Gear ratios

[Ziegelsteinfaust]

I am swapping rear ends in my truck going from 4.88 with a Detroit locker to a 3.08 open rear end.

The truck is a 73 C10 with a gen2 LT1 with a carb and full bolt ons. The transmission is a th700r4, and 31" street tires.

I am worried I may be going to far. Since I wanted 3.42, but they are rare to find on the budget I have to play with. Since I won't be able to do anything in the rear end for a year or switch tires. Partially to lower it further.

How do you expect it to be as a driver. Or am I just over thinking it.

[Keith Seymore]

I think that's way too far.

I do 3.73 with an overdrive trans and P235/75R15 tires. 3.42 with a non overdrive trans.

The combination of the overdrive, large tires and 3.08 is going to be a total dog, IMHO.

But - since you are locked in to the ratio and tires, may as well try it for a while and see if it works for you.

K

[bnorth]

I agree with the above. Since you are running big tires and already have a locker for the 4.xx+ gear ratio what if you just swapped to 4.10's in your existing diff? That shouldn't break the bank especially if you can sell the 3.08 diff and then you get to keep your locker as well.

[Ziegelsteinfaust]

Originally Posted by bnorth

I agree with the above. Since you are running big tires and already have a locker for the 4.xx+ gear ratio what if you just swapped to 4.10's in your existing diff? That shouldn't break the bank especially if you can sell the 3.08 diff and then you get to keep your locker as well.

I scored a G10 rear-end for $100 which also gets rid of the spacers.

My Dad who I am reinventing the truck for is 82, and in the beginning stages of Parkinsons. Plus when you back up or turn to tightly the truck slides sideways. Which he already hates.

Although it was great when I had to drag a stuck project car 200 foot in the dirt/sand of the high desert.

Tied into the fact to redo what I have is well over $1000 I don't have as it is.

I am mostly worried he will fall out of his van.

[AussieinNC]

Here is a link to an online calculator, using trans type, rear gear ratio and tire height...makes it simple

https://www.blocklayer.com/rpm-gear

Using your 3.08 gears, I get around 1590 rpm and 70 mph.....engine better have some deep torque curve for this to work ok....

Leave the 4.11 gears in it for a decent driver....

[Ziegelsteinfaust]

Update.

I just did a 1000 mile trip with 31" tires, 2500 rpm stall converter, and close to 800 pounds in the bed. Along with a roof top cargo carrier mounted above the bed height. While I had a head or heavy cross wind nearly every time I hit the highway. It was still better on mpg then the 4.88's did.

The only downside was the converter. As it flashes to high for that kind of load.

Today I had to put new tires on since my 285/65r17 were to wide for the new rear axle. 235/60r16 front and 255/60r17 rear. They stretched perfectly to my rims, and look better then I hoped.

So the rear tires are considerably lighter then the old ones, and measured height is 28.5 inches. The truck now feels more then fast enough empty for a DD, and well it did handle the old tires with weight good enough. Atleast for a temporary fix.

Unfortunately my fuel pump died this morning, and I only drove with the new tires for 10 miles.

Will do another update when I fix the pump this weekend.

[Ziegelsteinfaust]

New update.

After I swapped in the new motor I have driven it for about 150 miles now. It has plenty of power for normal driving, and doesn't feel stretched to accelerate the truck. Even with a engine in back. Also I know have a 1800 rpm stall converter vs the old 2500 version.

Old motor 10-1 gen2 LT1 with a Corvette cam, 1.6 roller rockers, and 1992 vintage AFR 195 heads. That I believe are the cnc version.

New motor. 10-1 gen2 LT1 with a B-body cam, factory steel 1.5 rockers, and factory aluminum heads. This was a Caprice engine I got with a carb intake for a price I could not pass up. So all I did was regasket the engine, and use the factory aluminum heads instead of B body iron heads. For safety with california gas as I run a quadrajet. Honestly I believe below 3000 rpm at part throttle to accelerate. This engine puts out atleast 25lbft more torque below 2500 rpm. After that it just doesn't carry, but who cares. This is a driver for my dad.

So if your building a similar truck and engine. 3.08's are fine if you want a driver or minor work horse.