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Old 04-02-2012, 02:06 PM   #1
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Post re-gearing questions

I finally got 4.88 gears put in my rig (6sp manual on 35" BFG KM2).

I have some questions that I haven't really been able to get straight answers for. I was assuming that after I got the gears put in I would be able to see somewhat of an increase in my MPG. I wasn't expecting anything dramatic but it is looking like it has got worse (<12 mpg). This is probably due to the way I'm driving and terrain, I'm not sure. The power is back which is nice! Would love to know how people with a similar setup are getting better MPG.

I am curious as to where I should be keeping my RPMs when I'm going through the gears. I mean, I've drove a standard all my life but never really paid attention to my rpm. I always shift when it feels right. But now, since I'm trying to optimize my mpg the best I can, anything advice would be great.

The other question is, do i need to recalibrate my speedometer and the computer to compensate for the gear change and tires? I thought that I would need to but the guy at the shop said that it will be near stock again so I would not need to do so, "it will even out".

Opinions?

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Old 04-02-2012, 02:25 PM   #2
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How are you calculating MPG?

Yes you need to reprogram the computer. Get an AEV Procal

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Old 04-02-2012, 02:29 PM   #3
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How are you calculating MPG?

Yes you need to reprogram the computer. Get an AEV Procal
This ^^

You have to reprogram the ECU.
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Old 04-02-2012, 02:31 PM   #4
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How are you calculating MPG?

Yes you need to reprogram the computer. Get an AEV Procal
Watching the gas gauge. I'm know about how much I use for this route I take. I'm burning more after getting the gears. I haven't done anything more reliable yet.

Will the reprograming help out with actually getting better MPG?
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Old 04-02-2012, 02:39 PM   #5
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Watching the gas gauge. I'm know about how much I use for this route I take. I'm burning more after getting the gears. I haven't done anything more reliable yet.

Will the reprograming help out with actually getting better MPG?
How are you driving? Are you bogging it down during the new gears break in?

You MPG shouldn't be much worse than stock, but there's also a lot of factors. You will loose MPG with a lift and 35s. The gearing should help gain some of the loss back.
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Old 04-02-2012, 02:43 PM   #6
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If you haven't changed your computer to 4.88 gears then there is no way to correctly calculate MPG's. Your speedo is way off at this point. Get a AEV Procal immediately. This also can cause ECU errors in some cases because the computer will notice something isn't right
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Old 04-02-2012, 02:45 PM   #7
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Originally Posted by Pikegulley View Post
Watching the gas gauge. I'm know about how much I use for this route I take. I'm burning more after getting the gears. I haven't done anything more reliable yet.

Will the reprograming help out with actually getting better MPG?
Absolutely.

It will change the shift points. A reference for 3.6L Pentastar:

pentastar gear chart


For 3.8L V6:

8/24 Updated JK gear chart
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Old 04-02-2012, 02:46 PM   #8
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If you haven't changed your computer to 4.88 gears then there is no way to correctly calculate MPG's. Your speedo is way off at this point. Get a AEV Procal immediately. This also can cause ECU errors in some cases because the computer will notice something isn't right
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Old 04-02-2012, 02:49 PM   #9
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I haven't regeared my Jeep, but my understanding is that if you spend a lot of time on the highway, MPGs generally drop after a regear. This is so because you can't simply upshift yourself into low RPMs anymore.

The MPG improvements I've seen associated with a regearing are generally said to apply for "around town" driving, as the new gears mean that you don't have to lug the engine as much when getting up to spend from a stop.

So if you put most of your miles on at highway speeds, I would expect a regear will drop mpgs overall.

And with a manual transmission you don't need a programmer to adjust for the new gears. "Shift points" and the like are determined by your left foot and right hand. You should however get a programmer to adjust for the new TIRES. Going to 35s has rendered your speedometer inaccurate, which translates to an inaccurate odometer, mileage tracker, and (I believe) can cause some funky ESP issues.

As for what RPMs you want to be in, the advice remains the same as it always is regardless of gears. The lowest RPMs you can do without lugging the engine is going to be the best for mpgs. Other "tricks" include trying to use the brake less, which translates to coasting to stop signs and off of exit ramps and the like. And generally just slow down.

