Gear ratio!!!!

With 255 tires it sounds like a usable set of gears to me.

 

I might be inncorrect here ,but my thinking on an 18 speed wae that the last two gears were overdrive and double overdrive. 2.93 rears with an 18 speed and 11RX24.5 tires should put you up around 90mph at 1500. A friend of mine has 6 geared with 3.55 rears and tall 24.5 rubber with the 18 speeds and they run 75 at 1500 in high gear. 90 mph is a guess maybe faster or slower but you could run and not use the last 2 gears and be about 60 depending on tire size.

 

You should factor in the smallest possible tires you can run so you are not screwing yourself trying to make height (vans, p/u's). But when you do decide on tires, be sure to check that they can be readily had out on the road.
Former company maint. director spec'd out new trucks from Cottrell with these odd size tires on them. Yeah you could put an E-350 van over the cab and make height. Problem...you could not get them anywhere when a truck is on the road and needs one. Only Swift was running that size tire. What a fiasco that was.

 

nothing like spending 135k and asking questions afterwards

 

It would depend on the 18-speed. A buddy's (and I'll have to see if I can dig up the model number and everything on it if anyone is interested) is technically a double-over, but, effectively a single over, as his first overdrive ratio is something like a 0.98, so, for all intents and purposes, a direct-drive, while the second overdrive gear is lower (0.86, I believe). His last 3 gears are very close ratios, but, no direct drive ratio, as the 3rd to last is just over 1:1, and the second to last is just under 1:1, while the "big hole" is more of a normal overdrive (though a short overdrive) ratio.

Of course, at the end of the day, you can always swap near gears into the punkin.

There's a question for a serious numbers-cruncher, rather run in a slight underdrive or slight overdrive when no direct-drive is available. Will the slight reduction in speed in underdrive overcome the reduction of RPM in the first overdrive? Then again, this is a 27,000lb. (bobtail) double-framed 379 Peterbilt, so, uh, fuel-mileage is kind of lower on the list.

 

This is a 379 Pete. 550 Cummins / 18 speed. Should be a nice driver.
More curious...who is building the headrack/stinger/trailer? I would get with a good Cummins rep to find the optimal trans ratio, rears, tire size to keep the 550 in it's optimal power band. Like between 65-85. Spec'ing for anything faster (and there will probably be a ton of disagreement) in my opinion is a waste. Over 85 in a big truck is way too fast.

 

WOW, unbelieveable isn't it!

Personally, I think it's geared too fast due to the aerodynamic drag that car haulers have.

 

I like my 3.55's in my truck but i run alot of hills and think that if i had 2.90's i would be running around with my 4-ways on too much...

I used to run a daycab with a 6nz 550hp cat and 4.11 gears, man that thing would've crawled up a tree!

 

i learn something new everytime i sign on to this place

i am thinking that Overdrive means the engine turns less than direct (1:1) and hence less turning, less fuel

i am not understanding how overdrive can reduce fuel mileage, except for lugging the engine