60 - 2200rpm
65 - 2400rpm
70 - 2600rpm

O/D off (for towing)
60 - 3250
65 - 3500
70 - 3800

And guess what, I'm within 3rpm of the chart when I use the 'actual' tire height.

It's not rocket surgery or anything. Plenty of resources to get people pointed in the right direction if they can be bothered to actually look...

 

thanks for the heads up on that chart

 

That's funny! I think I spent about 20 minutes on the thing and then spent about another 10 minutes for edits. Of course gathering the knowledge base from lots of user posts, looking at hp requirements at various speeds, where the engine makes power and rates of grades people needed to cope with took a lot longer. Then my long boring flatland jaunts across the midwest with lots of time to think, watching mileage differences in different gears and at various speeds.

Anyway, bottom line is you won't have too many people complain about having too much gear off-road and more than anything aero drag plays a much bigger role than most think when they are cruising on the highway.

 

I read the threads where you were talking about doing a chart, and you had it posted like the next day. I was just trying to give you some extra credit for the effort and obvious usefulness of it.

 

i have an auto like you and 5.38s

at 70 i run around 2500 i believe. i don't do much highway driving so i'm not positive. it's far superior to running without overdrive in stock gearing though