Changing rear end gears.

[SuperBuickGuy]

what size tire?

I have 4:10s in my truck, and love them because it's not like I'm setting land speed records with it; and for pulling a hill (loaded) the gear ratio is perfect for staying at 60 over most highway grades. Of course I also have approximately a 33" tire.

[FoFittyFoSS]

My tires are 235/75 r15.

[TC]

This is what I'm talking about how race gears don't hold up to street driving.......

[SuperBuickGuy]

The best way to figure gear ratio is to do math. Your tire is roughly 28" in diameter.
You don't have to pull the cover to figure out the gear ratio. There are a couple methods to figure ratio - 1st, pick the rear tires off the ground (presuming open diff), put the transmission in neutral, and rotate a tire once counting the revolutions of the driveshaft... of course, that doesn't always work; If you have a tachometer - drive put your truck in "3", not overdrive, drive on the highway and observe the rpms. Without doing the math - 2.73 gears will be about 1600 at 60 mph, 3.08 is roughly 1900, 3.73 would be 2800 rpm.

In my experience, the best fuel economy with a carb'd motor driving mostly highway is (again, presuming stock electronic q-jet), 3.08 or 2.73. However, if you pull a trailer, carry large loads, do a lot of stop and go driving 3.73 is probably a better choice - and with overdrive you may not see any drop in mpg. If you mostly tow 4.10 is the better ratio. There are other benefits of the higher numeric gear is you have better accelleration, and you save your 700r4 (they have weak overdrive clutches - meaning they burn up under load without gear swap).

Anyway, back to math. Find the target mph you'll drive - in other words, city or highway - do the math to find what RPM you want to drive at (to a point, the lowest the rpm that you can drive at without moving the throttle for most of your driving). Figure out where you motor develops its power - then match the gear ratio to that "best" rpm.

A couple calculators
gear ratio calculators
http://www.rocky-road.com/calculator.html

free desktop dyno software

http://www.virtualengine2000.com/Products.htm

[squirrel]

This is what I'm talking about how race gears don't hold up to street driving.......

I've seen that happen on several sets of OEM GM gears. I think race gears usually are softer so they won't fatigue crack like that, but they will wear faster on the surfaces of the teeth.

[A/Fuel]

Back when a lot of cars I worked on had the old 9 1/2" rear, the pinion would crack just like that. Every week I was putting togather a new center section. The race gears have a higher nickel content that makes them softer, and in the catalogs they are usually labled. Driving them on the street for a long perriod of time will cause them to get hot and the lash will change. Lots of gear manufactures make street gears which hold up fine.

[TC]

I've seen that happen on several sets of OEM GM gears. I think race gears usually are softer so they won't fatigue crack like that, but they will wear faster on the surfaces of the teeth.

The pinion there is a Moser unit which is made by US Gear and they are race gears....... I think the difference is the OEM gears that do that have 10's of thousands of miles on them, where these Moser gears only made it 10-15 thousand miles......

[TC]

Back when a lot of cars I worked on had the old 9 1/2" rear, the pinion would crack just like that. Every week I was putting togather a new center section. The race gears have a higher nickel content that makes them softer, and in the catalogs they are usually labled. Driving them on the street for a long perriod of time will cause them to get hot and the lash will change. Lots of gear manufactures make street gears which hold up fine.

The thing that got me was that for the same gear ratio(3.73) the GM gears were physically bigger in size than the Moser gears where...... Put it this way the guy at the shop that redid the rear actually called the dealer to make sure they sent him the right gears because the size difference was so extreme.........