Rearend Gears?

[rmccloye]

I have a 1965 chevy C10 with a 2-barreled 350 and 3spd manual tranny and 30.5in tires, what gear ratio should I use in the rear end for highway use? I want to be able to cruise at around 70-75...

[markeb01]

Per the attached link,

http://www.csgnetwork.com/multirpmcalc.html

using the "Transmission Final Gear Ratio Manual Entry" - the results show:
With a 1-1 high gear, 30.5" tires, and 70 mph, the following gear ratios would result in the engine rpm shown:
3.70 = 2853
3.55 = 2738
3.23 = 2491
3.08 = 2375

It's simple to change the speed to 75 mph, or the tire size, etc and recompute.

It's then a matter of matching the torque curve and power band of your engine to the speed you will be traveling most often. Low engine speed usually means better mileage, unless it is running in an rpm range where it doesn't make torque, which will really kill mileage.

There are dozens of gear and speed calculators available via Google if you want to compare results to a different calculation system.

[TwoFiftyShifter]

Go with 3.08 for highway use. I have 3.73's and at 70 I'm humming a little too high.

[truckdrivinsob]

I'm gonna run 3.07s .

[rmccloye]

Thanks, I'm gonna go with the 3.08 gears, whenever I get another engine... Blew my other 350 up trying to run 75 in the middle of the summer.

[rmccloye]

Yeah I'm gonna go with the 3.08 gears, whenever I get another 350. Blew up the other one tryin to run 75-80mph during the summer...

[blackedoutharley]

I have the same size tire with an SM420 trans and after having 3.73's (real fun around town), 3.08's (rocked on the open freeway) I ended up settling for 3.42's and couldn't be happier!

Best of both worlds.

[lakeroadster]

Here is a chart showing ratio', tire size and rpm's: http://www.sierrabronco.com/TireGearRatio.htm

I don't think you will be happy with 3.08's, it will be a real dog off the line and you will have to slip the clutch to get rolling.

Use blackedoutharley's hard work and cash and real world experience to your advantage and go with the 3.42's, you'll be glad you did!

[Clyde65]

I 2nd Lakeroadster!

[onequickchevy]

I'd go with the 4:11 gears and a 4200 stall like me. It's lots of fun. Oh wait, your cruising....nevermind

[raycow]

I have run a 3.07 with a 3 speed and liked it just fine. This was a factory combination you could order from GM, and GM didn't like to sell trucks that were going to generate buyer complaints.

"Dog off the line" is solely in the mind of whoever is doing the driving.

Ray

[lakeroadster]

Originally Posted by raycow

"Dog off the line" is solely in the mind of whoever is doing the driving.

Ray

What rear gear did you have in your factory 307 3-speed truck? That info would be helpful to the original poster as that was his question?

Your statement about "Dog off the line is solely in the mind" couldn't be more untrue. It is a simply matter of mathematics. Any street vehicle with a numerically low rear end ratio and a clutch will be difficult to get rolling without slipping the clutch a significant amount. And with a truck this is a huge deal if you plan to haul or pull anything 'cause the clutch slipping will get worse, not better. That is why our trucks were available with a 4-speed, really just a 3-speed with a granny gear 1st gear to get the truck rolling.

It's all about overall driveability. A 3.08 gear will be miserable around town with a 3 speed manual tranny.

[raycow]

Originally Posted by lakeroadster

What rear gear did you have in your factory 307 3-speed truck? That info would be helpful to the original poster as that was his question?

3.07, just like I said. Some people call it a 3.08, but on the 12 bolt it really is 3.07

Oh and the 4 speed trucks I have had, I always start off in 2nd gear, unless I am headed uphill, towing a trailer. Now I have to agree, that's the one place you probably wouldn't want a 3 speed.

Ray

[TwoFiftyShifter]

But with the 4 speed manual tranny, the 3.08 woulf be a great combo. You wont win any races, but you can take off in 1st from a dead stop or uphill or when carrying a load and still run about 2k rpm at 60mph.

[az65]

Originally Posted by Clyde65

I 2nd Lakeroadster!

me too, i have a very similar setup with my 327 powered truck. I run 3.42's and think it is the perfect ratio for my driving.

[TagMan]

I decided to go a different route. My '63 has a 3.73 rear end and is WAY too busy at 70mph.

I bought an A833 3-speed/ OD transmission out of a 1982 Checy 1/2T and will install it this winter. That'll drop the RPMs down to around 2200 @ 60mph and still have a decent 1st gear for city driving.

[blackedoutharley]

Originally Posted by lakeroadster

Use blackedoutharley's hard work and cash and real world experience to your advantage and go with the 3.42's, you'll be glad you did!

It was an expensive adventure too! But I never have to worry about the rearend ever again (other than routine oil related maintenance).

Originally Posted by TagMan

I decided to go a different route. My '63 has a 3.73 rear end and is WAY too busy at 70mph.

I bought an A833 3-speed/ OD transmission out of a 1982 Checy 1/2T and will install it this winter. That'll drop the RPMs down to around 2200 @ 60mph and still have a decent 1st gear for city driving.

Don't put a great deal of power or abuse through that tranny!

Although it is an "833", the venerable trans from Chrysler during the muscle car era it is not exactly the same. To give you an idea of what else these were installed in (the OD version) my dad had one that was factory installed in a late '70's Volare wagon behind a Slant Six.

To others: while it may be true that GM offered the 3.07/3.08 gears in trucks with a 3spd trans I can tell you that there weren't any tire mfg's making a radial tire that was nearly 31" tall and weighed 45 lbs back then.

Gear ratio (rear end and trans) and tire diameter can play havoc on a clutch assy when they are not matched. Don't forget that flywheel weight can also cause issues as well.
That 15lb flywheel will certainly make your new engine rev to the moon in a split second but the 40 lb flywheel is going to make it pull much better.

YMMV

[rmccloye]

btw, looking to go back with the original 60s chevy look, so does anyone know if any place that may sell bias ply 7.00-15 or L78-15 tires other than coker, universal tire, etc. And anyone know if any town co-op would still carry these size tires, other than "mudders"...