trans. swap, gear ratio or lower profile tires?

[67c10flatbed]

I have a 67 C10 that used to have a straight six with a four speed tranny, that was swapped out for a mild 350 and TH 350 trans. Well the long and short of it is that when I am traveling down the freeway at about 65 to 70 the engine sounds like it is about to let go. I dont have a tach so not sure bout the rpms. Not sure about the rearend ratio as well. My question is...would it be better to change the gear ratio on my rearend , swap out my 350 for a 700r4 or to get a lower profile tire since I am running a 31-10.50/ 15? Which would lower my rpm to where it will travel down the freeway without sounding like its on the brink of destruction?

[EyeH8the24]

wouldnt a lower profile tire increse rpms at highway speed?a taller tire would lower rpms

[CPNEJR]

[67c10flatbed]

I am trying to lower my rpms and yes a lower profile tire would raise them I didnt think about it til just now. I plan on putting a 2/4 drop kit and bed on it so I will end up putting a lower profile tire on it anyways. I just dont like how much low end torque this truck has. I break the 31s lose on a dry road easy.

[WorkinLonghorn]

My tires are 31" and I have a 350 motor,but with a manual trans.At 60 MPH I'm doing a comfortable 2500 RPMs with 3.73 rear. At 70 I am still below 3000,and not sounding too bad. I would guess that you have 4.11 or even 4.57 in back (you should check and see).Maybe swapping in a 3.07 from the junkyard would be the ticket.That's what the 350/350 came standard with I believe.

[67c10flatbed]

I really dont feel comfortable on pulling the diff cover. I found what the diff codes mean I just cant find the code on my truck to define them and figure out what ratio i have

[LMN8UR]

I think you need to get a tach! Then bring your rear gear ratio down.

Do you have a posi rear? If not, jack one wheel off the ground, mark the driveshaft and diff with a piece of chalk, while watching your drive shaft, now slowly turn your rear wheel slowly 1 full revolution and count the number of times the DS turns. You can get a real close guess on what your gears are.

3.08 A little more than 3 revs
3.43 Not quite 3 1/2 revs
3.73 About 3 3/4 revs
etc. . .

Yeah, don't forget to do this on level ground with your other three wheels chalked and the tranny in N. BE CAREFUL!

If it will help you, and you have Excel, I can send you a little spread sheet I made that will calculate RPM with different gears and tranny's. It also has an index of different tranny ratios. Email me at lmn8ur@gmail.com and I will send it to you.

[WorkinLonghorn]

Do you have a posi rear? If not, jack one wheel off the ground, mark the driveshaft and diff with a piece of chalk, while watching your drive shaft, now slowly turn your rear wheel slowly 1 full revolution and count the number of times the DS turns. You can get a real close guess on what your gears are.

You have to multiply by 2 the shaft revs.

[dave3156]

Depending on what you want your truck to do, you may not be happy with 307 gears in the rear. I switched to 373 gears from 307 to get off the line better. With the 373 gears it still runs fine on the interstate. At 65 mph your at about 2800 rpm.

[68gmsee]

I had a 6 on my 69 and installed a 350 with 350TH. Had the same problem you did-high rpms. I swapped the rear end with one from a 70 automatic with a 308.

Since I wasn't interested in drag racing, this has worked well for me. I've used it as a daily driver and used it for towing a trailer. Once with a full sized tractor and brush mower on it with no problems.

[67c10flatbed]

OK I found the code on my drive shaft and according to GMC Pauls I have 3.73 gears in it. Now what gear should I go with?