I recently completed a 24 valve cummins conversion on my 1990 Ford F250. I had the 351 gas engine in my truck with the ZF5 speed manual trans. I couldnt use this trans due to cummins adapter plate differences so I found a ZF5 speed manual out of a 1994 F350 with the 7.3l diesel. I had 4.11 running gear before and converted down to a 3.55 running gear after. I am running a 35" tire. My problem is at only 65 MPH my cummins is cranking out at 2900 rpm. I dont want to get a ZF6 due to the need for a different adapter plate to the cummins, and I would like to stay with the Ford drivetrain as much as possible. My question is: Is there something I can do with my current trans to get better gear ratios. I have heard of gear vendor and torque splitters. Are these feasable conversions and if so what does it take to install one (longer drivetrain and need for drive line mods ect...) or should I really start looking at a different trans all together? I have no plans of racing or sled pulling. Just daily driver with occational towing of my 5th wheel coming in at 13k lbs. Any suggestions and or cheap ways to fix this issue would be helpfull. Thanks

 

Those numbers do not add up. If you have 3.55 gears 35inch tall tires and 4th gear which is 1 to 1 gear ratio you should be going 85 miles per hour. Overdrive should be even faster. Check your numbers again because those just don't add up. Are you sure you are in fourth or overdrive I could see you doing those speeds in 3rd gear.

 

Thats the right info... 5th gear 65 mph and the tach reads 2900 rpm, Im gonna go to an auto shop and see if they can measure the RPM by other means. Perhaps my tach is reading wrong. Ill post later if its different. I did buy the conversion harmonic balancer sensor from ford cummins but still would like to be sure. Anyone do some type of torque splitter or gear vendor set up that works well for the ZF5, would still like to drop the R's down during long cruisin.

 

Yeah you have to have something wrong in that equation because 4th gear is 1 to 1 ratio and the numbers don't lie.

 

i agree your tach must be off, i am running a ZF behind my cummins with 4.10 gears and 35s and am only turning 2200 at 65

 

Checked the speed of the motor with an external snap on tach and found my tach on the dash was running 500 rpm higher than what the engine was really doing. Gotta fix that quick. So really 2400 rpm at 65 mph in 5th gear instead of 2900. What is the red line rpm for the cummins 24 valve?

 

that still seems a little high

 

Borrowed a snap on tach reader. put the sensor magnet on the harmonic balancer and clamped the reader into position. Drove the truck and through all the gears and driving at different speeds I saw a 500 rpm average drop from my interior tach. Don't know what else I can do. Is 2400 rpm high for the cummins to run at?

 

i believe it should be lower. i'd say around 2000 but i could be wrong, although it would be the first time

 

Yeah those numbers still don't seem correct. If you go on the this site Engine RPM Calculator and type in the bottom calculator all your numbers (the gear ratio of 1.00 sould be entered) that should tell you your rpms for 4th gear. If they so not match you have something wrong with your aet-up.

The only way I was able to get your numbers you first stated was to enter in your 4.10 ratio in 4th gear and 31 inch tires.

Also are you sure you have your spedometer corrected for the tires and gear ratio?

Also if all your numbers were correct for everything but your speed at those rpms in 5th should have been 120mph!!!!