seabass

I upgraded to toyo mt's LT285/75R17 which technically is a 34" tire but I like to call them 35's. I had the stock 4.10 gears. Overall it was very livable with the 6spd. I think I was so excited by new tires that it didn't matter if performance suffered. Last summer I purchased a small camper to pull, that is when I decided I needed some more power. 1st gear would almost stall out in some situations and I always had to use 4LOW park it.

I thought supercharger for sure but, the reviews are so mixed that it is obvious that the power to $ spent is off a bit for most folks. Gearing seemed to be the oblivious answer but 4.88 or 5.13 is a raging debate.

I really could not find a review of someone who went 5.13 and regretted it. That makes me believe the 4.88 folks just don't know what they are missing so they are happy over their previous gearing.

Overall the lower gear ratio makes the Jeep much easier to drive. I wouldn't call it fast but it accelerates without having to go to the floor all the time. In fact it seems like I am using only a quarter throttle most of the time. I was absolutely amazed but my fuel economy has improved as mentioned in many other posts. I double checked my hypertech program cal versus another vehicle and gps. I was consistently getting 15 mpg on the highway and city with the 4.10's, now with the 5.13's 16 to 17mpg in the city and 19 mpg on the highway. I haven't seen numbers like that since the Jeep was brand new with 32" tires! The only drawback is more gear shifting, you will easily be in 4th by the time you hit 35 mph.

Very happy overall. Below is more information collected on this that help me make the decision.

For the math types check out this post. Change the engine hp if you want the result is the same.

Gears will make all the difference when you go to bigger tires. Let's look at these formulas.

drive wheel torque = flywheel torque x first gear x final drive x 0.85

wheel thrust = drive wheel torque / rolling radius

Now let's put these into play on our original posters Jeep. He has 3.73 gears in his diffs, and a 6 speed with a 1st gear ratio of 4.46. And we'll assume 230lb/ft of torque, although that number is a bit off. We enter them into the formula's.

drive wheel torque = 230 x 4.46 x 3.73 x 0.85 = 3,252.3 lb/ft

wheel thrust = 3,252.3 / 30" = 108.41 lb/ft of thrust to the pavement

Now let's give him those 35" tires with the stock gears.

wheel thrust = 3,252.3 / 35" = 92.92 lb/ft

So let's see what numbers we would get if he did some engine gains to improve his torque by 20 lb/ft.

drive wheel torque = 250 x 4.46 x 3.73 x 0.85 = 3,535.1 lb/ft

wheel thrust = 3,535.1 / 30" = 117.84 lb/ft for the stock tires

wheel thrust = 3,535.1 / 35" = 101 lb/ft for the 35" tires

That shows us that is is slightly less than stock. Now let's see what happens if he regears to 4.88

drive wheel torque = 230 x 4.46 x 4.88 x 0.85 =4,255

wheel thrust = 4,255 / 35" = 121.57 lb/ft for stock engine and 35" tires after a regear.

So if a stock Jeep on stock tires only puts out 108.41 lb/ft of torque to the ground from the tires, and you can get 121.27 lb/ft of torque after going to 35" tires and regearing, which would you do?


Use these gear calculator with the information below to find out what the gearing change will be like. http://www.izook.com/gearcalc.htm

NSG 370 Transmission Gear Ratio
1st gear ratio 4.46
2nd gear ratio 2.61
3rd gear ratio 1.72
4th gear ratio 1.25
5th gear ratio 1.00
6th gear ratio 0.84

Ratio with 4.10
1st gear ratio 4.46*4.10= 18.286
2nd gear ratio 2.61*4.10= 10.701
3rd gear ratio 1.72*4.10= 7.052
4th gear ratio 1.25*4.10= 5.125
5th gear ratio 1.00*4.10= 4.100
6th gear ratio 0.84*4.10= 3.444

Ratio with 4.88
1st gear ratio 4.46*4.88=21.7648
2nd gear ratio 2.61*4.88=12.7368
3rd gear ratio 1.72*4.88= 8.3936
4th gear ratio 1.25*4.88= 6.1000
5th gear ratio 1.00*4.88= 4.8800
6th gear ratio 0.84*4.88= 4.0992

Ratio with 5.13
1st gear ratio 4.46*5.13=22.8798
2nd gear ratio 2.61*5.13=13.38893
3rd gear ratio 1.72*5.13= 8.8236
4th gear ratio 1.25*5.13= 6.4125
5th gear ratio 1.00*5.13= 5.1300
6th gear ratio 0.84*5.13= 4.3092

if you are currently running 4.10s, 5th gear is almost exactly what you are going to get as 6th gear with 4.88s.
if you go with 5.13s, 6th gear is going to be a higher gear than 5th is currently with the 4.10s, kinda between 4th and 5th.

There is also a gearing chart in the help information posted on this site.

slowpokie

Nice! I was looking for something like this. Thanks for all the details.

ethang1977

Did you calculate your mpg by dividing actual miles driven by gallons of gas used or did you use the estimate on the instrument cluster? This may have me considering 35's and 5.13s now...

fattony21

very good info helps alot but have you considerd any of this for a 2 dr would be a little differant do to waight n such? no?

seabass

The mileage is from the computer. But that is what I used before the gear swap so it is a direct comparison even if the numbers are off a bit. Quite a few other threads on MPG that you can check out. Several people saw 1-2 MPG increases after re-gearing.

As for a 2 door, all the gear ratios and RPM's would be the same so I believe the 5.13s would still be a good choice. You would think the lighter 2 door would accelerate better, possibly get better gas mileage and need less gear to work efficiently? However check out Jeep.com, not even from the factory do the two have distinguishable MPG ratings, so the performance is likely the same as well. I remember reading several thread will door Jeeps having 5.13's, and as I said above not much for regrets from anyone.

fattony21

makes sence im stuck between 33s and 35s right now so i was going to hold off on gears for a bit my 4.10s are not bad for my setup. when i do get ready to throw some serius money down what would be the best order of install for
1. 5.13s
2. lockers
3. dana 44 up front

seabass

For serious money, you would do it all at once, they are all related. Doing it bit by bit would only cost more $$. imo.

fattony21

thats what i figured just wanted to double check, so would 4.10s and stock "x" 29s be the same feel as 5.13 and 35s