What is the perfect gear ratio - for tire size?

So you bought a Jeep - you're all excited and you start to do some modifications and want those big tires. Then you find your new-found toy a little sluggish in the power department. Not really thinking about the Jeep other than the colour, the options and that you liked it and want to do some off roading - you find it has a 3.21 axle ratio or some other low number gear set. My first Jeep a 1980 CJ7 was acquired via a vehicle trade. I had no real clue about gear ratios, or Rock Crawling or what a lower ratio can do to help you off-road. It had a 3.08 rear axle ratio. I remember the comment from someone on the Rubicon my first big trip - your Jeep seems to have pretty tall gears! I thought that was a good thing at the time! It was not.

The first thing people think about for gearing is the axles. But let's step back a bit and talk crawl ratio for off road - you will hear that a lot as well.

Your crawl ratio is:

The lowest gear in your transmission

The lowest gear in your transfer case

The ratio of your axle

IE: 4:1 first gear, 2.72:1 transfercase low gear, that 3.21 axle ratio

4X2.72X3.21= 35.31 overall low ratio

The diameter of your tire effects your total gearing to the ground. When you add larger tires that ratio will reduce.

This will affect how slow you can go off road, giving you vehicle control while managing tire spin climbing hills and over rocks.

Let's look at how axle ratio affects highway speed:

We now subtract the transfer case - as it will be in 1:1 ratio (high range) on the road and you will obviously be using all the gears in your transmission. In this case we will look at total RPM and that seat of the pants feel taking off from the line, and fuel economy.

New car manufactures have fuel economy targets for their fleets. And 90% of new car buyers are certainly interested if not concerned about this part. Running an engine higher than optimum RPM for a given speed - will burn more fuel. This is managed by multiple over drive transmissions these days and axle ratio. A gear set, say that 3.21 gear set would be a rather "high ratio" for a Jeep giving it adequate power to move with a stock tire size and return the best possible fuel economy. Add a lift and larger tires and the Jeep will start to struggle to use all of the gears in the transmission at highway speed and feel like it is lacking power as the tire size will reduce the overall ratio at highway speed and are heavier for the engine to turn - falling out of that optimum power curve of the engine.

That same Jeep if ordered or equipment with an off-road package can have a 3.73 ratio, 4.10, 4.56 ratio even these days a 4.88 ratio from the factory! These higher numbers are actually called "low ratio" gears. They also tend to come with packages that have larger stock tires sizes - up to 35-inches now from the factory with the Rubicon Recon package where the 4.88 ratio is a $1000.00 option. The larger tires reduce that gear ratio to the ground, still keeping RPM at a given speed reasonable to give some fuel economy while compensating for the larger tires.

This also changes your crawl ratio - the slow speed you want off road for trail control. Let's look at a Jeep Rubicon Recon where you bucked up for those 4.88 gears.

The Rubicon comes with a lower gear ratio in the transfer case as well, at 4:1 ratio.

4:1 first gear 4:1 transfer case 4.88 axle ratio

4X4X4.88= 78:1 overall crawl ratio - nearly twice the overall low ratio of that first Jeep!

This will make a huge difference in all areas of how your Jeep drives on road and its control off road and it will also affect fuel economy as will those larger tires.

How can I tell what gears are in my Jeep or a Jeep I want to buy? if you don't have an options list there is a pretty easy way to tell via the tag on the rear axle - that tag can sometimes say ratio or it may just have two numbers like 44/9 - divide those and you get 4.88. But someone has said they change the axle ratio, or they did a gear lube change and didn't put the tag back. So get a jack lift up the rear axle, put the Jeep in neutral and spin the rear tires - both at the same time and count how many times the yoke on the driveshaft turns around. A little over 3 turns 3.21 - almost 4 turns 3.73 - just over 4 turns 4.11 etc...

Now it took a while - but what is the perfect gear ratio. we talked about how gear ratio effects engine RPM and crawl ratio. And now I'll say - there really isn't one - it all depends what you want to do with that Jeep. if you want to rock crawl - you want low gearing, if you want to race around the sand dunes or just have some lower gearing for back roads, and don't want to destroy any semblance of fuel economy - those super low gears are probably not for you. You want huge tires - you will need to add or buy those lower ratio gears to keep on road performance reasonable. Don't fret if you are now using your Jeep in a way you first did not intend - ratios can be changed but they do come at a price. as a ball park $2000.00 would not be unreasonable for a gear change. Remember that $1000.00 cost for the 4.88 gears in the Rubicon model? Not really all that bad now.... and under warranty for a while.

I concentrated on axle ratio here - but there are plenty of ways to have your gearing and street manners too, I'll cover that in a future Blog!

Albert V