ratio gear problems

[lamarrana32]

Hi, i have chevy pick up 69 c10, with motor 350, and transmission 700r4 (originally has manual transmission), the gear ratio is 3.071, but the max. speed is 55 MPH at 3200 RPM with 4 th transmission shift and waste a lot fuel, im thinking the problem is the gear ratio and i want to chage for a chevy 1985 gear ratio 2.56 (43-14),

This modification improve the speed in the road? what do tou think?

thank you for your help.

regards.

[mr48chev]

What size of rear tire are you running?

I went back and ran the numbers through this site http://www.csgnetwork.com/multirpmcalc.html and your numbers don't jive

Unless you have extremely small tires or an extremely low rear end ratio that rpm in .70 od is going to have you way up over 100 mph.

My calculations show that with 3.071 gears and a 27 inch tall tire that truck would be dong 120 mph at 3200 rpm with the 700 r4 in 4th or od. At least in theory or on paper.



Most of these trucks only get between 10 and 16 mpg no matter what you try to do with them with the stock carburetored engines.

[Longhorn Man]

something is way wrong.

[lamarrana32]

the wheels are 27 inch. i do not, but im very confuse, i do not think, that problem was the transmission, because i had 350 with 3rd. shift transmission in the same truck and was the same problem the truck feels as if need another shift. and i take the desition to change the transmission for a 700r4 and do not improve nothing. what gear ratio uses this truck originally c10 1969 manual transmission? what happening?

[bosco200]

My top speed in my C20 is 55mph...but I have 4.57 in the rear end...

[Shyguy]

My '71 runs about 2250 rpms when motoring down the road at 55, it has an old truck 4 speed with 3.73 gears and 31 inch rear tires.

So I figure something is amiss in your situation.

[lamarrana32]

and if i change the rear differential gears for a chevy 1985 2.56 gear ratio, do you think improve the speed?

[68gmsee]

I agree with the rear end ratio probably being the problem. Check it carefully and if the truck used to be a six in some previous life then your gearing could be off.

I had a manual trans six on my 69 and when I swapped a 350 with th350 tranny in, it did the same. It would pull stumps but the road driving was outrageous. To solve the problem I took the rear end off a v8 truck with 3.08 axle and it was much better highway driving.

[treveiger]

My truck is a C20 with a 4.56 rear end and bigger tires(not sure how big)and i walked it well over 90 the day i got it and was driving it home my dad was driving the prius and was wondering what the hell i was doing lol.

[Longhorn Man]

a 3.07 gear with no overdrive will go well over 100 MPH. If memory serves, 100 MPH comes in at just over 3000 RPM, PLENTY left over before redline.
If you had a 3 speed automatic, and converted to a 4 speed automatic, and your cruise RPMs are the same, then you are not going into overdrive. Once you do get it working right, you'll probabky find that the RPMs are too low and you'll be getting poor MPG.

[gonzo6869]

Originally Posted by Longhorn Man

a 3.07 gear with no overdrive will go well over 100 MPH. If memory serves, 100 MPH comes in at just over 3000 RPM, PLENTY left over before redline.
If you had a 3 speed automatic, and converted to a 4 speed automatic, and your cruise RPMs are the same, then you are not going into overdrive. Once you do get it working right, you'll probabky find that the RPMs are too low and you'll be getting poor MPG.

X2. Its not your rear end ratio, check your tranny, its not going into OD or is slipping A LOT.

[gonzo6869]

Originally Posted by 68gmsee

I agree with the rear end ratio probably being the problem. Check it carefully and if the truck used to be a six in some previous life then your gearing could be off.

I had a manual trans six on my 69 and when I swapped a 350 with th350 tranny in, it did the same. It would pull stumps but the road driving was outrageous. To solve the problem I took the rear end off a v8 truck with 3.08 axle and it was much better highway driving.

I agree but disagree. My truck used to have a I-6 and was converted to a V-8 it was always a auto (th350) with a rear end ration of 3.73, and it is not a truck to be driven on the highway (with 275/60/15 tires [with a th350 and the 3.73 @60MPH I am @2900RPM). But even if he still had a low rear end ratio, with the size of the tire and the OD he should be able to get a whole lot more out of the truck. Something is wrong with the Transmission. I also think if you mix a highway gear, like a 3.08 with an OD Tranny, you'll not be happy with it ( RPM's will be way too low in city traffic). Check your trans and make sure everything is fine with it.

[68GMCCustom]

combination of more then one problem? A rear ratio that isn't what its supposed to be...and an OD thats hangin in 3rd?