As to RPMs for shift points, if I'm looking to move, I usually shift around 3500 RPMs or even higher. If I'm looking to casually go forward without burning any extra fuel, I'll shift around 2500 RPMs or so. I don't think changing gears really alters any of that, so I'd recommend you do the same if you're comfortable with it.
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Old 04-02-2012, 02:52 PM   #10
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I believe can cause some funky ESP issues.
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Old 04-02-2012, 02:53 PM   #11
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If you haven't changed your computer to 4.88 gears then there is no way to correctly calculate MPG's. Your speedo is way off at this point. Get a AEV Procal immediately. This also can cause ECU errors in some cases because the computer will notice something isn't right
So it is a significant deal. Correct me if I'm wrong, If I think I'm going 65 mph, am I actually going faster than what the speedometer is telling me?
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Old 04-02-2012, 02:56 PM   #12
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So it is a significant deal. Correct me if I'm wrong, If I think I'm going 65 mph, am I actually going faster than what the speedometer is telling me?
Yes much.

It depends on what you were running before hand but if you moved to a larger tire then your speedometer is going to run slower than what your actually traveling. Just for reference, moving from 29 to 32 tires cause your speedo to be off by about 6mph. The same could be said for 32 to 35 etc...
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Old 04-02-2012, 02:57 PM   #13
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I'm fully stock but Like MTH I think it would be the same. I shift mine at 2200 if it's normal roads, 2500 if I'm accelerating up hill and 3000 to get up to speed on the highway.

After the regear the only way to get lower RPM on the highway would be to drive slower. Any chance you are cruising at 80? 65 or 70 would yield better mpg if that was the case. I think your mpg will increase after break In tho.

Also you could go to 33 AT's for your next set of tires. They'd be lighter and have a better road pattern
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Old 04-02-2012, 03:03 PM   #14
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I'm fully stock but Like MTH I think it would be the same. I shift mine at 2200 if it's normal roads, 2500 if I'm accelerating up hill and 3000 to get up to speed on the highway.

After the regear the only way to get lower RPM on the highway would be to drive slower. Any chance you are cruising at 80? 65 or 70 would yield better mpg if that was the case. I think your mpg will increase after break In tho.

Also you could go to 33 AT's for your next set of tires. They'd be lighter and have a better road pattern
Lol. I shift at 4000 or 4500 merging on the highway . . . . "RAWR!!!" Says my 3.8 . . .

33s would help with weight, but with 4.88s he'd be at nearly 3k RPM at 70 mph in 6th gear. I think he's committed to 35s at this point.
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Old 04-02-2012, 03:06 PM   #15
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I'm fully stock but Like MTH I think it would be the same. I shift mine at 2200 if it's normal roads, 2500 if I'm accelerating up hill and 3000 to get up to speed on the highway.

After the regear the only way to get lower RPM on the highway would be to drive slower. Any chance you are cruising at 80? 65 or 70 would yield better mpg if that was the case. I think your mpg will increase after break In tho.

Also you could go to 33 AT's for your next set of tires. They'd be lighter and have a better road pattern
Well, if my speedometer is a little off, then I am probably going faster than I think I am. Especially since I haven't recalibrated for the tire change from 32 to 35. That might help out.

I figured going to a less aggressive tire will help as well, but I like my BFG's. Lol, would love to figure out how people with 37s are getting excellent gas mileage.
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Old 04-02-2012, 03:07 PM   #16
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Lol. I shift at 4000 or 4500 merging on the highway . . . .

33s would help with weight, but with 4.88s he'd be at nearly 3k RPM at 70 mph in 6th gear. I think he's committed to 35s at this point.
I'm very happy with 35s.

What kind of MPG do you get?
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Old 04-02-2012, 03:10 PM   #17
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I'm very happy with 35s.

What kind of MPG do you get?
When shifting above 4,000 maybe around 9mpg?

x2 on the "he's committed to 35s at this point". If he goes smaller his mpg are gonna get worse. The only option is to go bigger!
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Old 04-02-2012, 03:11 PM   #18
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Lol, would love to figure out how people with 37s are getting excellent gas mileage.


Excellent gas mileage and 37" tires don't belong in the same sentence.

