general question about gear ratios

[rick0312]

I've bee reading alot and have learned alot frorm the board about gear ratios...I think. I am still confused a bit, but I think i am getting it. Is this explanation correct?

1. If the gear number is larger (for example. 3.73 vs 3.08) 3.73 is a larger number, but the diamater of the gear is smaller than the 3.08, so the 3.73 takes more revolutions to turn the axle one time which equals high rpms at highway speeds, but more torque off the line?

Am I getting close or am i totally wrong.....Thanks .... Rick

[rwgregory]

You are basicly on the right track. Let,s compare 3.08 vs. 4.56 gears. On the 3.08's the driveshaft/engine will have to turn just a fraction over 3 times to to turn the rear wheels one revolution. On the 4.56's the driveshaft/engine will have to turn just over 4 1/2 times to do the same thing meaning it will get rolling from a dead stop much easier than with the 3.08's.

[Bucky71]

Originally Posted by rwgregory

You are basicly on the right track. Let,s compare 3.08 vs. 4.56 gears. On the 3.08's the driveshaft/engine will have to turn just a fraction over 3 times to to turn the rear wheels one revolution. On the 4.56's the driveshaft/engine will have to turn just over 4 1/2 times to do the same thing meaning it will get rolling from a dead stop much easier than with the 3.08's.

I disagree. The 4.56 gears will get you rolling easier and provide quicker acceleration. The 3.08 gears will give lower engine rpms on the highway and therefore probably better mileage and a quieter ride.

[davidh]

compare it to a 10sp bike, remember 10th gear was a mother to get rolling but 1st gear was easy but topped out quick.

[rick0312]

bump

[rick0312]

so in order to get the best of both worlds I kind of want a rearend that is in the middle say a 3.42?

[67ChevyRedneck]

Depends on what you have.

A "low" rear-end gear is typically a higher number, like 3.73, 4.10, etc.

A "high" rear end gear is typically a lower number, like 3.08.

If you have a 700R4 you'll want a 3.42 or 3.73 to get good highway mileage and still have some "pull" off the line. Some say a 3.08 with a 700R4 is too weak and will make the rpm's in OD too low, outside of the range a 350 likes.

If you don't have an OD transmission, you'll want a set of 3.08 gears. You'll still get decent pull off the line, but will also get fairly good fuel economy. If you tow with this set-up, you may want to keep the 3.73.


Essentially, a higher rear end is better for fuel economy but loses "off the line" performance (but if you go too high, fuel mileage will actually suffer.) A lower rear end will give you better performance off the line but will run higher rmps on the highway and get less fuel economy than higher gears. The trick is to pick something in the middle of what you want.

From experience, a 3.73 is great with a 700R4 and I've heard a 3.08 is good choice for a non OD transmission.

[michael bustamante]

did our truks ever come with a 3.08 rear end? I`woldnt mind running one of those. that would really help out in gas.

[67ChevyRedneck]

Yes, they did.

[68Stepbed]

Originally Posted by rick0312

I've bee reading alot and have learned alot frorm the board about gear ratios...I think. I am still confused a bit, but I think i am getting it. Is this explanation correct?

1. If the gear number is larger (for example. 3.73 vs 3.08) 3.73 is a larger number, but the diamater of the gear is smaller than the 3.08, so the 3.73 takes more revolutions to turn the axle one time which equals high rpms at highway speeds, but more torque off the line?

Am I getting close or am i totally wrong.....Thanks .... Rick

OK, Rick. The actual diameter of the gear isn't bigger or smaller. It's the number of teeth on the ring gear according to the teeth on the pinion gear. For example, a GM 12 bolt truck rearend with a 3.73 rear gear will have these measurements: 8 3/4" dia. ring gear with 41 teeth, and the pinion gear has 11 teeth. You get your final drive ratio by dividng the ring gear teeth by the pinion teeth which would look like this: 41 / 11= 3.727 (3.73 rounded out).

A 12 bolt truck with a 3.07 gear will have a different amount of teeth but the ring gear is still 8 3/4" in dia.

