cctroupe11

Hey guys, so driving to and from Atlanta this weekend, using my GPS, I noticed that my speedometer is not correct, so what I previously thought was my gear ratio is not true, my '12 JKU is still running with stock 3.21 gears...on 37" tires. My fuel economy SUCKS. The most I can squeeze out of it is about 14.3 MPG.

So, from what I have been able to find searching for info, I will get a little power back if I put in some 4.56 or 4.88 gears. Will this HELP or HURT my fuel economy?

Also, can you guys recommend a good brand of gears for front and rear and a carrier for my front D30?

Thanks a lot,
Chris

Cjsmith4

Are you using the jeep computer to say your getting 14 mpg. I'd be pretty surprised if you were really getting that with 37s and 3.21s.

Re gearing will help your mpg as long as you don't go too low. If you go too low then you are just going to be running at high rpms which is bad for mpg too. It will also get you some torque back in the jeep.

Best and most accurate way to check your mpg is manually. Fill er up. Drive and keep track of actual miles (again not using the odometer because it isn't correct). Fill er up again and divide the miles you drove by the gallons of gas you put in.

cctroupe11

That was according to my Jeep computer and my math that I did on the trip. Seemed to match up.

I'm thinking of 4.56 so that I will have a good daily driver but can still wheel a little with it. Can anyone confirm that 4.56 would be good for that with 37" tires? Any estimates of what fuel economy I could expect with 4.56's?

ride4life1990

I can imagine your mpg is correct I'm running 35s on 3.21 and best I've seen is 16mpg but gutless like 2000rpm in 5 at 80mph. U running a auto or 6 sp?

Tooadvanced

Well if your Speedo isn't correct than your calcd mileage isn't either

Cjsmith4

Exactly. If the speedo is off, so is the mileage in the odometer. Can't use that to keep track of mileage when doing the math.

brighton

Need to recalibrate the speedo with a programmer like a ProCal to get accurate mpg's.

I have 4.88 gears with 35's, I can't even imagine what a 3.21 with 37's must be like. My mpg improved slightly when I regeared from 3.73.

Matthewd5

Ok. 2012 4 door with 37's and 3:21 and a d30 up front?

there are more important things to address here besides mpg

Hybrid Cars | Toyota Prius 2014

have you replaced yor factory axles? Your factory driveshafts?

your drivetrain is only as good as your weakest link...

i just went to 37's from factory 3:21, I spent a lot of time looking at the gearing charts, for better on road you generally want something like 4:88 or 4:56, I was leaning more towards better off road gearing so I chose to go with 5:13's and its amazing, my Jeep is like a big 4wd go kart!

factory it was good, when I upgraded to 33" bit stayed with 3:21 it did get a bit sluggish, especially on the highway...

but you should read some of the build threads, there are a lot of changes/modifications between factory d30 and 3:21 to 37's

matthew

cctroupe11

I'm going to be gusseting and sleeving my D30 here in the near future, as well as getting new driveshafts.

Anyone used the AEV Procal to adjust the speedo? I'm thinking of picking one up.

Mark Doiron

I have friends who do. It works well, if a little awkward user-interface compared to other options. But it is not something you should need to use very often, and it does set some things that others don't. So, why are you getting it? Nothing you said you're changing will affect speedometer calibration.