Differential Question?

[Dadandson]

This may seem like a silly question but what do the numbers of a differential signify? For example what difference would a 4:10 rear end be to say a 3:75? Is it more speed or more torque?

 

[GB1]

The lower the number the less the engine needs to turn to spin the rear wheels at the same speed.

So a 4.22 is faster off the line, but on the highway it runs at higher RPM's.

The 3.9's is what is mostly used for street cars that will be driven on the highway. A 3.7 is probably better but they get pricey. A 3.9 over a 4.2 will reduce highway rpm's by about 300. This is preferred to keep the temps down.

For autocrossing, a 4.5 or higher is preferred to keep the rpm's in the power band at a lower speed.

For racing it greatly depends upon the track, the length of the straight, whats before it, how many twisties, etc.

But basically for a street car a 3.9 is the preferred rear ratio.

 

[Dadandson]

Got it. Thanks

 

[Roger]

They signify the gear ratio. In other words, 4:1 means the input turns 4 times for every single turn of the output. A 3.75:1 (which I guess is what you meant) means the input turns 3.75 times for every turn of the output.
So a 4:1 is lower geared than a 3.75:1 - more RPM for the same MPH if you want to look at it that way. The 4.1:1 will give you a lower top speed than the 3.75:1 at maximum RPM, but will deliver more torque to the wheels.


Please also note that, though often used (or mis-used!) it's quite wrong to speak of the ratio of a differential. The differential is the component that allows one driven wheel to rotate at a different speed than the other, and it doesn't have a ratio per se. What you're referring to is the final drive ratio; pinion to ring gear is the term over here I think. I'd call it crown wheel and pinion ratio, being from across the pond.

Ocaasionally you'll see the ratios quoted as, e.g., 8:42. The means 8 teeth on the pinion and 42 on the crown wheel, for an overall 5.25:1 ratio, which will be familiar to most Austin Seven owners, incidentally.

 

[JPSmit]

specifically, it is the ratio of input to output. I.E. the the driveshaft has to turn 4.22 times to turn the wheels once