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Regearing / Fuel Economy Question

X

xj2win

NAXJA Forum User
Location
MonTana
I'm running 31" tires on the stock 5 speed with 3.07 gears. I have heard that you can gain fuel economy by regearing. However, from what I understand, about 2500 rpm gets the best mileage at highway speeds. On a road trip this weekend, I notice that in 4th gear at 70 mph, I'm running 2500 rpm. The fastest I ever drive is 75, and at that point the tach is only up to 2700. A shift to overdrive drops the rpms near the 2000 range. So my question is, if I just stop using overdrive will I get better mileage? I know with the gearing of 4th I won't get as many wheel rotations per RPM, but I would be getting less anyways with lower gears.
 
Your highway mileage should be close to stock using 4th gear...however your city mileage is the one that takes a dump after bigger tires unless you regear.
 
another probably cheaper option you have is to swap in the axles from an auto. 3.55 would be stock for an auto (mine was) and it would certainly help you. I put 4.10's in for my 33's because I do a lot of hiway driving, makes a difference indeed.

if you haven't done anything to your axles look into swapping yours for some from an auto. depending on your axles you may get some upgrades... d35 to 8.25 or 27 to 29 spline, bigger u-joints up front.
 
if you want better fuel milage dont worry about your RPMs all that much and do what i did. Put in a vacum gage in your vehicle. Drive by vacum the more vacume you have the better MPG your getting
 
beakie said:
another probably cheaper option you have is to swap in the axles from an auto. 3.55 would be stock for an auto (mine was) and it would certainly help you. I put 4.10's in for my 33's because I do a lot of hiway driving, makes a difference indeed.

if you haven't done anything to your axles look into swapping yours for some from an auto. depending on your axles you may get some upgrades... d35 to 8.25 or 27 to 29 spline, bigger u-joints up front.
Click to expand...

I have a full width 9" and D44 off a 78 bronco sitting in the shop. I don't feel like taking that on at the moment as the jeep is currently my DD.
 
88RIxj said:
if you want better fuel milage dont worry about your RPMs all that much and do what i did. Put in a vacum gage in your vehicle. Drive by vacum the more vacume you have the better MPG your getting
Click to expand...

Who told you that? Obvioulsy you're getting bad mileage at idle when vacuum is low, but vacuum should be highest at WOT - and you're not getting the best mileage at WOT. To further the point, a turbocharged car running .5 bar of pressure is going to get a hell of a lot more vacuum than an NA engine - but not necessarily better mileage.
 
3.07 gears are a bit high (numerically low) for 31" tires, IMO.
 
xj2win said:
Who told you that? Obvioulsy you're getting bad mileage at idle when vacuum is low, but vacuum should be highest at WOT - and you're not getting the best mileage at WOT. To further the point, a turbocharged car running .5 bar of pressure is going to get a hell of a lot more vacuum than an NA engine - but not necessarily better mileage.
Click to expand...


I think what he means is that once your vacuum gauge goes to zero, anymore throttle is just wasting fuel. It can't get anymore air (unless forced induction=boost).

I had a subaru 2.5 RS (before the turbo WRX) that had a vacuum gauge and it would accelerate until the gauge hit zero, then it didn't matter how far I put the pedal to the floor, it wouldn't go any faster.

Anyway, I have a '92 XJ, 5-speed and I just went from 3.07 to 4.10 with 32" BFG muds and its pretty rapped out on the freeway. Before the gear swap I had about a +11% speedo error (speedo 60mph, true 67mph), now it goes the other way, around -14% (speedo 60mph, true 52 mph). I used gps to get my true speed. Its mostly a wheelin rig for me though, so it's better. I'm going to 34" ltbs for tires now. So I guess it depends on two things, what size tire your running and if its a daily driver. Oh, and now I can accelerate up hills in 5th, been awhile for that.
 
beezzerr said:
I think what he means is that once your vacuum gauge goes to zero, anymore throttle is just wasting fuel. It can't get anymore air (unless forced induction=boost).

I had a subaru 2.5 RS (before the turbo WRX) that had a vacuum gauge and it would accelerate until the gauge hit zero, then it didn't matter how far I put the pedal to the floor, it wouldn't go any faster.

Anyway, I have a '92 XJ, 5-speed and I just went from 3.07 to 4.10 with 32" BFG muds and its pretty rapped out on the freeway. Before the gear swap I had about a +11% speedo error (speedo 60mph, true 67mph), now it goes the other way, around -14% (speedo 60mph, true 52 mph). I used gps to get my true speed. Its mostly a wheelin rig for me though, so it's better. I'm going to 34" ltbs for tires now. So I guess it depends on two things, what size tire your running and if its a daily driver. Oh, and now I can accelerate up hills in 5th, been awhile for that.
Click to expand...

What do you mean by "rapped out"?
 
xj2win said:
Who told you that? Obvioulsy you're getting bad mileage at idle when vacuum is low, but vacuum should be highest at WOT - and you're not getting the best mileage at WOT. To further the point, a turbocharged car running .5 bar of pressure is going to get a hell of a lot more vacuum than an NA engine - but not necessarily better mileage.
Click to expand...

I'm guessing that you have never actually watched a vacuum guage on a runnig engine have you?, a NA engine greates high VAC at IDLE and LOW vac at WOT, and turbo/ supercharged car at boost has PREASURE in the intake and does not have VACUUM ( thats why it's called PREASURE)
 
beezzerr said:
higher rpms
Click to expand...

How many RPMs are you turning at 70mph? I'm currently running 31's and will have my 4.10s in by this weekend (hopefully). Just curious wat it will be like. The XJ is my trail rig / extra vehicle when one of the DDs are down, so it's not that big of a deal.
 
I couldn't tell you for a few days. I fixed some rust holes in the floor and my herculiner is drying as I type this. I'll try to remember to write down some numbers once I get it out of the garage and take it for a spin.
 
xj2win said:
in 5th, 2000 rpms @ 70 mph
4th, 2500 rpms @ 70 mph
3rd... haven't gone there yet. Probably 4000-4500
Click to expand...

But that's with 3.07s right? I'm interested in the numbers for a 5-speed (AX15) with a 4.0 and 4.10s.
 
smccollamjr said:
But that's with 3.07s right? I'm interested in the numbers for a 5-speed (AX15) with a 4.0 and 4.10s.
Click to expand...

i have 4.10 gearing and 31's with the peugoet tranny on my 89.
@65mph its right at 2500rpms
@75mph its about 2800rpms
 
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