3.73 vs 2.89 ratio with a Detroit ??

go with the 13speed.......

 

The 13 speed would give you more flexibility. You can split high range which will give better fuel mileage in the hills and mountains. You might check on the 9 speed to see if it can be converted to a 13 speed. If so, you could run it in direct for excellet fuel mileage. You would still have two overdrive gears if you ever got freaky!

 

I'm guessing 2.89 with a 9 speed is probably a direct drive 9 speed i think i would go for the 13 though for drive-ability.

 

according to the calcutlations they are about the same final depending on drive tires. by final i am not talking of the tranny talking at the wheels tranny rears and tire. but if it where me i would take the 13 speed since it is a 13 and run it a while maybe drop some 3:55 or alittle smaller would got to low though you will be dropping top gears to maintaine highway speeds and some rpms as 12.7 do best around 1450 - 1600 rpm when you get them down around 1400 you will hear motor rattle. i think 13 speed with 3:55 with 11r22.5 or LP24.5s would give you good all around results. since it seems you will be light you dont need alot of power but you will have ok power for pulling the long big hills still beware alot of cummins guys say run low rpms...newer cummins are made to 12.7 detroit aint. to get best results everything needs to match motor tranny rears and tires. and matched to what you are doing...thats the reason i gave what i gave now for me that would not work i would need the 3:73 as i am starting flatbed again and will be heavy hauling so i need more power. the less a motor works the better the MPG

 

Maybe, it could be a single over with 8th being direct.

 

i cant picture anyone specing a truck with a overdrive 9 speed with 2.89's and a 12.7 of course i'm sure stranger things have happened.

 

That one with the 9 spd and 2.89 sounds like what Swift used to spec in a lot of their road trucks. Those 9's were 9/13 convertable trannys. Running a 13 with the 2.89 wouldn't be too bad. Just use single overdrive. If you really wanted to step out, then go to double over. Either way, it would work. And if it is one of those convertable tranny's, have it converted as part of any negotiated deal. Not that expensive. Then it would be just a matter of which truck you like best.

The one that already has a 13 and 3.73 rears is not a bad choice, especially if you are one who doesn't run like his hair is on fire. That one, run at around 60 mph would be a solid performer and should get good mpg. The engine would be turning 1450 rpm on low pro 22.5 rubber or 1400 rpm on low pro 24.5 rubber at 60 mph. My 12.7 seems to have a real nice balance of power and economy at 1400 rpm.

 

That will depend at what speed you run and what gear you are using. Running in direct gear will give you better fuel mileage than running in one of the two overdrives.