4.11 gears and 287/75/16

[18436572GMC]

What would rpms be at 70 mph with this gear tire combination and an sm465

[kwmech]

3000 plus, The 285/75 is about 31'' tall. I've got the same tire on my 68 with 3.73s and a big block........I put in an overdrive

[18436572GMC]

Thanks for the response. I am debating a lift and 315/75/16. I figure that would run better with 4.11s and no overdrive

[budman k20]

My Cooper 285-75-16's are 32.7 inches in diameter per the manufacturer specs?

With a turbo 350 70 mph is about 3200rpm per the factory tach should be close since I have a 2800 stall.

A rpm calculator puts the RPM at 3035 for an auto and 2956 for a manual trans based on tire diameter of 32.7"

[18436572GMC]

Originally Posted by budman k20

My Cooper 285-75-16's are 32.7 inches in diameter per the manufacturer specs?

With a turbo 350 70 mph is about 3200rpm per the factory tach should be close since I have a 2800 stall.

A rpm calculator puts the RPM at 3035 for an auto and 2956 for a manual trans based on tire diameter of 32.7"

What is your ring and pinion ratio?

[budman k20]

Sorry,

Factory equipped with 4:11 3/4 ton 4x4

[18436572GMC]

thanks

[18436572GMC]

ttt

[special-K]

Yeah,just under 3 grand is accurate with manual & 4.10s. 285/75-16s run between 32" & 33". 235/85s are closest to original 7.50-16s,which are more like 31".

[Cali72K20]

budman k20
you have about the same set up as mine:
TH350 / 4:10 gears / 265/75R16
and mine is at about 4500 RPM's @ 65 mph.
Does the factory tach make a difference?
I have a 350 engine with a 5k tach, that is what is in it now, i have seen an 8K tach in others.

[Sicklajoie]

Originally Posted by Cali72K20

budman k20
you have about the same set up as mine:
TH350 / 4:10 gears / 265/75R16
and mine is at about 4500 RPM's @ 65 mph.
Does the factory tach make a difference?
I have a 350 engine with a 5k tach, that is what is in it now, i have seen an 8K tach in others.

4500?? You sure about that?

[jocko]

18436572GMC, spot on on the 3000 estimate - it's just a hair under that for your tires, like Tim mentioned. If you switched to the 315s, your 70 mph rpm would drop to 2790, so, a little reduction.

Budman - As for SM465 - I might be wrong, but thought it had a 1:1 high gear (?) - if so, then it would be the exact same rpm at 70 as a TH350 or TH400.

Cali72K20, something not quite right in your numbers there - 4500 is not correct for the combo you listed. Perhaps incorrect speedo gearing - have you double-checked your speedo with a gps? Or perhaps different rear end gearing than you think. I have the exact same combo on 2 trucks (TH350, 265/75/16, and 4.10s) and 65 mph = approx. 3000-3100 (per truck and per calculator). It's also possible that your tach is out o whack. Factory tach was 5K, some folks stuff aftermarket or other vehicle gm 8K tachs in. I think LMC makes an 8K tach kit also.

[budman k20]

Jocko,

It is true that a SM465,Turbo 350 and 400's have a 1:1 final drive ratio but the automatics don't have lock up TC's so that is why they have higher rpms at a given speed. My truck rpm is a little higher at 70 mph due to the TC having a higher than stock stall speed.

[Cali72K20]

Is a 5K tach for a big block and 8K tach for small block? I drove it again @ hwy speeds and it is about 4500 @ 65-70 MPH.

[Cali72K20]

the tach did not work when I got it, the speedo didn't work either. I had to splice the tach wiring to get it working. The speedo cable was broken/cut, that was an easy fix. It does have a Mallory HEI in it from the PO.

[Cali72K20]

The harness in the engine bay is new and has the tach wiring incorporated in it, the wiring under the dash looks OEM and the connector to the tach port in the fuse block was corroded beyond repair, all I did was splice a new connector from LMC truck.

[Cali72K20]

the speedo is dead on according to local police signs that show your current speed.

[RANDY COX]

I'd like to run a 33x9.50-15 with 373's would be good to run..that would help with rpm's some.

[budman k20]

Cali72K20
The factory tach's are known to be inaccurate that is why I mentioned I have a factory tach. You really need to know what your speed is using a gps source. It is very possible that your truck has 4:56 gears and with an auto the rpms you are reading from the tach could be close. Yours still sounds high but that could be from an inaccurate factory tach. Another possibility is you have a 6 cylinder tach. All the factory tach's I have seen are 5K. It's a popular thing to put a 8K Camaro tach in the truck dash if you have a performance engine.

[Cali72K20]

Thank you budman k20!
I need to pull off the diff covers and check them to verify what gear ratio I actually have. The tach does look original but I don't know for sure. Thanks again,

casey

[jocko]

Originally Posted by Cali72K20

Is a 5K tach for a big block and 8K tach for small block? I drove it again @ hwy speeds and it is about 4500 @ 65-70 MPH.

Factory is 5k for all.