hi... i have a 1978 4x4 f-600 and ime triyng to put a nissan fd6t on it... but my problem is that the differentials seem to be 7:1 and with the diessel engine i think it wont go over 55mph.
Is there any overdrive ot something i can get for it?
thanks

 

That engine made 95hp in its day so getting to 55mph will be test on its own don't plan on going any faster than that. The diffs if it was a marmon conversion should be 6.48:1

 

the front dif is a rockwell fds 75... I spinned the wheels and i counted about 7+:1
Ime not triying to get that much speed... just think the diessel will be too loud at that speed with this ratio (about 3200 rpm)... I would prefer it to be around 2500-2600rpm... ive heared about some chevrolet spicer auxiliary gearboxes that came on series 70-80 trucks that had low-direct-overdrive gears... something like that is wath i need... but those are rare...

 

Did you have one or both wheels off the ground when you turned the wheels?

 

did it both ways

 

If you did it both ways, you should have got two different readings. If both wheels are off the ground and both wheels are turning that should give you a true count. If one wheel does not turn at any time when you are doing it, you will get a false reading. If you do it with one wheel off the ground and the other solidly on the ground then you divide by two or double it, can't remember for sure which it is. From the high reading you got I would guess that you would divide it by two.

 

7.something sounds about right to me too. No way they would have 3.50 gears in a big truck back then. How about some HEMTT 16.00X20's? A tall tire would lower the engine speed, but at 95hp, it isn't going to work. If you go to a truck wreckin yard, and get a brownie box, it may help. Some guys stick em in backwards, and get a 200% overdrive. Why such a low hp rating? Get it at a good price?

 

with both wheels up i got 7.2:1 and with one wheel off the ground i counted 3.5:1... i think those 16.00x20 might be a solution... the engine i have is a 175hp turbo diesel nissan, and i`me going to put an intercooler to it. might get about 10hp more.

 

O, ok. That sounds better, 175 will move it pretty good. Those Michelins are approx 54-56" tall too. If you dont tow too much weight, you may be fine. Stopping is another story. Going with the trans that's bolted to the nissan? Might have an overdrive option there too.

 

yes... ime looking for a nissan trans with overdrive... i`ve heared there are some bus trans with a overdrive at the back

 

1971 was the first year Ford offered factory installed 4WD in F600's.

The way to determine if it's factory installed 4WD is by the AXLE code found on the Warranty Plate, and/or by the first 3 digits of the VIN.