An axle ratio is the number of revolutions the output shaft or driveshaft needs to make in order to spin the axle one complete turn. The number is expressed in a ratio, which represents the number of teeth on the ring gear divided by the number of teeth on the pinion. For example, a vehicle with a 3.73:1 gear ratio means that the driveshaft will turn 3.73 times for each complete wheel spin. Many people simply refer to the axle ratio as 3.73, rather than express it as the ratio 3.73:1. However, they both mean the same thing.
A lower (numerically higher) gear ratio provides more low-speed wheel torque, which makes it easier and faster to get the vehicle moving when accelerating, pulling a trailer or carrying a load of passengers and cargo. (Note- Acceleration is improved up to 60 mph. Passing performance and speeds above 60 mph, however, is a matter of horsepower- not axle ratio).
Typically, axle ratios are chosen by manufacturers for performance, capability and fuel economy. In a standard passenger car, the stock ratio is usually biased more toward fuel economy. By selecting a final drive ratio that lowers engine rpm, under most conditions, manufacturers can more easily meet government mileage standards. In contrast, optional axle ratios tend to provide quicker acceleration and more towing capability by increasing the amount of available torque delivered to the axles.
Until fairly recently, one of the best ways to improve drag strip performance, on a stock car, was to install a lower gear ratio. In fact, in the 1960s and 1970s, buyers of factory muscle cars had the option of ordering their cars with lower gear ratios. For example, in 1971, Dodge Challenger R/Ts, with automatic transmissions, were equipped with 3.23 rears. Optional final drive gears were 3.91 (High Performance Axle Package) and 4.10 (Super Performance Axle Package).
In order to calculate axle ratio, you count the number of teeth on the ring gear as well as the number of teeth on the pinion. Then, divide the number of teeth on the ring gear by the number of teeth on the pinion in order to determine the axle ratio of a vehicle.
If you don’t want to take apart your differential, then here’s a quick “Tech Tip” for you. Most modern differentials will have a sticker located on the outside, so you can easily have access to your axle ratio information. If you have an older vehicle, or the sticker peeled off, then there’s another relatively simple way to calculate your axle ratio. Grab a marker and mark the inside of the tire along with the driveshaft. Put the vehicle in neutral while it’s lifted in the air and pin the wheel one full revolution to see “roughly” how many revolutions the driveshaft spins. While this will just give you a ballpark figure, you can estimate the axle ratio by judging the choices available. For example, some of the gear ratios for a 1971 Challenger R/T include 3.23, 3.55, 3.9 or 4.10. If you count 4 revolutions, then you probably have a 4.10 axle ratio.
Another, quicker way, is to use this online Gear Ratio Calculator to find the correct gear ratio for your ring and pinion gear set:
Traditionally, many owners of muscle cars chose to install lower axle ratios for performance benefits. However, there are some downsides to going lower when it comes to axle ratio. Although having a higher numerical axle ratio will give you more power, it will affect your top speed. However, even though you may not be able to go as fast, you will have better initial acceleration and better “seat-of-the-pants” feel. Along with decreased top speed numbers, having a higher axle ratio will also hurt your fuel economy because your engine’s rpms will be higher and the differential will take longer to complete a full rotation.
A torque converter can take the rotation of the engine and convert it to a lower speed with higher torque. Note that a higher torque always means lower rpm.
There is something which is often forgotten- you can't choose the speed at which the tires of a car rotates at a certain speed. This means that you can't choose the output rpm of a transmission given the speed of the car. As speed increases, you have to increase the transmission ratio to keep the rpms constant on the engine. This means that as speed of the car increases, the torque at the wheel will decrease. So if the speed of the car increases, the transmission will have to increase the ratio and the output torque will be lower. This is true regardless of the type of the transmission used.
In the end what matters to get maximum torque at the wheel is to keep the engine rpm at peak power. The torque converter can help you, for example, when you are at low speed, around 5 mph. In second gear, it will double the torque and you will go more than twice as fast, compared to a manual that will also need some clutch slippage until it gets to a good rpm like 2,000 rpm. It’s also good when you are in a high gear and accelerate. It will give you good acceleration without having to downshift, compared to a manual that will immediately cause engine lug. Basically, the torque converter is like a mini CVT with 1:2 gear ratio, at a cost of some power loss. It helps by instantly giving you more rpm.
Chrysler 8-Speed Automatic Transmission
Modern, computer-controlled automatic transmissions, like the ZF 8-speed in the Dodge Challenger, are far superior to the old 3-speed Torque-Flite and 5-speed NAG1 transmissions, as well as manual transmissions. Its fast locking torque converter allows for quicker engagement and limited “slip.” Because of this, it does not need lower drive gears. In fact, the final gear ratio on an automatic 392 Challenger Scat Pack is only 3.09. And, with an automatic Hellcat it is a very high 2.62.
Here are the gear ratios for the 392 Scat Packs and SRT Hellcats:
Effects of Rear Axle Ratio and Tire Diameter on Drag Racing
For those of you crossing the ¼ mile finish line at an engine speed higher or lower than the desired rpm your engine likes, you can change rear end gear ratios, or you might consider changing the tire diameter. A stock 8- speed auto Hellcat is used as an example. The formula is as follows:
Rear Axle Ratio x MPH x Transmission Output Ratio x 336/ Tire Diameter.
Rear axle ratio= 2.62:1
MPH (trap speed) 125 mph
Transmission output ratio (5th gear ratio is 1.29:1)
Multiply by 336
Divide by stock tire diameter (28.7 inches)
Here are the results:
(2.62 x 125 x 1.29 x 336)/28.7=4,946 rpm. This is the rpm in which the driver finished the race. Based on the dyno sheet, the Hellcat’s torque curve started declining right at 5,000 rpm.
If you plug in a 140 mph trap speed into the equation, for a 900 hp. modified Hellcat, using the rest of the same numbers, the rpm comes out to 5,539. This is well past the point where the torque curve drops off. But if you change the tire diameter to 30 or 31 inches, it brings the rpm back down to right at 5,000 to keep it in its power band. In general, you want to put the tallest, widest tire you can on the car for maximum traction. But sometimes, depending on gear ratios, changing the tire diameter will help you tremendously depending on the set-up.
If you switch to higher 3.25 gears, it will push the car into 6th gear about 100 ft. before the traps. This is not ideal, so the solution is either a taller tire, if it will fit, or a change back down to 3.09 rear gears.