The Stovebolt Site Forums The Shop Area Engine Shop th350/ rear end info please

hey guys i dont know much about rear ends and transmissions but i have got to save some money on my gas bill!

my current setup is the drive line from a 79 caprice. 305v8 (supposedly has a R.V. cam), th350, and 10bolt chevy 2.56:1 rear end (thats the specs from a website)

now what concerns me is that i do a lot of highway driving, 35 miles a day, and the th350 shifts into 3rd at 40-45mph... i dont have a tach but im not comfortable driving any over 60mph... just feels like im dogging it out.

any suggestions? different gears? an over drive? i want this truck fun and quick but also comfortable on the highway.

the 2:56 should be good for the hiway.. why cant you drive it over 60.......the shift points dont seem that bad off but it has been a long time since I drove a 350 auto...........

I can drive it over 60, hell ive had it over 80, it just seems to be running too high rpms, dont want to beat the crap out of it. maybe im just being paranoid? this is my first auto trans in many years perhaps im too used to being in control of 4+ gears manually???

Are you sure it's a 2.56 then?
Have you jacked up the rear end and counted wheel rotations to drveshaft rotations or counted the teeth?
Good Luck.

colin... no not sure. can you further explain ways to determine what ive got. thanks

With a 2.56 rear end you should be loafing down the highway. Pull the cover off the rear end & get the numbers off the edge of the ring gear. It will have something like 36-9. Divide the large no. by the small no. & you have the ratio. 2.56 would be an ideal ratio in that combo for highway use. A 305 won,t hardly pull anymore than that.

I have a collection of pieces from a '78 Monte Carlo that are similar to what you describe. 305, 350 turbo and 2.29 rear gears. This setup had 70 series series tires on 14 inch wheels and when I drove it, before taking it apart, it seemed to work reasonably well.

What tires are you running? Your effective ratio is taking tires into account may be so high that your motor is lugging going down the highway at 60. Find one of the ratio calculators and check it out.

You don't need an overdrive to make this truck race down the highway. If it won't do it now you either need more power or lower gears, not higher.

good point old sub. im running stock size 63 chevy truck wheels/tires. 15inch 7wide... forget the heigth... same as the front. I assume the rear end is the stock from the 79 caprice because the rear drum brakes are as well, had to play the guessing game on searching for replacement brake cylinders). after i insulated the cab with peel & seal and double foil bubble wrap its much quiter now and not seeming so bad. the part im still a little concerned about is shifting into 3rd at 35-45mph... is that ok or normal? if not how do I go about changing w/o a shift kit? somethign with the vacuum to the trans??? planning on a new tach install this weekend and that should show me whats really going on and put my mind at ease hopefully.


thanks for the info guys

With big enough tires and a true 2.56 gear ratio, 35 to 40 may be right. Are you using the kick down cable on the turbo 350? If you are try unhooking it, it won't hurt a thing, just won't down shift. But if the cable is hanging up, it fool the trans into shifting late. It should be shifting sooner when driving very easy, get that tachometer hooked up and let us know some rpm shift points and cruize rpms. Joe

kick down cable is not hooked. would hooking it up help anything? i dont know much about the function of them suckers

The cable on a 350 is for kickdown (passing gear). The vacuum modulator controls the shifting. Replacement mods. have a tiny adjustment screw inside the vacuum opening. It doesn,t change a whole lot though. This doesn,t apply to 200-4r or 700-4r. The cable adjustment has to be perfect on these.

Troy-
Sounds like you have a way higher rear gear than a 2.56

My '67 has a Turbo 350 and a 3.73 rear end ration.
At 70 mph I turn about 3100 RPM. Way too high for
today's gas prices.

My plan is to install a 700R4 overdrive which will bring the cruising RPM down to 2100. That is in the range my '95 Silverado had and it had excellent power and mileage.

Here is an excellent link to help you decide on gearing and so forth. It accounts for tire size, tranny and all the variables.

http://www.classictruckshop.com/garage/shopmathgearcombo1.asp

Good Luck!

_________________

Guy

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'67 C-10 283/350

Thanks for the page Guy. I like the extra info on other possible gear ratios, accross the bottm, once youve calculated for your stated combo. - MV

Best one I have seen so far - there are several out there but this one seems to work the best.

You can run through a ton of "what if" scenarios
in just a few minutes. Sure is a lot easier than swapping parts till you get it right!

Love your project - keep us posted on the progress...

______________________

Guy

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'67 C-10 283/350