This has to be one of the most talked about subjects of any 4x4. At some point in time anyone that modifies their rig will have to decide is gearing for tire size, drive-ability and trail performance. We will have a look at tire size and how it affects the performance of your 4x4. gearing options, crawl ratio and what all this means to our trucks and how it can help or hinder the performance of your 4x4. This is all something we have had to learn but if you are new to the sport of orr-roading approaching a bunch of wrench heads with what seems like a simple question can be intimidating.  With a little simple math and a tape measure it is pretty simple. As always though numbers and real world experience are a little different but not as bad as you think. Well will get more into that later.

On with the basics

Well you have your first 4x4. You have the basics down and now you are bitten by the build bug. The choices for builds are endless so we will go with the typical, basic build.
You have a lift in and picked out those awesome tires and wheels but now your truck is a slug, running hot and sucking up fuel like it’s free. What happened? When you put those bigger tires on you effectively changed the gearing of your truck. A larger tire does not have to turn as fast to achieve the same speed and distance traveled as those tiny stockers.
What to do? Well it’s pretty simple, you have to change the gears to get your engine back into the powerband, but here is where things can get confusing. Knowing the factory gearing can give you a bit of an idea where the best economy and street performance was with the factory tires so this will give you a base line. There are a few ways to find this, RPO codes, a tag on the differential or by removing the cover and finding the numbers stamped on the ring gear.

In the photo you will see the numbers stamped on the ring gear at the end of the sequence. 40 13
The 40 represents the number of teeth on the ring gear, the 13 represents the pinion gear. 40 / 13= 3.07, basically the pinion will turn 3 times to turn the ring gear once. If you are running a 33” tire from a stock 27” you will have a big loss of power. To calculate the gears you need, yes this is a math lesson, there will be a test later……… The equation is pretty simple but you will need to know the ratio of your transmission. If you have a 3 spd auto or an older 4 speed with a low first your final drive will be 1:1. With an overdrive it could be 0.75, this reduces the rpm at cruising speed for fuel savings but it does complicate the equation a little. The other thing you will need is tire size. Do not assume your 33” tire you just bought really is 33” tall. Many tires do NOT measure their claimed height. For this you will need the true tire height.
At normal tire pressure place something with a flat edge protruding over the tire and measure the height. You will find tires can be up to 2” smaller than what is claimed. Once you have all that you are ready to start.

What you are looking for is the best gearing to achieve the rpm you want at your highest cruise speed. This formula is in MPH not in metric but that is a simple conversion. The engine you have will decide the target RPM you want. As an example a V8 will have the torque to run at a slower speed than a small 4cyl.
For comparison right now we will use a V8 and a 4L60E trans which has an overdrive of 0.69
If I put a 35” tire under this truck I will need to decide what gears to use.

The formula goes:  speed x overdrive ratio x gear ratio x 336 / by measured tire size.  So this is what it will look like.
  65mph x 0.69 x 4.10 x 336 / 33”  (remember measured tire size)

Now with this gear ratio it will give me a RPM rate of 1872rpm at 65mph. Now this might not be bad for a big block V8 but if you want a little more pulling power you could try a different ratio. If I try a 4.56 ratio I get:  65 x 0.69 x 4.56 x 336 / 33”.  Now I will have 2082rpm at 65mph. With out metric system 100kph is actually 62mph.   (the conversion is 100 x 0.62)
However there is a little more to consider. The other thing to think about is the weight of your new tire and wheel combo. This is where you can save a little weight with aluminum wheels but that is another article all together. The heavier tire and wheel will take more power to get rolling and keep rolling. If you have a lower power engine like a small 4cyl you will want to get the rpm a little higher than where it was running with stock tires.

Gear swaps can be an expensive undertaking so you may also want to consider any future changes. You have just bought 33” tires but what if you decide to move up to 35’s?  This is the time to give yourself some room to grow.

In the next installment we will look at transfer case ratios, what can be changed there and crawl ratios and what affect they will have on your 4x4 when you take it out on the trail.

Thanks to: JBZukin

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