Thread: rear gear

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i have a 8.2 10 bolt chevy rear with a 2.73 gear. can i install a 3.73 gear in it and if so how much improvement in my ft lb torque can i expect.

Huge "feel" difference. You'll love it.

It won't increase your torque since you're not changing the engine. However, it will increase the seat-of-the-pants feel somewhere around 35%. The ratio of the two gears gives a pretty good idea (3.73 / 2.73 = 1.37 = 37%).

thank you henry. that is what i was trying to ask. learning new things every day.

how much will my mpg be reduced if any.

By somewhere around 25% - 35%. Same numbers work here. At the same driving speeds, your engine will be turning faster and burning more fuel. It's not a perfectly linear relationship, but it's close. I changed the gears in my 'Vette from 2.90 to 3.55 (22%). My gas mileage dropped from around 22-23 to about 19-20.

i wish i knew as much about all this as you do. but i am enjoying learning it all.

Originally Posted by jyardgirl

i wish i knew as much about all this as you do. but i am enjoying learning it all.

I'll explain something to you that you can wow the boys with. You know that a shorter gear will provide more acceleration than a taller gear, but at the expense of lower top speed with the shorter gear. But how does it do it?

It's a matter of applying power pulses from the motor (or work) to the tire.

An 8 cylinder motor like yours will have 4 power pulses for each 360 degree revolution of the crankshaft. It doesn't have 8 like you might think because it's a 4-cycle motor. With any given cylinder in your motor, it takes about 180 degrees to pull in the fresh fuel/air mixture, 180 degrees to compress the mixture, 180 degrees to push down on the piston and transfer work to the crankshaft and 180 degrees to push the burned mixture out of the cylinder. 4 times 180 degrees equals 720 degrees to complete a cycle, or 2 complete 360 degree revolutions of the crankshaft. So, for each 1 revolution of the crankshaft, only 4 of the 8 cylinders in your motor will have fired to produce work.

I don't want to confuse you, but I have to explain it that way in order for you to do some calculations on your own.

Now, let's pick any given rpm of the motor, we'll use 3,000 rpm's. The crankshaft is turning 3,000 rpm's, so 3,000 times 4 will equal 12,000 power pulses that the motor is producing in 1 minute.

Now, divide the 3,000 rpm's the motor is producing by the gear ratio 2.73 and you find that the tire is turning 1,099 rpm's. If we assume a 88" circumference tire (tire diameter times 3.14159), then the tire is travelling 96,712 inches at the given 3,000 motor rpm's with a 2.73 gear. (88 times 1,099). If it's getting 12,000 power pulses per minute at that motor speed, then dividing 96,712 by 12,000 reveals the tire is getting a power pulse each 8.059 inches of its circumference.

Using the 3.73 gear (shorter), the tire is turning 804 rpm's under the same conditions (3,000 divided by 3.73) and rolling 70,752 inches (88 times 804). Getting the same 12,000 power pulses, divide 70,752 by 12,000 and find that the tire is getting a power pulse each 5.896 inches of its circumference.

You see, with the shorter gear, more work is being applied to the tire in the same amount of time. More work equals quicker acceleration.

As I said before, for the same crankshaft speed, the taller gear will allow the car to go faster, but not get there as quickly. The shorter gear will allow a slower top speed, but will get there in a hurry.

Hope this helps.

wow that is alot to digest. i will study it thoroughly. The boys are not so wowed by me as they take me as a woman in a mans world. There are a few that take me seriously and let me help them out when they need it. Once they see that I am willing to get grease under my nails and I have a smile on my face they begin to treat me like one of the guys. It is truly amazing how much the people on this site know about anything automotive.

jyardgirl---I agree with all above and just want to go one step further with the 35% increase in feel comment. If your 2.73 is "bogging down" your engine at 0-25MPH the feel can be much greater. In fact with enough engine you may have tire spin with the 3.73, if your so inclined to "tee it up at a red light"! Also the improvement can be more than 35% from 0MPH to wherever your engine gets into its torque band.

The "bog down" is where the engine overloads with gas and runs too rich for maximum power. Try nailing it at 25MPH in 2nd and you will get a big bog down. My automatic with a "jacked up" start pulling a 2.73 bogs a little at around 15MPH.

I'm going down the same road right now and have choosen a 3.42 replacement for a 25% improvement. I want to keep the option open of running the interstate at 70MPH open. There are many RPM Speed calculators on the WEB so give it a lot of thought and use a calculator and a tack to run your engine at your proposed highway cruise with the 3.73 (use 2nd gear) to feel where your going with your change. Above 3200 RPM at highway cruise speed gets old quick.
You will also need to change your speedometer gear in your tranny to have it read right after the change.
Good Luck with the change!