My 48 is also mostly all original, except when I went to 16.5 wheels brakes and tires off a 75 F-250 doner truck, the tires are obviously much smaller than the original 17 x 750's. I went with the 4:11 ring and pinion set from Chucks Trucks which brought the engine rpm to mph ratio back to what it would have been with the original taller tires. Made the truck much more fun to drive.

 

assuming i have 486 gears, changing to 411's what would i expect in increase speed.. now it looks like 55 is really max and 50 is a better speed.. wonder if the expense is worth just a small change in gearing ????

 

If you are going to the trouble of the D60 swap, I would look for 3.73 gears.....4.11's would help though. There are several online tire size/ratio/RPM/speed calculators to help you.
Also remember, these trucks were not designed (in stock form) to go 70 mph on the freeway. The steering, brakes, etc just aren't up to it.

 

i wanting to keep the same rear but doing a gear change.. if i change rears then the D-shaft needs to have mods too.. i'm wondering if i do a rear change can the D-shaft be a one piece or needs to have carrier like it has now ??

 

Changing from 4.86 to 4.11 ratios will drop your engine r.p.m. 15%.

If you have a 32 inch tire rolling diameter, with a 4.86 ratio, at 2550 r.p.m. you will be driving 50 m.p.h.

If you have a 32 inch tire rolling diameter, with a 4.11 ratio, at 2550 r.p.m. you will be driving 59 m.p.h.

I think you need to keep the 2 piece drive shaft. Too long of a drive shaft will "whip" and cause vibration problems.

 

Keep in mind that the engine has to make enough HP at a given speed to push thru the air, and climb any reasonable hills. Based on 38's numbers, the 4.11 looks reasonable. If you over-gear, you'll be into the gas pedal enough to trigger the power valve to open and gas mileage will plummet.

Ditto on the 2-pc driveshaft; one long shaft will have a lower critical speed where it resonates and that will tear up U-joints and bearings very quickly.

 

very good info.. tks