Some might be running Hemis though; which would probably even out the MPGs with more power (assuming you're running a V6 at higher RPMs).
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Old 04-02-2012, 03:14 PM   #19
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You are going to have to do a diff fluid change at 500miles(pretty sure that's the number) of the gears. If you go with a lightweight gear oil it'll help. If you tow a lot tho this isn't the best idea. Plus before you recalibrate if you stay at "60mph" it'll actually be 66 which would be better mpg
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Old 04-02-2012, 03:14 PM   #20
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Lol, would love to figure out how people with 37s are getting excellent gas mileage.
Lies lies lies lies. Either that, or they only drive downhill.

Bottomline: Pushing a heavy brick with heavy tires and a high center of gravity down the road with (assuming you've got a pre-2012) a 202 hp engine designed 20 years ago ain't going to get good mileage. Just ain't gonna do it. Ever.

If you've got a 2012, you've got some newer tech pushing the thing, so mileage gets better. But it still isn't going to be groundbreaking.

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I'm very happy with 35s.

What kind of MPG do you get?
I have a 2010 with very lightweight 33s and 3.21 gears, so my situation is going to be pretty different. Overall, I average about 18 mpg with mostly highway driving. I never use 6th gear, so that keeps my RPMs around 2200 to 2500 on the highway in 5th.
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Old 04-02-2012, 04:20 PM   #21
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You are going to have to do a diff fluid change at 500miles(pretty sure that's the number) of the gears. If you go with a lightweight gear oil it'll help. If you tow a lot tho this isn't the best idea. Plus before you recalibrate if you stay at "60mph" it'll actually be 66 which would be better mpg
lol that may be the problem too since I think running 65 means actually going 65. I'll probably end up getting an aev procal or borrowing one to correct the speedometer and tire size.
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Old 04-02-2012, 04:24 PM   #22
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I'm fully stock but Like MTH I think it would be the same. I shift mine at 2200 if it's normal roads, 2500 if I'm accelerating up hill and 3000 to get up to speed on the highway.

After the regear the only way to get lower RPM on the highway would be to drive slower. Any chance you are cruising at 80? 65 or 70 would yield better mpg if that was the case. I think your mpg will increase after break In tho.

Also you could go to 33 AT's for your next set of tires. They'd be lighter and have a better road pattern
Definitely not cruising at 80.
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Old 04-02-2012, 04:24 PM   #23
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lol that may be the problem too since I think running 65 means actually going 65. I'll probably end up getting an aev procal or borrowing one to correct the speedometer and tire size.
You can't borrow one. Once used, they remain linked to that Jeep's VIN. They won't work on another Jeep until they reset the first Jeep to stock settings. This prevents whole Jeep clubs from buying just one for each member to "borrow."
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Old 04-02-2012, 04:41 PM   #24
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You can't borrow one. Once used, they remain linked to that Jeep's VIN. They won't work on another Jeep until they reset the first Jeep to stock settings. This prevents whole Jeep clubs from buying just one for each member to "borrow."

That's good to know.
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Old 04-02-2012, 05:24 PM   #25
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i lost mpgs regearing.

demographically, most people gain mpgs but some people lose mpgs
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Old 04-02-2012, 05:30 PM   #26
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I gained under 55mph and lost over 65 mph but I drive a lot more under 55 anyways. But I think the better MPG really applies more to autos that are way under geared and not as easy to pick the gear that best fits the conditions. Really 4.88 in a manual needs a minimum of 35" tires.
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Old 04-02-2012, 05:39 PM   #27
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YUP, my mileage after regearing, increased between 5 & 10%, unless I driveit like it deserves to be driven, with the new power/torque !

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Old 04-02-2012, 08:06 PM   #28
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YUP, my mileage after regearing, increased between 5 & 10%, unless I driveit like it deserves to be driven, with the new power/torque !

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Old 04-02-2012, 08:19 PM   #29
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Heh Heh, yeah and I don't USUALLY drive very aggressively, but--NOW & THEN----BAM !


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Drive it like your stole somethin'!
Kinda kewl !

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Old 06-18-2013, 09:44 PM   #30
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Ford 8.8 installing 4.88 G2's, backlash is .011 and here is my pattern. What do you think, more shim for the pinion (deeper)?
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