The Stovebolt Site Forums The Parking Lot Big Bolts changing rearend ratio

I have a eaton 7.17 single speed rear end on my 59 8500. I have a top speed of about 40mph. I am not looking to burn up the road but I would like to be able to cruise at 55 - 60mph. I know I can look for anothe rear axle but I want know if it is possible to change the gears in the one I have to get a higher ratio? If so is it cost effective to do it and what would I need to do to get proper speedometer calibration. OR would some kind of add-on overdrive be better? I would like to hear what others have done.

If you haven't set up a rear end before it is rather time consuming. Getting the preload correct and pinion depth can be extremely frustrating. I'm not sure what options you have for your year, but I would almost bet you are better off finding a complete rear end rather than finding a ring and pinion to install yourself!

Hy Gene_Spartan80, could you please tell us what engine, transmission and rear tires you have. Most of the charts in my Truck Sales Data book show top speeds in the mid fifties at least with the engine turning 3700 to 3800 r.p.m..

I have 46 1/2 ton and went to 3.55. The one caution I have is you can go faster but you have the old brake system and shocks. I have had to stop quickly from 55. Any faster in a hard brak might not come out so well.

This truck has a 366 big block in it with a sm465 transmission and 9.00 - 20 tires. With all the noise in the cab when riding down the road it is hard to tell how much the engine is revving. maybe I have been babying it too much. I bought a tach this morning so I can know for sure what my rpm are running. I am pretty sure it was not at 3700 RPM when I backed off. Like I said I am not looking to get a lot more speed and if this combination will make 50 MPH, I could live with it. because you are right, the steering and braking has its limitations that make high speeds unnerving.

Hy Gene_Spartan80, according to the charts in my Truck Sales Data book, you should be able to achieve 59.4 m.p.h. with the equipment you have and the engine turning 3700 r.p.m., hope that helps.

Thanks 3B, so I can get to almost 60, what rpm would you say this engine would want to cruise in 4th gear? I don't know that 3500-3700 is where it wants to be constantly (or is it)? The less I need to do to this truck the better for semi-daily driving (I work very close to my house), but I do want to take this to the lumber yard when I need to get supplies when needed (approximately 60 miles round trip)

I installed a tach in the truck then took it on the road. At around 3100 rpms I was around 50mph. I can live with this. it just felt like the engine was wound out. My lack of experience with this size truck and this engine shows. Maybe high RPMs at cruising speeds is the norm for this truck

Hy Gene_Spartan80, a couple of years ago I purchased a 1969 C50 series it had been repowered with a 1972 car 454, it was 300 miles from my home, to get it to 60 m.p.h. required I run the engine at 4000 r.p.m. according to the tach. With the car Quadrajet carb it did tend to run a little warm, but it did fine only stopping for gas.

That is good to know. Maybe I am babying this thing.

Gene,

If you 366 has a two barrel carb it makes 220 hp @ 4000 rpm, if it has a 4 barrel it makes 235 hp @ 4000 rpm. It would be goverened at 4000 rpm, so wind her up!

Mike B

I drove gas burners back in the day when we made a living with them. With a 5 and 2 I upshifted at 4200 and down shifted at 3800. On an upgrade I might even let her go up to 4400. Get used to the noise, and enjoy driving it.