You've added the lift and big tires, but you're being outrun on the freeway by aged Yugos, and your 4WD sucks gas faster than an earthmover. Part of your woes may simply be due to the gearing in your axles. We'll explore a bit about those ratios and their affect on drivability, and also discuss overall drivetrain design with an eye towards both performance and strength.
Manufacturers design vehicles with a final drive ratio that can provide a good compromise, providing reasonable acceleration (lower gearing) and good highway speed at a practical engine speed (higher gearing). The final drive ratio is what you end up with when you factor in the transmission and transfer case gearing, and the axle gear ratio. Tire size also plays a big role in the final results you get.
When tasked with working hard to meet government mandated fuel mileage standards, the OEMs will often choose a ratio that provides improved fuel economy at highway speeds. While this may work well for a stock truck with light loads, it's often not optimal for modified trucks or those towing heavy loads.
What do the proper gears do for your rig? They help maintain decent fuel economy, good acceleration off the line, better highway passing performance, greater torque, and smoother application of power to the tires. When dealing with an existing vehicle, it's most common to swap only axle gears when needed to accommodate larger tires or other changes. When you're building up a complete drivetrain, you may have the option to choose other gear ratios in your drivetrain as well, and we'll discuss later the implications of those choices with respect to strength.
For the case above, suppose we have stock 3.73:1 gearing in the axles. We could swap to a numerically higher gear ratio such as a 4.56:1 gear ratio. This would provide about a 22-percent change in the gearing and put our 60 mph engine speed at 2,096 rpm, closer to our original speed.
While the gearing change can correct the engine speed problem, there is one aspect concerning a change to larger tires that cannot be corrected. This is the added weight and inertia, that come with larger tires. It simply takes more power to get a heavier set of tires rolling (and consequently takes more braking force to stop them). This is one reason that some owners choose to slightly over-compensate when changing gear ratios by going a little numerically higher than the direct calculated ratio.
Determining Your Axle Ratio
If you have open differentials, block the tires on the other axle to keep the vehicle from rolling, then jack up one tire on the axle in question. Place a mark on the tire and on the driveshaft. Place the transmission in neutral and spin the jacked up tire exactly two revolutions and count the number of times the driveshaft spins. This number equals your gear ratio.
If you have a limited slip or locker in the axle, you will need to jack up both tires on the axle as you cannot turn one tire without the other turning. With tires jacked up and your marks done, turn the tire one revolution and count the times the driveshaft spins. This equals your gear ratio. A driveshaft that spins 3.5 times per one wheel rotation equates to a likely 3.55:1 gear ratio.
Which ring-and-pinion sets offer the best strength? As axle gear ratio becomes lower (numerically higher) the number of pinion teeth diminishes, as does the overall diameter of the pinion gear. Typically, a numerically higher ratio gear set is weaker than a numerically low gear set due to the fact that the higher set has fewer pinion teeth in contact with the ring gear.
Helpful Formula For Gears
To figure out your engine speed based on gearing and tire size use:
|ENGINE RPM||=||SPEED (mph)||x||FINAL DRIVE RATIO*||x||336|
|TIRE SIZE (inches)|
|*WHERE FINAL DRIVE RATIO||=||AXLE RATIO||x||TRANSMISSION GEAR RATIO||x||TRANSFER CASE RATIO (1:1 in high range)|
There is a fair bit of misunderstanding and sometimes fear associated with changes in gearing. Some people worry what effects gear changes have on the engine or transmission life. For some, there is concern over affecting the computer controls or ABS functions of the vehicle.
Changing gears to a numerically higher ratio to compensate for the addition of larger tires should not harm engine or transmission life. As long as these components are not rotating excessively fast, their lives will most likely increase with the gearing change. Engines will operate within their optimal powerband, providing smoother power delivery and avoiding excessively low end lugging that can wear bearings over extended time periods.
Automatic transmissions are generally benefited (by a gear change) due to less slip and heat build up. For those trannies with lock-up torque converters in the higher gears, a properly geared rig will more quickly shift up to these gears and lock up the converter, reducing heat buildup in the tranny. Manual transmissions can also benefit as clutches will last longer and provide better performance with big tires if the proper axle gear ratios are used.
Late model vehicles use the speedometer reading as one input to the onboard computer or ECM (electronic control module). Whenever you add larger tires and change the expected speed, as seen by the ECM, this will most likely affect your vehicle engine performance. Swapping to gearing that corrects the overall axle ratio to compensate for the larger tires will also correct the signal to the ECM.
Gearing & Torque
Torque is defined as the product of a force multiplied by the length of the moment arm to which that force is applied. For instance, a 10-pound force exerted over a one foot distance is 10 lb-ft of torque.
If we double the length of the moment arm, we double the available torque. This is why a stroker engine with a longer stroke distance often delivers greater torque than an engine of similar displacement but bigger bore diameter. By the same physics, a given torque acting through an axle shaft can exert less road force using a larger diameter tire than a smaller diameter tire. Imagine lifting a bucket of water with your outstretched arm, with your hand one foot from your shoulder (small tire) versus two feet out from your shoulder (large tire).
Additionally, whenever we send power through a gear change, we affect the torque at the other end. For instance, if we have an axle ratio of 4:1, then for every four times our driveshaft spins the axleshafts spin one revolution. Our output torque at the axleshafts is the input torque at the driveshaft multiplied times the axle ratio (4), making it four times greater than the input torque.
We can see that if we have the opportunity to design our drivetrain makeup then we might possibly have the option to choose components to adjust where our torque gains are and consider their impact. Transmission and axle gearing affect all driving modes, whereas low range transfer gearing affects slow speed driving but not high speed. How you setup your gearing will be determined by performance, budget and desired drivetrain strength.
Fourth Gear Tranny Or Axle Gearing
There was a time when most all transmissions had a 1:1 top gear ratio, but overdrive transmissions have been the norm on new vehicles for the last couple decades. Off-roaders that use their rigs on the highway will often consider swapping to an overdrive transmission if they don't already have one. What does this gain us when compared to a simple axle ratio swap?
Most overdrive trannies have a similar or low first gear and then, of course have the final drive gear ratio that is less than 1:1. This often gives an off-road vehicle the best of both worlds. Our high gear offers less torque output from the tranny but helps to lower engine rpm at high speeds.