rear gears

[gafiredog]

does anyone have an idea what the gear ratio would be in my 75 c-10, 250-6 cyl. 350 turbo. I can't find a tag on it, or a stamp in the housing. maybe I'm not looking in the right place, Anyway, It sounds like the rods are gonna come out at about 60 mph., and the best it will do on mileage is about 11-12 mpg.

[mooseknuckles]

thats prob normal mileage!...lol......not 100% but I thought the 6cyl's had 3:73's?....find a 700r4 and some 3:42's...what size are your tires?

[gafiredog]

235/75/15

[mooseknuckles]

Originally Posted by gafiredog

235/75/15

think thats like a 29" tire, so you prob cant go much bigger...

[wilkin250r]

When you say "It sounds like the rods are gonna come out at about 60 mph", I'm assuming you mean it's revving really high? Sounds like it's probably 4.10 gears then.

A lot of us are getting 11-12mpg, but we're running bigger engines. I would think a 6-cylinder would get better than that, and the gears might be the culprit.

[Keith Seymore]

Originally Posted by gafiredog

does anyone have an idea what the gear ratio would be in my 75 c-10, 250-6 cyl. 350 turbo. I can't find a tag on it, or a stamp in the housing. maybe I'm not looking in the right place, Anyway, It sounds like the rods are gonna come out at about 60 mph., and the best it will do on mileage is about 11-12 mpg.

Here's the info packet for a '75 Chevy pickup as found in the GM Heritage Center archive:

http://www.gmheritagecenter.com/gm-h...olet-Truck.pdf

Per the powertrain chart on page 28 base axle is a 3.73 with an optional 4.11.

Of course, anything could be in there now so the only sure way would be to do the driveshaft/tire trick or to pull the cover and count teeth.

K

[gafiredog]

Thanks for the help guys, I have had people tell me they had the same or close to what mine has, and they put in a 308, or 2 something gears, and mileage improved to around 17-20 mpg. this sounds like they are full of s^%t, or is that possible to gain that much mpg?

[LONGHAIR]

Originally Posted by gafiredog

I have had people tell me they had the same or close to what mine has, and they put in a 308, or 2 something gears, and mileage improved to around 17-20 mpg. this sounds like they are full of s^%t, or is that possible to gain that much mpg?

Gearing has a lot to do with the intended use of the truck. If you are a highway driver that sees the majority of driving at 75+ MPH, a higher gear will help you out. But....in the city, it will be a complete dud. Not only sluggish to drive, but mileage will suffer because it requires more throttle to move it around.
Those high gears (308, 273, etc) also are not so friendly to carburetors. Running down the road at too low of an RPM gets them into all kinds of metering issues. Injection handles this better, but it is still sub-par. You really need to be geared to hit your engine's "sweet spot" and your most used speed. That ideal RPM is Dependant on the engine's size compared to the truck, the cam, etc....