transmission

I like 18's. Don't need every gear all of the time, but when you do they are nice to have. Fleets run 10's because that's really all you NEED in most cases hauling legal freight. You're basically looking for something that'll do the job. A 1st gear low enough to get you rolling, and a top gear high enough to keep your RPMs in the sweet spot driving down the highway, paying attention to the steps between the gears. That's where 18's are nice...smaller steps between the gears...but if you're running flat ground or lighter loads, even an 8 or 9 speed wouldn't be unmanageable. 13's are wannabe 18's...can't split the low side, but then again I rarely split the low side anyway.

Most important factor is your tire size and rear gear ratio, and where the engine operates most efficiently. You want to spec a trans that works well with the combination to optimize fuel efficiency at highway speed. As long as you've got that, next look at startability...make sure 1st gear is going to be low enough to get you rolling. If you can get the load moving, and you are efficient once you get up to speed, what happens in between is more or less personal preference. 8, 9, 10, super 10, 12, 13, 15, 18...they'll all get you there.

 

The other thing to factor in is the final drive ratio of your transmission... AFAIK, all 18s are either .73 or .74.
You kinda have to figure out where you want to cruise at. So, let's say you're looking at a truck with an engine cruised at 1350 RPMs... you'll want to see where you'll be at in relation to where you want to run and where that 'sweet spot' on that engine is.
So, you do (A x 60)/(B x C x D) = your speed at those RPMs
A = your RPMs
B = transmission final drive ratio
C = rear end gear ratio
D = tire revolutions per mile

So, let's say you're looking at a truck with 3.55s, a .73 OD transmission, an engine whose 'sweet spot' is at 1350 RPMs, and... we'll say 295/80R22.5 drive tires... your equation would look something like this...

(1350 x 60)/(.73 x 3.55 x 491) = 63.657.... so you'd be running about 63 and change at 1350 RPMs. I got the 491 number from here, and I can't vouch 100% for the accuracy of the website.. if you want to manually figure out your tire revolutions per mile, you'll first measure the diameter of the tire in inches. Then you multiply that by 3.1415 to get your circumference. Then you divide that number by 63.360 (the number of inches in a mile) to get your revolutions per mile.

Now, say you wanted to compare it to a truck with 3.36 rears and everything else being equal..
(1350 x 60)/(.73 x 3.36 x 491) = 67.257.... so you'd be running about 67 at those RPMs.

As for the 9/10/13/18 speed question, I'm an advocate of the 18 speed, but it wouldn't be a deal breaker for me. If you're running mostly flat ground in Alabama, the benefit would be less than if you were doing, say, logging trails in Washington State. You just want to make sure the torque rating matches up to your engine.

 

Measure twice. Cut once.

Youre a new driver. Where do you see yourself in two years? Don't know? Well, plan on going wherever there's money. Where's the money? Freight lanes change (for example, because of the price of oil, the oilfields are slowing down), so don't just buy a truck for one particular area. You want a good overall truck with decent fuel economy and good performance numbers. For that, I always recommend 500 HP and a 13 speed. 3.36s will get the job done, and with 500 hp, you can cruise those western speed limits if you needed to make time and still walk up most hills without too much effort. The most common ratio is the 3.48-3.55.

Usually,you don't see the 18 speeds until you get to the bigger engines, like the 550 hp range. Those bigger motors don't get good economy with the light loads, but get better economy than the smaller engines when pulling the bigger loads. So, if you're not planning to pull big and heavy, save the money and stay around the 500 hp mark.

A 500 hp motor normally comes with a 10 speed or the 13. Don't even bother with the 8-9 speeds...the gears are spread so far apart that it's frustrating. You wouldn't think so, but there's a huge difference in performance between a 9 speed and a 10.

 

I pull a flatbed with a Detroit 500hp and 10 speed. I get crap for fuel mileage but when I run in 9th at 58mph my MPG goes way way up.

 

Sheesh, we all know that 2.64's and a direct 10 speed is THE way to go for Gawds sake......... Some very smart (?) people said it's so, so it must be true.

Martin

 

Thank god we don't have to hear that retarded drumbeat anymore. "All the company drivers are jealous" never before have I considered blindly yelling at a person for the way they communicate thru written word. So very thankful for that "ignore" function the forum offers.

 

One is on permanent vacation many thanks to a mod/admin here. Not a minute too soon, as it would have been me or them before too long me thinks!
Of course, KR said it too, so it must be true....
Martin

 

Don't just look at how many gears, need to look at the torque ratio as well, 1650, 1850

 

10 works, but once you have a 13 or 18 speed, you will not want to go back to a 10.