3.08's or 3.42's

[ruffrida2005]

Ok i'm looking for better highway miles. I thought I had 4.10's and comes to find out they are 3.73's I still get bad highway with the 350/350.

what I want to know is what you guys think about 3.08's with about a 28-29 tall tire. that will put me in the 2400-2700 range with my trans.

Guys with 3.08's comment please.

[62gmc910]

you need to find your engines sweet spot . Find a quiet level road and run the truck up at 1/4 throttle til you hit drive then feather out of the pedal till you find the rev range that everything runs free.
Most late 70s 350 trucks ran 3.08 gear
3.42 was common with 700r4 trucks

[markeb01]

My 1960 GMC came stock with 3.73's. After I installed a 350/350 I switched to 3.08's and it was sweet on the highway. I commuted 72 miles round trip a day in it for many years and could turn up 16+ mpg if I didn't use the back barrels. For my setup it was right in the torque curve. Once I went to a 4 speed manual I went to 4.09 gears and traded mileage for fun. I was running 255/70-15 Radial T/A's with the automatic which list at 29.10" tall.

[ruffrida2005]

Sounds good guys, wish I could drive the truck now but it looks the way my pic does. I might trade Gage my gears for some 3.08's if he wants to.

[Winola73]

Originally Posted by ruffrida2005

Sounds good guys, wish I could drive the truck now but it looks the way my pic does. I might trade Gage my gears for some 3.08's if he wants to.

You'll need to trade carriers too. 3.42 and "higher" (numerically lower) use a different carrier than the 3.73 and lower (numerically higher).

With a TH350, 3.08's and 235/75R15's your RPM at 55 MPH should be right at 2100, maybe a little higher with converter slippage but nowhere near 2400 with 28-29 tall tires. Overall diameter on a 235/75R15 is 28.88 but the actual "rolling" diameter under load is 27.145.

Here's a website with some great calculators for tire diameter, speedo gears, etc.

http://tri-5.chevyrides.com/speedo.php

I have an '86 D150 with a 318, 727 trans, 235/70R15's and 2.94 gears and it turns just about the same. It's nice on the highway but I sure wish I had an extra gear in there down low because it's a real slug around town. Due in no small part I'm sure to being a Dodge....

I'm considering putting a 3.08 in my '66 C10 but with the stock 4 speed. I'll have a 355 built for low and mid-range tq. in front of it and I'll be running 235/75's on it.

[ruffrida2005]

Thanks for the info Winola I knew I had to swap carriers also.

I got on Richmondgears website and at 70 MPH with 3.08's they had me little over 2400RPM

I will keep the 3.08's for now till I can get a set of 3.42's for a little better low end.

[Winola73]

Originally Posted by ruffrida2005

I got on Richmondgears website and at 70 MPH with 3.08's they had me little over 2400RPM

I missed the 70 MPH part...

I wish they made a 3.23 for the truck 12 Bolts.