Final Gear Ratio vs Differential Ratio

[JimboHostetler]

Hey everyone! I understand how a final gear ratio works, it is the relationship betwen engine speed and the wheel speed. How does the differential ratio effect the behavior of the transmission? Both of the transmissions that i am considering have a 0.73 final gear ratio, but one has a differential ratio if 3.55 and one has 3.25.

[CaptainHazzard92]

if a differential has a 3.55 ratio it means the drive shaft turns that many times per rotation of the tires. so a 4.10 would get more leverage but lower top speed

[Hylton Trucking]

3.25 is faster, 3.55 pulls better

[Trucker_71]

3.25 will give you more top end speed, 3.08 even more so. ;-)

[JimboHostetler]

Originally posted by Super Kami Guru:

if a differential has a 3.55 ratio it means the drive shaft turns that many times per rotation of the tires. so a 4.10 would get more leverage but lower top speed

So, the differential ratio is another ratio, this one effecting the driveshaft after the transmission? Effectively lowering the diff ratio would have a similar effect to lowering the final gear? Thanks for the insight!

[Robinicus]

Originally posted by JimboHostetler:

Originally posted by Super Kami Guru:

if a differential has a 3.55 ratio it means the drive shaft turns that many times per rotation of the tires. so a 4.10 would get more leverage but lower top speed

So, the differential ratio is another ratio, this one effecting the driveshaft after the transmission? Effectively lowering the diff ratio would have a similar effect to lowering the final gear? Thanks for the insight!

The driveshaft speed is affected by the transmission output ratio; you divide the engine RPM by the output gear ratio to find driveshaft RPM...so on a .73 double overdrive final trans ratio, at 1500 engine RPM the driveshaft speed is 2054 RPM.

The diff ratio doesn't affect driveshaft speed, it is the relationship between the number of driveshaft revolutions it takes to make one revolution of the drive axle. The ratio is calculated by using the number of teeth on the pinion gear (splined to the driveshaft yoke) and the ring gear (splined to the axle shaft).

For example, on a 4.0 ratio (for ease of display) there would be 10 teeth on the pinion gear and 40 on the ring gear. To turn the ring gear once, the pinion would have to rotate 4 times. Typical pinions are in the 10-13 teeth ratio but there are a few oddballs with less or more.

[AA 005]

if you are after top speed, just multiply the top gear ratio by the diff ratio on the different transmissions. The smallest number of the options is faster assuming you also have a good enough engine for it.

[Hylton Trucking]

The engine turns the transmission, which turns the driveshaft, which turns the differential, which turns the wheels

[JimboHostetler]

Thanks for all the input guys!