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Gearing for Gas Mileage

98XJSport

98XJSport

Destiny is the rising sun
Location
Western Maine
I have a 98 XJ 4.0/Auto with the stock 3.55s. Im looking into putting on 32s and then 33s with a bit of trimming in the next couple months. Ive searched and found tons of threads on regearing, with the general rule being to go with 4.56s, since I have a 8.25 rear. But Im not interested in pure offroad performance, this is my daily driver. I drive 110+ miles a day mostly around 55-60, but frequently take longer trips that I run around 75. There are many mountain roads, and the offroad stuff I do is usually mud or old back road logging trails. Not much creeping, Ive actually had enough creep with the stock gears on 31s. Gas mileage is my main concern with 32s/33s. Is it possible to keep my gas mileage around 18mpg with 4.56s? Or is there a better ratio for this type of driving. Thanks.
 
I run 33's and 4.56's on a '99 Auto and the city mileage sucks.

However, I just got 18.25 mpg highway doing 65-70 for 300 miles each way on I-80 across PA.(Allegheny mountains)

edit: that chart is a little high, as noted at the top regarding automatics. 70mph puts me at about 2850rpm
 
Yeah, that chart doesn't help for auto's. OD gearing for aw4s is around .75 or .8 depending on year, not 1:1.
 
55mph is fairly low speed, mix hills/mountains into the equation and having a higher ratio number will most likely help your mileage. You may pay a penalty at 70+, but what percentage of your travel is at that speed. 4.56's with 32/33 is just a bit lower geared than stock. 4.56's with 35's is roughly stock. Personally, I would go with the 4.56's. I ran them on 31's here with the mountains, and it make driving a lot nicer.
 
My heep suffered a huge loss in fuel economy when I went to 33's with stock gears...after regearing the fuel economy went back up to normal.
 
MudDawg said:
My heep suffered a huge loss in fuel economy when I went to 33's with stock gears...after regearing the fuel economy went back up to normal.
Click to expand...

What did you regear to?
 
I went with 4.88's since i have Dana axles. My tranny is a non-OD auto. With no changes the speedo is within 2mph at 60mph running new 33" TSL bias.
 
not trying to change the direction of the thread. I have 35s and stock gearing, need to regear badly. 4.88s? or 4.56s? It is my Daily driver
 
I say 4.88's. I have 4.56's and a stroker and it is fine, but with a stock motor and highway driving, the lower gears will make those large tires a lot more manageable.
 
Another thing to factor in is the weight of the tires. Someone running bfg all terrains is gonna have a lot different opinion and results than a person with boggers of the same size.4.56 with 33's is good for me with an auto. "Super Swamper Trxus"

I live in the appalachian mountains and i tell ya what, you need those gears here with all the hills that are on the interstate. Also i like the power.
 
4.56 with my 33s turned out to do great.... but i'm running a 8.25 so that was my only option anyway....still happy with my choice
 
DeftwillP said:
Yeah, that chart doesn't help for auto's. OD gearing for aw4s is around .75 or .8 depending on year, not 1:1.
Click to expand...
Pre 91 AW-4 used a .705 OD, 91-up AW-4= .75
 
Basically then my question to those with 4.56s and 33s, gas mileage around 55-60? Thats the speed I do most of my driving at. Also, how much does tire width fit into all of this?
 
The larger the foot print, the larger the frictional drag, the harder on fuel economy. This is one reason why I went with a 265/75R16 instead of 32x11.5x15 (WJ brakes being the other).

The other biggie is the weight of the tire. Example: a BFG is considerably lighter than a Truxus MT in the same size. It takes power to accelerate that spinning mass and keep it spinning, which means more throttle. If you "over gear" slightly you can offset it a little tiny bit. This was my logic for going 4.56s with 32s.

HTH
 
BrettM said:
4.88s

with 33s on and driving 65mph I got 20mpg on the freeway
Click to expand...

Then what the fawk am I doing wrong with my 4.88s, 33" BFG MTs and lucky to get 13-15 mpg?
 
DeftwillP said:
Yeah, that chart doesn't help for auto's. OD gearing for aw4s is around .75 or .8 depending on year, not 1:1.
Click to expand...


This chart is based on 65 MPH and a gear ratio of 1:1, on a manual transmission in 4th gear. Please NOTE: If you have an automatic transmission your RPM will higher due to slippage in the transmission and the torque converter. With an overdrive transmission your RPM in overdrive will be 15% to 30% less than indicated.
 
Oops, better turn off the TCC then.

:)
 
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