FWIW, the difference between 3.27's/3.73's/4.10's in the stangs was almost exactly 200 rpm @ 70 mph.

2200
2400
2600

 

I have an auto on 31s with 4.56 gears and I run ~2750 at 70 I believe, I know that low range has a 5 speed with 4.56 on 33s and he says 80 is about 3k. My highway mileage dropped (if I go over like 60-70) but my in town went up 2 mpg almost.

 

I had the same question and it looks like the consensus is to regear.


Can someone recommend what gears would be best for me? I drive perhaps 50/50 road/off road. I currently have 3.73's and an auto trans with the 4.0L motor.

 

what tires, trans, armor, towing? There are a lot of variables.

 

Simple math for you whether you have an auto or 5spd.

OTD/NTD X CGR = EGR
OTD: Old Tire Diameter (in inches)
NTD: New Tire Diameter (in inches)
CGR: Current Gear Ratio (Whats in the differential now)
EGR: Effective Gear Ratio (What your rig acts and feels like it has)

Now the math is to plug in different available gear ratios until the EGR number is as close to stock as possible (Note: the final results will be slightly higher or lower than actual stock numbers)

EX: You have a 3.07 geared, 5spd XJ with 28" (avg) tires stock and you want to run 33" tires without regearing..

28/33 X 3.07 = 2.60:1 That's a lot higher than when you had stock tires so your acceleration now is less and you have to rev your engine up a lot more to get to speed resulting in crappy fuel mileage.

Now, let's "regear" to 3.55's or 4.10's
28/33 X 3.55 = 3.01:1
28/33 X 4.10 = 3.47:1

Both of these "regears" will give you back your performance and acceleration. 3.55's will bring you closer to stock performance while the 4.10 will hive you a little extra "Umpfh", perfect for if you have heavier bumpers, etc.

Any questions on how this works?

 

With which sized tire?

 

42re, 33" tires, no towing, and not much more than stock for armor.

Thank you for asking.

 

Nice! So 28" is the stock size? I have no idea since mine All ready came with larger tires.

 

The stock size XJ tire is ~28" (different manufacturers are always +/- the actual advertised size) so that's the base number.

 

so you're a grand with a 4.0? I believe people typically go 4.56 with 33s on grands

 

Shoop I thought you went FI? Was that pre or post regear?

 

Huh?

 

I find this interesting as My daughters xj auto
has 33s and we want to regear. According to the chart 4.11 to 4.27 was recommended not 4.56 as I thought. I have always been told 4.56 for 33s. Now I'm confused.

 

28/33 x 4.10 = 3.47:1
28/33 x 4.56 = 3.86:1

4.10's will bring your daughters XJ back to stock performance.

4.56's will give you a little extra especially if you have a heavy bumper or are loaded with gear/people.

Regearing isn't always about power.
With an automatic transmission like hers, it keeps the transmission from working too hard and overheating from constantly high revving to get up to speed.

 

that is what the math is saying since I have 3.73's in there now (stock) that calcs to a stock equivalent of 3.16 (with the 33's) and to bring it back up to near what the 3.73's felt like as stock I will need 4.56 gears.


I heard that going that low decreases the pinion strength and can cause issues??