Final Drive

2k2TahoeSS · April 22, 2003

Ok, so can someone explain to me what this final drive thing is? My friend was kinda telling me that its like a gear that multiplies the gear ratio of the car, but at the same time does not function like a gear. I think he is on crack becasue it doesnt really seem like he exactly knows what he's saying. Just that it feels like you get alot more torque becasue of a better final drive. Anyone? Explain please!

Ben

fenderbender · April 22, 2003

??? :werd:

robsawatsky · April 22, 2003

Final drive = axle ratio.

Your vehicle has a transmission with some number of gears, each with a different ratio. Depending on whether auto or manual tranny, 1st gear will be around 3/1 or 4/1 ration and top gear or overdrive around .75/1. This is the ratio from the engine to the driveshaft. i.e. in 1st gear your driveshaft turns one complete revolution for every 3 or 4 revolutions of your engine. This provides more torque in lower gears to get you moving, and more rotations in higher gears to give you reasonable road speed.

Now the driveshaft goes to a differential at each driven axle (1 for 2WD, 2 for 4WD) that will have a ratio varying from 2.something/1 to 4.something/1 or possibly higher. This is the ratio from driveshaft to axle. i.e. your axle turns one complete revolution for every 4 turns of the driveshaft.

A higher axle ratio, just like the transmission low gear, will provide more torque and pulling power. But, at the expense of a higher RPM for any given speed and possibly lower fuel economy. A lower axle ratio does just the opposite.

Typically, for a truck, you want a 3.73 axle ratio as a minimum for any significant towing or heavy loading and even 4.10 or higher for frequent and heavy towing applications (assuming a gas engine). Note that large diameter tires will reduce the effective axle ratio as compared to stock tires.

2k2TahoeSS · April 23, 2003

wow that clears things up quite a bit thanks. So here's my next question: would changing the gear in the differential do much good or not really. and do you know what gear comes on a a 3.73 auto trans tahoe? What would be a good size to upgrade to to get a bit of extra umphf without sacrificing too much top speed? I still want to have great ease and relative speed getting up to about 80-90 mph. I rarely go faster than that. Also, does having larger axle:driveshaft ratio mean i will have less top end or top speed? Thanks

Ben