A gear ratio will read like this: 3.73:1. This means the pinion gear turns 3.73 revolutions per 1 revolution of the ring gear. A 3.07:1 ratio means the pinion gear turns 3.07 revolutions per 1 revolution of the ring gear. The quicker the ring gear revolves in accordance to the pinion gear the quicker the vehicle can accelerate, but it takes more rpms the faster it turns.

That's all I can type now before I actually confuse you or myself. I'll chime in later with more info.

[Ackattack]

Originally Posted by 67ChevyRedneck

If you have a 700R4 you'll want a 3.42 or 3.73 to get good highway mileage and still have some "pull" off the line. Some say a 3.08 with a 700R4 is too weak and will make the rpm's in OD too low, outside of the range a 350 likes.

I switched from 3.08s to 3.73s after I put in my 700R4. My mileage actually increased cause I was too low in the RPMs with the 3.08s.


Of course you could run similar rpms with 4.11s and a T56 (50% OD in 6th)

[rick0312]

here's what i am gonna run
350/290hp SBC
3 speed transmission saginaw no groove 2.851st 1.68 2nd and 1.00 3rd
Tires I currently have 31x10.5 R15LT's
Might go a little smaller on the tire,but not by much.
Any suggestions on a rear gear ratio that will get me decent power off the line and accepatable highway cruising....thanks

[davidh]

3.42 or 3.55 with a 30x9.5 15's would be good.

[sick69]

Don't forget in the equation is tire size.

Running a tall tire with a higher gear ratio(like 3.08s) will kill the bottom end torque,,,,but running a tall tire with a low gear ratio(3.73 or 4.11) will kill the highway milage.


Personally I would be inclined to look at a 30" or 31" tire with 3.08 gear ratio for a good balance. If there were any towing or hauling involved I would look at the same size tire and a 3.73 gear ratio.

Just my .02 though


Frank

[68Stepbed]

Rick, I would go with a 3.42 gear. If your truck was originally a 3spd truck it should have 3.73 gears in it now. You can buy a 3.42 R&P set that is machined to fit your 4-series carrier. If you don't plan to run a tire less than 30" tall, I wouldn't go with a 3.08 unless you just want better mileage.

The only thing that will hurt your performance now will be the 3spd tranny. It's not that it's a bad tranny, but it's the gap from 1st gear to 2nd. The engine has to work harder to catch up to the next gear. A 4spd would be better suited if you could find one and afford it. I'm not recommending a Muncie unless you just want one. A Saginaw 4spd would work great in your truck and they're easier to find the right length shifter for. The 3spd should be fine if you just want a dependable tranny and you want to play around a little here and there.

[rick0312]

thanks guys.great info!

[67_C-30]

Originally Posted by 68Stepbed

Rick, I would go with a 3.42 gear. If your truck was originally a 3spd truck it should have 3.73 gears in it now. You can buy a 3.42 R&P set that is machined to fit your 4-series carrier. If you don't plan to run a tire less than 30" tall, I wouldn't go with a 3.08 unless you just want better mileage.

The only thing that will hurt your performance now will be the 3spd tranny. It's not that it's a bad tranny, but it's the gap from 1st gear to 2nd. The engine has to work harder to catch up to the next gear. A 4spd would be better suited if you could find one and afford it. I'm not recommending a Muncie unless you just want one. A Saginaw 4spd would work great in your truck and they're easier to find the right length shifter for. The 3spd should be fine if you just want a dependable tranny and you want to play around a little here and there.

+1

[corn]

3:42 is the perfect all around ratio...

[dave3156]

If you are going with a Saginaw 4 speed make sure you get one with the correct gear ratio for your set up. They are not all the same. They were made in 4 different gear ratios. You can tell the ratio by how many lines are on the input shaft. They have no lines, 1 line, 2 line or 3 line. I don't have the ratios handy but I have seen other posts on here that have the ratios. The transmission and rearend ratios need to be matched up to get the desired results.