Ratio question

[Ruffin]

I just got a 71 C10 that was used by Mississippi Game and Fish Dept.
It was originally a 6cyl 3speed but was swapped to a v8. When I'm drive at 55mph the tach reads 3000rpms. Would this be 373 0r 411 gears? My Spid has posi on it but no gear ratio....

[trac209]

Originally Posted by Ruffin

I just got a 71 C10 that was used by Mississippi Game and Fish Dept.
It was originally a 6cyl 3speed but was swapped to a v8. When I'm drive at 55mph the tach reads 3000rpms. Would this be 373 0r 411 gears? My Spid has posi on it but no gear ratio....

Need to know tire size to tell you for sure

[A1971Blazer]

there should be a tag attached to the cover that tells the ratio....unless it's been removed....it will be under one or two of the bolts that hold the cover on....
On another note...my Grandparents used to live between Reform and Millport...been there many times....

[Ruffin]

275/60/15 on 8 inch rim....Cool A1971Blazer!!

[Shinysideup]

4.56 with that 28" tire gives you 3090 @ 55mph


Makes sense game warden needed low gear more than highway gear.

[Ruffin]

Crap! Going to be swapping the Sbc out to a 454 and be looking for some more friendly highway gears...going to put this rearend for sale....lol

[Ruffin]

Thanks Members for all the help and info!!!!

[Shinysideup]

Correction, my post should have been 2996, not 3090 for the 4.56
Same difference though.

[Ruffin]

Thanks Shiny!!!!!

[Ruffin]

I'm going with a 454 with turbo 400 out of a 1983 motorhome and thinking that 3.08 will be what I'm going with.

[Mike C]

You are making a lot of assumptions trying to come up with the right gear ratio this way. Pull the cover and know for sure. One of the reasons that newer cars are so nice is the transmission. Overdrive is a game changer in the land of 70 mph speed limits and still wanting to have good low gear acceleration.

What does the SPID say? GM axles didn't have a ratio tag on them under a cover bolt, only the Dana axles. The GM axles would have a posi tag on them however. Usually a metal tag under cover bolt but sometimes a plastic "flag" behind fill plug.

[James the III]

there is a code on the pass side axle tube at 9 o clock..

[Ruffin]

As stated before the spid only says posi.....

[halfstep]

My observations fwiw...
- Seems most 6 cylinder trucks defaulted with 3.73 gears
- "If" you can get the codes off the axle tubes you may get the answer.
- Easy to figure out yourself by raising rear and spinning tires and counting driveshaft revolutions - look up procedure online. Done it many times.
- There will always be people with "magic" trucks but I've found these trucks get 9-14 mpg no matter what carbureted engine (6 cyl or big block)is in them or how they are driven. So if mileage is your concern - forget about it - especially with the big block.
- Many newer cars have surprisingly low (high numeric ratio) rear gears to offset the overdrive element and keep performance up. So if you go to overdrive transmission obviously your cruising rpms will drop without touching the rear. TH400 is nice and durable though.
- I like to play with my hotrods and cruising at 3000 rpms has me right in the power band and I can "stab it" and walk away from most anything without the kickdown or downshift drama.
- If you don't like the drone I'd work on the exhaust and insulation.
- Gears (higher numeric) are easy way to tangible performance improvement without touching the engine.
- I have had several well built big blocks and unless your rear is higher than 3.73, I wouldn't change it. Rebuild your posi and enjoy. If it is a 4.+ rear, I still wouldn't go below a 3.42 or 3.55 which is what many of the early era Super Sport big blocks came with.
- Do what you want, no matter what any of us say - your truck.