HD / MD transmission swap

HI Walter,

I have a new 33GVW single axel truck with the cummins ISB 6.7 in it and I have some gearing issues too.

I use it to haul stacks of hay for my small square hay bale business. It replaced an older Ford F700 with a (very) thristy under powered 370 gas motor. That was geared too low for local highways these days, we're talk'n like 55mph flate out!

Anyway this new truck I have the Fuller 6 speed FSO6406A in it with a 4.88 rear end ratio. 6th is an overdrive of .78 and 5th is direct drive. At 62 mph in 6th the RPM is only around 2000 rpm. Well when I'm loaded I can't run in 6th!! Empty it even has to work some untill I get the RPMs up to say 2250 which seems to be the sweet spot for this engine. But then of course I'm traveling a bit fast for the local 2 lane highway which is 62mph (100kms) on the 4 lane highway by me its fine as the speed limit is faster on it (68mph). There is a pretty big gap in the gearing between 4-5th and 5-6th.

When loaded it weighs around 25K-27K lbs. but has to buck wind with the load on. The load is 10ft. wide and sticks above the cab quite a bit sort of like a van body except wider. Empty it's sort of like a flat deck with some arms sticking up.

Most of the trucks on the lot I found came with the standard 6 speed with no overdrive and the 4.11 rear end. Which would give you about 2150 Rpm @ 65 mph. Or they came with the same trans and the 3.90 rear end which would be sort of like what I have now for ratios.

So I find now I have to drive in 5th when loaded with the RPMs up around 2400 to do 62 mph an hr. on my local highway that runs by the farm. AFter a using it a week or so I now have decided 2400 in 5th loaded is not so bad after all. I have really good power at this RPM so if I face a head wind or a hill I don't have to worry about downshifting etc. I think if I had gone with the other tranny and a 4.11 rear end I'd be down shifting constantly because I think I'd have problems hauling a load at only 2100 rpm. The engine is rated 300hp at 2600RPM at 2400 RPM I believe its rated at 260hp.

So when I come back to the farm empty I run it in 6th and idle along at 2000-2200 rpm. It kinda has to work some at 1950-2000 RPM but seems to handle it fine once you get over 2100. A 7 speed would probably be about right with an overdrive of .85 instead of the .78 I have now. With a .85 with my gearing I would do 2150 in 6th which would really cut down that shift from 5th to 6th.

A friend of mine suggested I go with a 9 or 10 speed. I figured though with this new truck and all the HP is was suppost to have (according to the salesmen) I wouldn't need the extra gears. But now I'm thinking it would have been handy to have a lower 1st and reverse at times on soft ground. NOt to mention a couple more of the higher ones.

The truck works pretty good over all and if running it at 2400 or so half the time isn't going to hurt anything I guess it should be fine. Cummins has a good website for gearing trucks called powerspec.

It says for economy gear the cruise speed at 2150 and performane at 2250. But it says to keep the cruise speed in the range of aprox. 2100 to 2400rpm.


What kind of mileage are you getting with your truck running at 2500 rpm??

What is your diff ratio?