I have now owned my 1978 f600 a few months and have hauled quite a few loads with it and I am unhappy with my top end speed. I have a 4 speed trans. with (i think when I looked it up it was ) a single speed 8.66 rear end with a 15,000 weight rating . Top speed with the petal to the floor in 4th gear is about 43 mph ( if I'm empty lol). it has a 330xd so I'm not sure if I should even try to get a highway speed by changing the rear end gear but I am getting frustrated with it.

Any advice about changing the ratio or going to a 2 speed would be welcomed.

 

We had a '58 with a 292/4/2. Since your 330 has a few more beans than the 292, I would expect you could make 60 mph with a 2 speed with the 6.something/8.something combo.

You will have to look for an axle--truck parts places I suppose.

Another thread with some good info for you, including ratio info:

https://www.ford-trucks.com/forums/7...peed-diff.html

Series numbers here:

Two-Speed & Planetary

If you can figure out the model of your current axle, you may be able to find out if simply replacing the carrier with a two speed set up will work, or if you're in for a drum-to-drum swap.

 

I've seen single speed rears both ways. Geared for highway only or off-road only. As I remember the Eaton was used for 2-speed applications. The axle tubes are round. Timken had square axle tubes and I don't remember any Timkenaxle having 2-speeds. If you get in a junk yard looking around I think the axle tube info will help. If the donor truck you find has a gas engine you'll probably be good. If it has a diesel engine it will probably be geared too low since diesels have half the RPM range. I do have some misc. 2-speed shift motor parts if that helps.
Let me know what you learn, Dennis.....

 

if I had to guess I would say its prolly more around the 5.83:1 range. If you have the six lug rear a diffrent axle sould not be a problem to find. However adding an auxillary trans with an OD would be a better solution as you can keep your pullin gears and run OD unloaded so you can have your cake and eat it too. I just added one to my truck a 5531 spicer three speed and went from about 50mph at 2500 to about 60mph at 2000rpm in high. If you have a torch the linkage is east to make. I used 5/8 round stock, threaded the ends and used air brake clevises found at any truck store for the linkage ends. I found a shifter at an online truck salvage yard. Ive been meaning to post pictures but I have been lazy. I used the torch to bend the round stock which included two 45 degree angles per rod and had a friend weld up a maount out of channel iron. The expensive part was getting new driveshafts made. that ran me $550. I prolly have $1000 invested in the whole set up. Or you can find you a five speed od with the trans mounted E brake.

 

My 63 f600, 292 2v, 5 speed Clark, 5th is direct so no different than your 4 speed that way...2 Speed rear 633/881, the truck weighs 10,000 pounds empty, 825-20 Tires, comfortable cruise on the freeway is 55/60.

I opened it up on the flats on the freeway one time, curious if would do 70...I let off at 72...still had some more, but I didn't see the point in going any faster.

When I'm loaded heavy, its backroads only.All the gearing in the word won't help if you can't pull it.

The 2 speed would be a nice bolt in upgrade, as you can also split all your gears when your heavy.

If you like your truck, and plan on keeping it I would recomend it.

The auxilary trans would be cool too, if you have the time, skills, money.

The rear swap would be fairly cheap and simple.

Greg

 

Also before you get to carried away...Is your truck governed?

If it is, how high will it rev before the governor takes away the fun?

Mabey it's not working right?

I have no idea what governed engine speed is on a 330 if so equipped.

Are you sure about the axle ratio?

Those are some things I would look at and research before buying anything.

Greg

 

His top speed sounds about right for the other info he gave. I would bet the 330 is governed, probably on the carb and the distributor. Good question though, it should be checked.

 

I double checked and according to the axle code it is a 8.66 rear gear. Yes it is governed at the carb. I don't have a tach but by the sound of the engine I'm in the high 4,000 rpm range above 40 mph. It does seem like the truck was originally set up for job site / off road use. I think if I switch rear ends to a two speed it will keep the low end I need for the hills around here but give me just enough to end to use the highway. Then I'll use that truck for jobs where the truck sits most of the time and get another with a higher top end and more horse power for hauling back to back loads.

Thanks
For the info everyone