Gearing for Taller Tires
Whenever larger-than-stock tires are installed on a truck, it will have a direct effect on the truck's performance. Why? It has to do with the effective gear ratio. Your truck comes from the factory with the optimum axle gear ratio to work with the truck's engine, transmission, and stock tire combination as well as provide a good balance between acceleration and fuel economy. When taller tires are installed on a truck but the axle ratios stay the same; the effective gear ratio is reduced. This means the engine is forced to operate below its power band, and performance and fuel economy suffer as a result. In order to restore the effective gear ratio (and the truck's performance), you'll need to have the axle gears swapped to lower (numerically higher) ratios.Fortunately, finding the correct gear ratio to run with the tires that you plan to install is simple using a mathematical formula. There are actually several different formulas available to determine the correct ratio with varying degrees of difficulty. The simplest and most reliable method we've found is to determine what engine RPM the truck runs with the stock gears and tires at a given speed, and then closely match this rpm with the larger tires. In order to use the following formula, you'll need to know the stock axle gear ratio as well as the stock tire size. It also helps to know what RPM the truck runs at a given speed with the stock setup, but it's possible to find this information with the formula. Keep in mind all RPM readings should be taken with the transmission in a 1:1 ratio (Fourth with most manuals or Third with an automatic).
The Formula
rpm = mph x gear ratio x 336 / tire diameter
To use an example, let's say we want to know what gears should be installed when putting 36" tires on a transmission, 3.08 gears, and 30" tires in stock form. Since the truck doesn't have a tach, we first need to find out what RPM the engine is spinning at a given speed, say 65 mph. With this information, the formula looks like this:
2242 = 65 x 3.08 x 336 / 30
At 65 mph, the truck's engine is spinning at 2242 rpm, which is right in the optimum powerband for most V-8 engines. Now all that's left to do is plug in the new tire size and a couple different ratios available for the truck to see which one closely matches the RPM.
2263 = 65 x 3.73 x 336 / 36
As you can see from the above formula, 3.73 gears with 36" tires is optimum for restoring stock performance with this truck. Keep in mind that not all ratios are available for every axle, so do some research to make sure there's a ratio close to what you need available for your truck's axles.
One last note regarding trucks equipped with overdrive. The Overdrive gear in a transmission allows the drive train to run below a 1:1 ratio, which brings down engine RPM for good fuel economy when running down the highway. Most Overdrive gears run somewhere around 0.73:1. As we said before, the best option for people who use the truck primarily on the street is to match the formula given above. However, Overdrive offers a unique opportunity for dedicated off-roaders. Since overdrive decreases the reduction at highway speeds, it's possible to "cheat" and select gears that are a little lower (numerically higher) for better slow gearing off-road without sacrificing highway driveability. This advantage becomes apparent when you do the math by multiplying a prospective gear ratio by the Overdrive ratio of your transmission. To use an example, 5.13 gears in a truck without an overdrive and moderate-sized tires would offer awesome slow-speed crawling ability for off-road situations but would leave the engine screaming at highway speeds. However, if the same 5.13s are installed in a truck equipped with an overdrive of 0.73:1 and we do the math (5.13 x 0.73 = 3.744), we find that the Overdrive gear gives the effect of a very streetable 3.73 ratio for highway cruising. In other words, the truck has the same good crawling speed of 5.13 gears (because Overdrive is never used off-road) combined with a very street-friendly effective ratio of 3.73 gears. Of course the engine will still rev higher on the highway compared to having the proper ratios found by following the steps we laid out earlier. Overdrive does offer dedicated four-wheelers the option of installing low, off-road friendly gears and still be able to drive around on the street with some fuel economy left intact. To plug overdrive into the gearing formula above, simply multiply the Overdrive ratio by the axle gear ratio.
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