Gear ratio/whell size?

seamonster · January 27, 2005

I was on another site and someone suggested that the

bigger 17 inch wheels make up for the 3:42 they have

now instead of the3:73 you cant get!

Can someone explain this?

I thought 3:42 revolutions of drive shaft = one axle rotation.

If the axle has bigger wheels doesnt if cover more "ground"

meaning you go further which to me would= less torque.

Can someone help explain it to me???

mjgjr72 · January 27, 2005

the rim size dosen't make a difference, it depends on the overall diameter of the tire used, the only difference it would make is the weight of the tire and rim combination

AG Bill 05 Z71 · January 27, 2005

Unfortunately, the folks on the other site have their math backwards... Everything being equal, when you increase the diameter of your tires, you want to increase the differential ratio numerically.

I can give you a real life example: My 2005 GMC Z71 has a 4.10 rear end ratio, that runs at 2150 RPM at 70 MPH. If I had purchased the same truck with the 3.42 rear end ratio the RPM at 70 MPH could be calculated as follows:

3.42/4.10 = 0.83, or the RPM would be 83% of the RPM for the 4.10

For the 3.42, the RPM at 70 MPH would then be 0.83(2150) = 1800 RPM or

350 RPM less than the 4.10 at the same speed.

Since engine torque typically rises as the RPMs go up (my L33 engine is rated at

335 ft-lbs of torque maximum at 4000 RPM) the available torque at 70 MPH would be less for the 3.42 than the 4.10. This is why the 4.10 is recommended over the 3.42 if you are going to do any significant towing with your truck.

You were right to question their logic on this issue...

04-silver · January 27, 2005

Unfortunately, the folks on the other site have their math backwards... Everything being equal, when you increase the diameter of your tires, you want to increase the differential ratio numerically.

I can give you a real life example: My 2005 GMC Z71 has a 4.10 rear end ratio, that runs at 2150 RPM at 70 MPH. If I had purchased the same truck with the 3.42 rear end ratio the RPM at 70 MPH could be calculated as follows:

3.42/4.10 = 0.83, or the RPM would be 83% of the RPM for the 4.10

For the 3.42, the RPM at 70 MPH would then be 0.83(2150) = 1800 RPM or

350 RPM less than the 4.10 at the same speed.

Since engine torque typically rises as the RPMs go up (my L33 engine is rated at

335 ft-lbs of torque maximum at 4000 RPM) the available torque at 70 MPH would be less for the 3.42 than the 4.10. This is why the 4.10 is recommended over the 3.42 if you are going to do any significant towing with your truck.

You were right to question their logic on this issue...

Good explanation, I have an 04 with 4.10. Now actually I understand why they say 4.10 is better for pulling.

Thanks

John

ROMAD · January 27, 2005

Well, since you guys were so helpful with his question...here is mine.

I have a 99 rado with a 7inch lift(2WD) and the stock rear in it still. I know that I have lost power with the 315/75R16s I put on it, and am probably suffering mileage loss too. Q1: How do I find out what size gear I have? Q2: What would be the best size and make of differential to replace it with?(some towing and mild off-road horseplay) Also, on the dyno it says I am running 418hp.....I need something that will handle my lead and angry foot...

AG Bill 05 Z71 · January 28, 2005

Here is the answer to question number one on rear end ratio...

These are the RPO codes for 1999 trucks:

GT4 3.73:1 Rear Axle Ratio

GT5 4.10:1 Rear Axle Ratio

GU4 3.08:1 Rear Axle Ratio

GU6 3.42:1 Rear Axle Ratio

Check the list of codes on the inside of your glove box door for one of the above codes.

Jacqson · January 28, 2005

Well, since you guys were so helpful with his question...here is mine.

I have a 99 rado with a 7inch lift(2WD) and the stock rear in it still. I know that I have lost power with the 315/75R16s I put on it, and am probably suffering mileage loss too. Q1: How do I find out what size gear I have? Q2: What would be the best size and make of differential to replace it with?(some towing and mild off-road horseplay) Also, on the dyno it says I am running 418hp.....I need something that will handle my lead and angry foot...

<{POST_SNAPBACK}>

Hey ROMAD, I left you an answer to your other question in the Suspension section.I hope it helps

ROMAD · January 30, 2005

well, I have a 3.42 gear and a G80...I assume this is the locker?

Ben · January 30, 2005

Well, since you guys were so helpful with his question...here is mine.

I have a 99 rado with a 7inch lift(2WD) and the stock rear in it still. I know that I have lost power with the 315/75R16s I put on it, and am probably suffering mileage loss too. Q1: How do I find out what size gear I have? Q2: What would be the best size and make of differential to replace it with?(some towing and mild off-road horseplay) Also, on the dyno it says I am running 418hp.....I need something that will handle my lead and angry foot...

<{POST_SNAPBACK}>

If the 7 inch lift was a suspension lift, that reduced the power delivered to the pavemetn. The increased U-Jont angle does that.

Stock tire dia for most GM full sized trucks are 30 inch dia.

Any change in tire dia changes the "effective" diff ratio by the same percentage difference. An inverse function. Meaning larger dia tire will make the diff ratio "effectively" lower numeric (lesser power).

Really rev's per mile, as tire dia is not an accurate measurement. Same dia tires from different manufacturers can have different rev's per mile.

That difference in dia/rev's per mile will affect the diff ratio with the same percentage. A 10% taller tire will change the diff ratio to be effectively 10% lesser. A 4.1 will effectively become a 3.7

The gear set is still whatever and never changes, as gears meshing is gears meshing. BUT, it's "effective" ratio changes. Same drive shaft to axle revs, but the vehicle has moved farther with larger dia tires.

One tire rotation of a larger dia tire will move the vehicle farther than a shorter tire's single rotation.

Higher numeric diff ratios has higher power multiplication. 3.73 ratio has the drive shaft turn 3.73 times to each tire rotation. Meaning 3.73 times the drive shaft power at the axle. 4.1 is 4.1 times the drive shaft power at the axle.

By going to very large dia, there is less power mulitiplication at the axle, so less power. If lifted, even less power delivered.

SteveSierra · January 30, 2005

Unfortunately, the folks on the other site have their math backwards... Everything being equal, when you increase the diameter of your tires, you want to increase the differential ratio numerically.

I can give you a real life example: My 2005 GMC Z71 has a 4.10 rear end ratio, that runs at 2150 RPM at 70 MPH. If I had purchased the same truck with the 3.42 rear end ratio the RPM at 70 MPH could be calculated as follows:

3.42/4.10 = 0.83, or the RPM would be 83% of the RPM for the 4.10

For the 3.42, the RPM at 70 MPH would then be 0.83(2150) = 1800 RPM or

350 RPM less than the 4.10 at the same speed.

Since engine torque typically rises as the RPMs go up (my L33 engine is rated at

335 ft-lbs of torque maximum at 4000 RPM) the available torque at 70 MPH would be less for the 3.42 than the 4.10. This is why the 4.10 is recommended over the 3.42 if you are going to do any significant towing with your truck.

You were right to question their logic on this issue...

<{POST_SNAPBACK}>

I'm not doubting you in the least, but how did you manage to get 4.10's on an 05 Z71 Sierra? I went to their website and the only option is 3.42's? Not that I want 4.10's, I had them in my 01 Z71 and didn't care for them, but just curious.

I know on Edmunds' site, you can spec out an 05 GMC with them, but GMC's site doesn't allow it.

Err, nevermind. I just checked on specs on the GMC site. I didn't know the difference between the 3.42s and 4.10s is only 1000 lbs towing. And the same capacities hauling.