Last week I posted that I have acquired a 1989 F-Super Duty with the 7.3L, 5 speed and Dana 5.13 rear end. I intend to transplant this drivetrain into my 1958 F600.
What RPM does the 7.3 like to turn when cruising down the highway at 60-70 mph with no load on the truck? In it's current configuration the engine turns 2600 RPM at 60 mph. Would 1800-2000 RPM at 60 mph be too low as in "lugging" the engine?
If this rear axle is a Dana 80 I see that there are 4.10, 4.63 and 4.88 gears available. I will be running 9R-22.5 tires.
I'm wondering what the lowest RPM the 7.3 will be happiest with for fuel mileage purposes.

 

They love 1600 and every thing under 2000

 

mine seems to like 1500 to 2200 at 2200 i am going down the highway at 75 mph

 

you will get your best fuel economy and power at around 1900 rpm

 

Yup ~1800-2000RPM is the power/fuel band.

 

Hmmmm, mine runs 2500 rpm at 60 and 2750 rpm at 65 no load. It's an 88 F250 7.3 with an automatic. I got the truck a few months ago and the kid I got it from said it has a shift kit (it definitely shifts solidly). Not trying to hijack the thread but now I am wondering if mine is not right. I got it for pulling a heavy 30ft 5th wheel.

 

Exactly, I cruise at 1800-1900 rpms (55-57 mph road speed with my gears and tires) and this gives me 19-20 mpg when empty and 14-15 mpg when loaded down with a big camper in the bed.

 

the relation between engine speed and road speed is heavily dependent on transmission ratios - the OP has an overdrive transmission, you have a C6, the C6 has no OD gear soit's normal for the engine to revv higher to keep up with an E4OD or ZF5-equipped truck.

 

Thanks, I believe the original window sticker that was in the glove box says it is a C6 transmission. Is it hard on this engine to be cruising at 2750 to 3000 rpm or should I just expect to drive it around 55 all the time to keep it at lower rpm???

 

the only thing it will be hard on is the fuel mileage. the diesel does not care if it is running 1900rpm or 3000 rpm.
my 88 is the same way. top speed is 75-80 mph at 3300 rpm.
i have the C6 trans, 4.10 gears and 36 inch rubber on it.

 

hwatx and LFdavidson

I think you both should investigate a Gear vendors system. It would give you the best of both worlds...

 

I have thought about something like that but I have been unemployed now for 18 months... Damages the cash flow big time... Just doing little repairs and fix up for now...

 

ive got 2 1999 7.3L diesel. Both tow trucks. 450 and 550 both manual. But my cruising rpm at 55 mph is about 2000 rpm in 5th gear so from what I’m reading your truck sounds like it’s where it should be. I got on here wanting to know if I’m lagging or rapping the truck out at 60 doing 2300 rpm. I keep it between 55-58 I get 13mpg. After reading what I’ve read I’d say you might have a shift kit in there an electronic chip most likely but sounds like it’s where it should be. All kinds of stuff to change that but if it ain’t broke don’t fix it lol

 

two completely different animals.
out trucks are old school, no computers, no electronics of any kind. all we need is 12 volts to the fuel shut off switch and they will run forever.
your trucks are totally computer controlled.

 

with the few exceptions of where some of us run E4OD on our IDI trucks and the Transmission uses a PCM but Never the engine

a 3.55 rear with E40D is nice on the highway


it gives me 1607 RPM @ 60 mph