Rear Gear Ratio?

[JTautomotive]

Originally Posted by russgt

Thats a good plan. I went this route and scored a set for $150. They are plentiful and sometimes the frame they are attached is included for free.

did you have to rebuild them at all?

[bowtiemanchevyman]

Originally Posted by JTautomotive

im looking lift my 1980 k10 4 inch and put on 35 inch tires , i was wondering what rear gear ratio would be the best with a SM465 tranny. Also what speedo gears would be needed?

Thanks
-Justin

It all depends on what you are using the truck for. my 82 with 4" lift and 33 i have 3.73 you might want 4.10 if it a daily drive. if its a play toy 4.56 or more. where is your power band on your motor. and the speedo gear you will have toplay with that it tok me three tries to get mine right.

[LONGHAIR]

The simplest way to remember the "High/Low" gear thing is about "speed".
Low gears accelerate quickly....but the top speed is limited by engine RPM.
"Quick" and "Fast" are not the same thing.
Low gears= low speed
High gears= high speed
There are trade-offs each way for going too far away from the ideal gear, which is generally determined by tire diameter. You have some lee-way based upon your intended use, but not much.
If you go too "low", your highway RPM my be too high...especially if you are a speeder This is not only bad for fuel mileage, it's not necessarily good for anything else either.
If you go too "high", acceleration suffers. This causes you to push the pedal harder....once again, not good for fuel mileage. It also seems sluggish and heavy. You might think that this low RPM on the highway would counter-balance this by giving you mileage gains, but that is not necessarily so either.....especially with a carburetor. They get confused in the transition between idle circuits and the main jets. It can be even worse with an over-drive automatic transmission, they don't like that either.

[Firebirdjones]

Originally Posted by mopar346

Man you guys think different than me, a truck needs a low gear and the bigger the tire the lower the gear. By the way a high numeric gear is referred to as low and low numeric gear is referred to as high. 4.56 is a low gear, 3.08 is a high gear. To move the mass of a truck you need leverage from a low gear, as you increase your tire size you make your final drive gear higher, 35s with 56s runs out the same as 29s with 3.55s.

Here is a chart that will give you info on your RPM 1800-2200 or so is the sweet spot for mileage.

We think alike. To me a 3.73 gear is rather mild and very streetable in all conditions even without an overdrive. I run 3.73's in 2 cars here and find it a very nice street gear without overdrives.

A 3.73 with only a 28" tire and a 1:1 trans gear,,,,you can cruise at 65 mph at only 2800 rpms. With an overdrive you could knock that down to the 2000 rpm range easily.

If you are going to run a taller tire then that 3.73 becomes a very lazy gear. I'd step that up to a 4.10 at the very least, maybe more if you really go crazy on that tire size.

[Ronald29609]

When driving my 67 C10 at 60 mph the tach says 4000 rpm and the motor sounds like its around 4000 rpm. 29" tires. The ratio should be in the 5s. Too much for a daily driver.
I pulled the differential cover and the gears are 41-11, 3:73 ratio. The trans is a 3 speed case number 8925647. I think if the clutch was slipping I would smell it. Something doesn't add up. Any ideas? Thanks, retired guy.

[mrpaintstick]

I have 4:10 in my c-30 and was going to go to a 3;54 in the next couple weeks with the turbo 400 have same size tires as you. Was thinking about an over drive next spring. do you think the overdrive would be over kill? Only do occasional light to med towing

[Ronald29609]

A 3:73 with 200-4R that I am going to install reduces the ratio to 2:50 or so. With 29" tires that's 69mph at 2000 rpm. About the same as our 2010 Impala. It gets 30+mpg highway. Your 4:10 might be OK with a OD transmission.