Choosing the proper gear ratio.
 

Ok CF, I get asked this a lot and constantly retype so I believe a Reference Sticky is in order.

Now, you just lifted your XJ/MJ/ZJ/WJ/etc... and you have added those mean looking 33" tires but now your wondering, "Why doesn't my Jeep have the same get up and go? or Why does it feel like I've lost power?" Well, let's take a look at why.

Your Jeep from the factory is designed with a good mix of economy and power. Part of this is having a gear ratio that is proportional to tire size and weight of that vehicle.

Let's use our XJ/MJ platform for our example. From the factory, they came with a 28" tire and 3.55:1 gearing on average. Not bad at all. But now you lifted and added those 33's. So now,

28/33 x 3.55 = 3.01:1

Your XJ/MJ now acts and feels like you have 3.01:1 gear ratio.

How does this formula work? Well simple.
OTD/NTD x CGR = EGR
OTD: Old Tire Diameter (stock, in inches)
NTD: New Tire Diameter (in inches)
CGR: Current Gear Ratio (what your axle now has)
EGR: Effective Gear Ratio (What your vehicle behaves and acts like it has)

Now that we have covered the basic math, "How do I regain that factory get up and go?" Well, that is where the topic of regearing has come into play time and time again.

For starters, do the basic math to find out what your EGR is. Got it? Good.
Now, let's take available gear ratios (ring and pinion sets) for your axle and input that number into the CGR spot. So, let's use a common 4.10:1 in the CGR.

28/33 x 4.10 = 3.47:1

Now this will give you back a lot of your acceleration and power by going to 4.10's.

The final math will always be off of stock by a little but the main point is to try and get as close to stock gearing with the EGR as possible. By doing so, your engine and transmission will still think you have the stock size tires and have little trouble in turning them without over working.

I hope this helps you in selecting the best gear ratio for your needs.

Yes, thank you! ... thought I had to do Math!... :-)

Sorry RacerX, due to system glitches, I am just now seeing your response. Now 4.10 was a standard gear in the 4cyl and 2.8L XJs and MJs with 5spd manual (AX-5), the 4cyl autos had 4.56 and the 4spd (AX-4) manuals had 3.55's. Some of the 2.8Ls had 3.31 gears. So depends on what you find.

My 85xj with 4cyl and 3speed auto came with 3.73 gears. The 4.56 was with the tow package and 4cly I believe. Those are not so easy to find.

Correct, 3.31 and 3.73's were an option but rare. They called it the "Fuel Miser". My old 86 XJ was originally equipped with a 2.8L V6, 3spd auto with 3.31 gears. I just posted the most common ratios available

Over gearing
 

So I've seen a lot of people do the 4.10 or whatever else to get "close to original " as you mentioned. What would be the benefits or problems of over gearing to get lower than original?

Better acceleration and Performance Off Road is the advantage. The only disadvantage is higher RPMs driving on the road.

So perfect for a trailer queen may be hard on a DD. Thank you for the help.

Not too hard on a DD if it's reasonable. Say if you went from 4.10 to 4.56, that's bearable. Now if you went from a 4.10 to a 4.88, that would be screaming and hard on the DD.

The other benefit of changing the gear ratio is that your speedo will also be nearly corrected without having to change the speedo gear.

So to keep close to the stock 28" with 3.55 gears- 4.0L Auto-
use 31" with 3.73 gears
use 33" with 4.10 gears
use 35" with 4.56 gears
use 37" with 4.88 gears

These numbers will keep you 'near stock' like 28" and 3.55

Now if you have more weight, armor, winch, bumpers, etc, you would want a little 'more' gear to move things along, Just move up one gear level (lower gear ratio, higher number).
Yes this will raise your cruise rpm, but will help your stop and go driving.

Thank you for your input Marty. Helpful amendments are always welcome. :thumbsup:

How much of this would change if you upgraded axles? Or is it still the same gearing just stronger? And if you go any higher than 4.88 or if you only have 35" tires with really low gearing, just makes for harder on the road?

All great info thanks. I have a question in regards to your front axle. Does the front have to match the rear? If I put 4:56 in the rear do the same for the front

Yes. Both must be the same

For those requiring very low gearing a doubler is used or another transfer case (3 shifters on the floor).


I run 35 and 33" tires with stock axles a D30 and Chy 8.25 29 spline both locked with 4.56 gears. With the 4.56 gear and 33" tires ( 32.7") I still had to replace the speedometer gear for correct reading but RPM's are the same at 65 mph when stock and with the 33's.