gear ratio estimates

[jethro]

I am installing my stock (I think) 5.9 into a 2004 F-350 with a ZF-6 tran and 3.73 diff -4 wd - also currently has
33 inch toyo fat tires.

I was asking myself if I needed to get stronger valve springs and faster governer spring for the injection pump.........

So I found the gear ratios for the zf-6 and the circumphrence of the tires - and tried to estimate the RPM vs. road speed.

I estimated that in 5-th gear and 1500 rpm I would be going 40 mph : (5-th gear is 1:1 ratio)
and in 6-th gear at 1500 gave 55 mph.

I am attempting to build this truck to achieve the best possible gas mileage.

I don't have a good intuitive idea what the ideal RPM should be for - say 55 or 60 mph - given the large torque available
from the 5.9 compared to how little it should take to go 55 or 60 with no load other than just rolling resistance.
(don't need to pull anything bigger than my 16 foot aluminum boat !!!!!!!)

Does anyone have some feedback on this question ? (not sure what I could do to easily change the ratios...... just wondering how far off ideal I might be)

Tim

[cmann250]

What's the weight of the truck? If it's a crew cab dually weighing close to 8000 pounds, I think 1500 rpm may be lugging it a bit. In my 5300 pound truck, I feel like I'm lugging it if I'm doing any less than 50 mph which is 1250 rpm in O/D.

[Tacoclaw]

http://www.grimmjeeper.com/gears.html

According to this, my ~5400lb truck turns ~1400 at 60, 1500 at 65, and 1600 at 70. It is extremely happy at 70, the rattles all quiet down and if it wasn't for the wind noise and blower howling it would probably sound about like the old man's 3rdgen.

1300 rpm starts making the trans a "little" upset and had a tendency of falling under the 67mm charger when accelerating, but when I had the 62mm the trans' death throes were the only problem. Even maintaining speed up hills with the 67mm at ~1500 would start hazing and bring up EGTs, but once on flat ground it wasn't a problem. Once again, with a smaller turbo it was always spooled before the trans was at a happy rpm.


I will say, as unlikely as it seems, that my truck seems to get much better mileage if I just stay in 4th when I'm driving local roads. 55 is around 1650 and 60 is 1800 in 4th, it's a really handy setup for when I'm towing something.

[oldestof11]

Best mileage in any of my trucks was around 1800-1900 rpm on a stock charger.

[RCCUMMINS89]

stock with 3.54... 33-35 with 3.73....37 with 4.10

I've found those seem to be the best all around mileage ratios. Anything else and one starts sacrificing city or highway mileage for the other.

[jethro]

It has an hx-35 - which my impression is stock for a 93 engine.
The truck is an extra-cab .... with the little 1/2 rear door. There is no regular box bed- it has a fairly light built
steel flatbed which they had a 5-th wheel arrangement on. (will remove that at some point if anyone needs one)

It has been lifted 6 inches and has the big fat 33 inch tires. I was considering removing some of the lift and going with a smaller tire.
sounds like I may have some room to possibly put on a somewhat smaller tire. At least with lots of forward gears
I can all ways downshift if 6-th is bogging down. or go to a smaller turbo housing (?)

Its pretty flat here in florida so not much in the way of hills.....

Thanks for the replies !

Be doing the KDP and repairing oil pan (removing metal screw and tigging up hole) next. Also ordering the flywheel and clutch/pressure plate stuff in the next month or so.

[PToombs]

Smaller tires will cause it to rev higher per gear than it does now. Maybe lose the lift and go with skinnier tires, not shorter.

[jethro]

yes a skinnier tire had occurred to me-
what would be good- like a tire which might be on an f-600 chassis ? Like a downsized version of a tractor-trailer tire ?

I will probably try it first with the big tires and see how it does.....

T

[CumminsPower59]

Like 22.5" heavy duty truck tires or 19.5" medium duty tires? Not sure what you are getting at...?

[jethro]

Sorry-just wondering aloud what application of tire would have pretty good height but be narrower-
are there common wheels in that range that would fit the hole pattern of the super duty ford ?

[cmann250]

I think you can get direct bolt on rims in a 19.5". Maybe American Force Wheels?

[CumminsPower59]

There are a few manufacturer's that make 19.5", 8x170mm wheels. Most 245/70 19.5's are a full 33" tall. 225's are a shade smaller and 265's a little larger. Pizza cutter 8R19.5's are about 33.5" tall.