4.11 gears and 287/75/16
 

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

Re: 4.11 gears and 287/75/16
 

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

Re: 4.11 gears and 287/75/16
 

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

Re: 4.11 gears and 287/75/16
 

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"

Re: 4.11 gears and 287/75/16
 

What is your ring and pinion ratio?

Re: 4.11 gears and 287/75/16
 

Sorry,

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

Re: 4.11 gears and 287/75/16
 

thanks

Re: 4.11 gears and 287/75/16
 

ttt

Re: 4.11 gears and 287/75/16
 

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".

Re: 4.11 gears and 287/75/16
 

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.

Re: 4.11 gears and 287/75/16
 

4500?? You sure about that?

Re: 4.11 gears and 287/75/16
 

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.

Re: 4.11 gears and 287/75/16
 

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.

Re: 4.11 gears and 287/75/16
 

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.

Re: 4.11 gears and 287/75/16
 

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.

Re: 4.11 gears and 287/75/16
 

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.

Re: 4.11 gears and 287/75/16
 

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

Re: 4.11 gears and 287/75/16
 

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

Re: 4.11 gears and 287/75/16
 

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.

Re: 4.11 gears and 287/75/16
 

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

Re: 4.11 gears and 287/75/16
 

Factory is 5k for all.