I'm the guy with the dead motor (7.3) and transmission killed by towing from the front with the driveshaft engaged. The engine from International seems fine, fuel delivery issues but under warranty. Great oil pressure everywhere and I was told Franklin the rebuilder doesn't sleeve.

The transmission is another story. I bought a remanufactured unit through a local Ford dealer. It came from United Ford in OK where they actually rebuild their own units. It works fine but it's not the same as my original. No neutral safety switch plug and the back up switch is completely totally different in terms of the plug. All of that can be dealt with but I also suspect the 5th gear ratio is different. I'm spinning 2500 plus rpms at 65 mph. With the old unit that would have been more like 75 mph for the same rpm's. Looking at the very nice chart Dave posted it looks as if there are only two ZF5 5th gear ratios to choose from, .76 and .77. I can't see how that could cause the disparity in rpm's at speed unless the difference from the transmission to the rear end is 10 to 1. The transmission I ended up with is a close ratio. It's great in town but terrible on the highway. Did ZF make any other 5th gear ratios early on? I can't figure this one out. any thoughts appreciated.


1988 7.3 IDI Superduty long chassis, normally aspirated

 

Never heard of a ZF5 without the overdrive gear. 5th gear should get your rpm at 62mph down to ~1,850rpm with a 3:55 rear end and R16 tires or ~2050rpm with 4:10 rear end and R16 tires. (give or take 50 rpm)

It almost sounds like your 5th gear is not an overdrive gear if you are reving 2500 rpm at 65mph.

Have you checked you speedo and tach? What is your highway fuel milage like?

 

I'd check the speedo and tach but this is a truck I've owned for a good long while. I know how fast it's going without really paying all that much attention to the speedo and ditto for the tach. I may end up getting the speedo calibrated just so arguing with United Ford is easier. I should probably get off my lazy butt and figure out what the rear end gears are and calculate what the rpms vs mph should be based on the gear charts for the ZF 5 that Dave posted a couple of months ago. How's my mileage on the highway? Lower than it used to be. I've seen 17 mpg in this truck but 14 was normal. Right now it's about 11. Of course the engine has maybe 2,000 miles on it so it's still pretty tight and I have a fuel delivery problem that needs to get fixed. Right now if I try and coast down from highway speeds in gear the engine surges as the rpms drop. I don't know that it's costing me fuel but I don't rule it out. By the way the tyres are stock sizing and none of that changed anyhow. Did I mention the truck is heavy? I weigh out from selling scrap at over 10,000 pounds.

1988 Superduty dually 7.3 IDI normally aspirated, long chassis, 11 foot service body

 

What axle do you have in the rear end, because I have the same issue and I believe that this year of truck only came with the option of 4.63 or 5.13 gears. What is the axle code on the drivers door post?

 

The axle code is 73 and the transmission code is a C. I was happy before the tranny got replaced. So what's the 73 decode to and ditto on the C? I hope C doesn't just mean ZF-5 that I already know.

 

Code 73 is an 11,000lb. axle with a 5.13 ratio. I'm trying to track down what a "C" Transmission is right now.

 

The "C" is for a close ratio 5 speed being the ZF 5 speed.

 

It's begining to sound like I got the correct transmission in terms of original equipment but the wrong one in terms of what I had previously. I wish I'd had the tranny in my posession when I needed the service tag numbers. I bet the guy who owned it previously had swapped it out for something with a taller 5th. Any idea where I can hunt up a plug for the reverse switch? My old transmission and my wiring harness have plugs for reverse and the neutral safety switch. My new transmission only has the reverse switch and the plug is entirely different. It appears to be only two pin. I'd love to find an elegant way to get my reverse lights back on. Before I back over someone too. Thanks for the axle ratio numbers. Are there any good web site tables or calculators for rpms and gear ratios out there?

 

Do you have a spreadsheet program?

I wrote my own in Lotus 123, Microsoft Excel works the same way.

 

gear ratio calculator