The Stovebolt Site Forums The Shop Area General Truck Talk I can't drive 55

not that I need to go faster on a day to day basis, but.... one day i'd like to make the trip from boulder, colorado to lake glendo in wyoming. I-25 at 55 doesn't sound like fun. I'm looking at ways to get my '50 chevy 3600 to go over 55 mph. As a newbie.... bigger rear tires, rear diff switch out.... you all know more than me. open to all ideas.

Gearing is one way to go;several vendors offer a new ring and pinion set that's more acceptable for highway driving. I found that a set of LT 235/85R-16's will probably give you approx. 70 mph at about 3,200 rpm.That's not exactly the best speed for gas mileage,but it'll be better than stock. You can often pick up a set of four good used tires that size for 75.00 to 100.00,whereas the ring and pinion will cost you around 300.00,and at least 150.00 to have it installed. BTW-if you go for the tire size change do all four.Then you only need one spare,and changing just the rear ones will make your truck twitchy on the highway by reducing the rake of the kingpins.
You may also notice a little tire rub against the front springs when you turn it full lock,but it won't hurt anything.

If your running the stock rearend in that thing you are geared way too high.Get your gear ratio down around 3:73 or 3:42.

I'm kicking around the idea of putting 235/85R-16's on my '50.. does the larger tire size effect the speedometer accuracy?

One possibility is an auxiliary tranny.
http://www.6066gmcguy.org/spicer-5831-b.htm

Fumblin 46, the answer is yes. It is a function of the diameter. The tire you mention has a 32" diameter. If you are now running 6.50X16, they have a diameter of 29". With that you get a difference of 3". So your speedo will be reading about 10% slow, from 3/29= about .10.

As an example, my 52 3/4 ton I went from 4.57 gears to 4.10. I also went from 7.00X15 to 7.50X16 tires. My speedometer reads slow by 18%.

4.11 rear end from 67-72 truck and a ranger overdrive (27%) from advance adapters in front of your SM420 and radials makes cruising or splitting gears a breeze for a lot less than a gear vendors overdrive.

All, how do I determine what gear ratio I have? I have moved to larger radials and it has helped, but the engine screams when I'm at about 60 - 65. I'd like to get more speed for fewer RPMs. I have a 54 3100 1/2 ton with a stock 235 and hydromatic tranny.
Second question is where would I go to get the new gears and have them installed? Do I need to find a specialist or can any garage do this work?