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One of the most confusing aspects of gearing is how gear ratios are expressed. It’s an inverse relationship. Low gears, such as a 4.10 to 1, are numerically higher; high gears—2.54 to 1—are numerically lower.  Low gears give the truck greater pulling power; higher gears improve fuel economy.
Changing tire sizes and gear ratios may play havoc with the On Board Diagnostic (OBD) system found on late-model trucks. That’s because the sophisticated computers that comprise the OBD system monitor a wide range of vehicle functions, including the engine, transmission, and anti-lock brakes. New wheels and gears also will affect the speedometer reading, and when the speedometer, tachometer, and wheel speed readings don’t correspond with the pre-programmed conditions in the computer system, the system can get a little testy. Usually a warning light on the dashboard will come on, letting you know that you have offended the sensibilities of the factory engineers; in more serious cases, vehicle performance may be seriously compromised. So before you change wheels or gear ratios on a late-model truck, check with an offroad shop that regularly performs these modifications,
Heir’s an example of what we’re talking about.
Let’s say you have a stock Ford Explorer with tires that are 28 inches tall and a 2.73 drive gear ratio. At 60 mph, the engine is turning 1,966 rpm. Now add taller tires, say 33-inchers. At 60 mph, the engine will turn only 1,668 rpm, a power loss of nearly 15 percent. You’ll notice it immediately; the engine will appear sluggish and fuel economy will suffer.
The Explorer needs a gear ratio of 3.19, but the factory gear ratios available for the Explorer (a 4×4 requires you to change front and rear axles) in our range are 3.07 and 3.55.The 3.07s are a bit on the high side, which entails a sacrifice of a small amount of torque for better Ihel economy; the 3.55s are on the low side and would be a better choice for offroad use, towing, and hauling heavy loads. The penalty here is higher fuel consumption.
If you don’t know your vehicle’s drive gear ratio, look at the vehicle specification tag, usually found in the glovebox or on the axle tag attached to the axle covet You can also check this manually by putting the vehicle on jack stands, blocking the front wheels, and Putting the transmission in neutral. Mark the driveshaft and tire with chalk, and rotate the tire one full turn while counting the number of turns the driveshaft makes.This will give you a close estimate. If the driveshaft spins 2I4 revolutions per 1 revolution of the tire, the truck has a 2.73 ratio. (This procedure will work only if the truck is equipped with a limited-slip or locking differential, if your truck has open differentials—see Chapter 7—you and a partner will need to rotate both rear wheels simultaneously.)
if you don’t want to fool with the math, that’s okay. Just keep in mind the basic concept: When you change tire size, you’ll need to change the gears in order to maintain the performance of the truck.