I bought an old 90 Uhaul w/ 7.3L. It is an F350 chassis w/14foot box. Actually a pretty nice truck. I just took it to an IH dealer and had them check everything out. They replaced the fuel filter, replaced all the little "O" rings and fuel lines, fast idle solenoid, and adjusted the timing to specs. BIG difference in the way it ran when I bought it. IH said it was a strong running engine. However, it is a 90, has a 3-speed C6 auto trans and 4:10 differential and set up to cruise at 50-55 MPH. IH did tell me that the trans fluid was burnt black and since it does act up a bit it needs to be replaced. What goodies would enhance transmission performance? I also want to change the axle ratio. I asked a technician about what would be involved in doing this and he told me that I needed to be careful not to get out of the power band or fuel milage would go way down. A review of a few posts in this forum led me to a site with a calculator to determine RPM at various speed using tire size, speed, and gear ratios My truck has new 215/85R16 tires and the calculator indicated a 3:08 gear ratio would put the RPMs at 70 MPH at a little more than 2500 RPM which is in the power band of 1400 to 2800 RPM. Question is, will this really work to give performance at highway speeds without dumping fuel economy? Has anyone ever done this with a truck this size and can you give me some idea what I might expect before I start spending the cash to do it?

Thanks very much for any response.....

Dennis

 

You need to realize that a 14' van box isnt no pick up bed, its a big ole parachute dragging behind that 7.3 IDI at highway speeds. 'specialy the 70MPH speeds your talking about. Therefore I reccomend that you drive it on the interstate at 70MPH and get some miles under your belt before you start changing gears. My experiance with the 7.3 IDI is that performance is going to be marginal and raising the rear end ratio will probably kill some power your going to want.

 

I don't know about the gear ratios but I think a turbo kit would probably benifit you more than anything since it will be under a constant load which is where the turbo works best at. Maby a turbo with 3:08s? That way you would have more power to handle that lower gear.

I would think if a gm 292 gasser can pull a 1000 cubic foot box truck loaded with packages at 60 mph with the right gears at ups, surly we can get that uhaul to do 70mph with that 7.3 idi in a comfortably range.

What you have is what I originaly wanted. I wanted a diesel box truck that way I could have a shop on wheels and plenty of storage. A pickup just doesn't have the space that I am looking for.

Well, good luck and post your results because Im still interested in possibly getting a box truck and selling my 93 f350. I would still like to stay with the IDIs though.

 

Thanks for the response... Good points.. However, with the 3-speed tranny and 4:10 gears the RPM is maxed at about 68 MPH. There must be someway to maintain reasonable highway speeds without running at max RPH which will kill the engine in no time.

 

they're a little pricey, but you might consider a gearvendors overdrive unit to cut your freeway rpm's down some.

speedrdr

 

Thanks, Ford Mega, exactly what I want this truck for.

 

What are you going to use the truck for? I ran my F250 for about a year with a C6/3.08 combo before adding the turbo and replacing the transmission with an E4OD. I towed at 70MPH, and hauled railings on top of the truck (similar drag penalty as a cube box) sometimes at the same time, without any problems. Working to hold pace with trafffic, I averaged 13-16 MPG at an average speed of 70 MPH while hauling.

What you have to bear in mind is that its actually impossible to run your truck below the powerband because you have an automatic with an open TC. The torque converter will not accept significant power untill you pass 1600 RPM usually, so you don't have to give that another thought.

What you have to check is the type of differential you have. The older (pre 1992) sterling 10.25" can accept 3.08 gears, but I'm not sure if your truck has that type. For what its worth, my truck first came home with 4.10s and it SUCKED for anything above 55 MPH. After 3 weeks I couldn't stand it anymore, and ordered the gears.

 

Old90Uhaul,
First welcome to FTE and the IDI forum.

Two things that also come into play with gear swaps are how much weight are you going to usually be hauling and what type of terrain you are located in.

Flat country at a low elevation would take a faster gear much better than if you are at high elevation and live in the mountains.
With a NA motor, as elevation goes up, less air means less power.

Pulling hills with weight brings up a different problem.
If you run 4.10 gears you can run 55 MPH at 2500 in high gear.
With 3.08 gears you can run 55 MPH at 2750 in second gear.
The difference though is the stress you are putting on the U joints.

 

How do you figure?

 

Leverage. By lowering the ratio (numerically), the driveline and transmissiion are working harder to get the truck to move. You're still moving the same weight, but now the rear end has lost a torque multiplication advantage, and it requires more throttle and more torque input to get it moving, and that torque has to go somewhere. By keeping the rearend ratios higher, you spread that load across the entire drivetrain. Plus, the added heat to the transmission is a killer too.

Jason

 

Thanks to everyone for the feedback.... Actually this truck won't be doing a lot of work. I have thought about transmission heat. What is involved in changing the C6 to a E4OD. I could split the difference and put in 3.55 gears and probably accomplish about the same thing while keeping the stress balance across the drive line.

 

Ahh yes, that makes sence. Can't say I had problems with heat so far, I once measured the temps of the tranny at the solenoid body during a heat wave last summer, the hottest it got was 170F with a slipping clutch (long story). U joints seem to be doing well too.








Converting to an E4OD is not cheap or easy, plan on anywhere from $500 - $3000 depending on how you go about it. You could maybe pickup a tranny cheap, but its rare, and its usually taking a chance. And a much bigger chance than with a C6, the E4OD is not to be trusted in used stock form. If you want to keep cost down, go with a gear swap, if you want to cruise at 70, go with 3.08s if you can, I never regreted the swap to this day. Ocasionally, used auxiliary overdrives can be found in the $500 range, that would also be a good setup, if you can find one. Otherwise they're $2500 new.

 

Remember I am in the mountains and tow heavy.

With 3.55 gears and 33" tires, my U joints take a beating.

Using simple numbers if you has 6.00 axle gears the driveshaft turns 6 time to one revolution of the axle.
With 3.00 gears the drive shaft now turns 3 times for one axle revolution.

The stress on the U joints and drive shaft with the 3.00 ratio is twice what it was on the 6.00 axle ratio.

 

This is a great forum. Who says and old dog can't learn new tricks. You all have given me good food for thought. Now that I have the engine all tuned and running right I'll address the transmission problem.. Once that is done and road tested I'll address the gearing issue. Not to get too far off subject, I am definately am a Ford kinda guy, I have 4 of them. My other project is my 91 Bronco (which I'd like to convert to diesel). It has a 3-in body lift as well as a 4-in chassis lift. (I love that body lift, makes it soooo easy to work on) When I figure out how to have a photo gallery I show some pictures. Anyway, thanks again for your help.

Dennis

 

Ever notice a piece of heavy equipment like a skidder or very large forklift has a very large hub sticking out of the middle of the wheel? That is a planetary gear reduction box that is located at each drive wheel, and it takes a lot of the torque off the rest of the drivetrain. The axles and tranny can be smaller, because of less torque. But they do have to turn faster.