Gear ratio!!!!

What's your transmission? I ran two trucks with 2.93s... one with a 9 direct, one with a 13 direct.. and they both had me turning 1500 right at about 60 MPH. With a big motor, fuel prices the way they are, and especially if you're only running five axles, might seem a bit excessive.

 

OD or direct, though?

 

Transmissions have gear ratios, too. There'll be an input gear and an output gear. When the two have the same number of teeth, they spin at an equal ratio (1:1). That's direct drive. A smaller drive (input) gear in relation to the driven (output) gear causes the output gear to spin more slowly than the input gear - this is gear reduction. A larger drive (input) gear in relation to the driven (output) gear causes the output gear to spin more quickly than the input gear - this is overdrive.
Eaton Fuller has their various nomenclatures for different transmissions... for example, you might see something like RTO18618 or something similar (there's also RTLO, RTO, RTX, etc.).
The "T" indicates that it's twin countershaft... what the rest means depends on what you have. I don't really feel like putting the complete list on here right now.
The first two numbers will indicate the nominal torque rating (x100). Third number indicates the series (in this case, "6" indicates multi-mesh gearing), and the last two numbers will be the number of gears in the transmission.

 

If you already ordered the truck. Asking now is a little late isn't it?

 

That was what I was thinking too....

 

And I think the final letter in the transmission may denote what amount of overdrive it has. With 13 speeds you can get double over drive, so I would guess that 18's would be similar.

 

If it's an RT (direct-drive) 18-speed transmission, 2.93s would be usable, but, you would be somewhat limited on your top-end speed (kind of like running 4.44s with an 18-over).

If its an RTO (overdrive) then you can forget the last gear. The final gear with those rear gear ratios will be so wide/ long that the truck won't be able to push it withouth either a.) a lot of power, or b.) a significant downhill.

There are guys these days who are doing this, however, as driving in direct-gear is the most efficient gear to have a truck in because there is no parasitic loss of power (theoretically speaking) as you have with under or overdrive gears.

Look inside the door jambs, there is typically an abbreviated build-sheet on one of the doors, it might list it there. If it doesn't, you'll either have to get a build-sheet pulled on the truck, or crawl under and see if you can find a data-plate on the transmission.

Be wary, however, of starting on inclines with that gearing. If you're not heavy, the truck should pick it up, but, you might be hopping a bit getting 80,000 started on the side of Vail. If you flat land a lot, its a usable set of gears, even with an over-drive 18-speed, just forget going beyond direct-gear in the transmission. And, trust me, with 2.93s and an 18-over, you wouldn't want to be in overdrive, that gear would have you well past 100 mph.

You can also effectively gear-down the truck by running smaller tires on the truck. A buddy of mine has 3.08 rear-ends with a 13-over in his truck (0.73 overdrive). In many trucks this would be egregiously fast gearing, but, he runs 255 rubber on the truck, which is a significantly smaller tire than an 11R24.5 (commonly referred to as "Tall 24.5 rubber").

So, whether or not an 18-over with 2.93 rears is a "good" set-up or not depends on a lot of factors.

Hope this helps.