How do you build a fast truck?

The ratio change in the factory built RTOO compared to the tradiational RTO in the low hole is 12.13 for the RTOO and 12.56 for the RTO.

In the 12513 conversion the gearing would not be different from a normal overdrive 12513 because L, 1, and 2 are not changed just 3 and 4 are switched around.

 

I have a 475 C-15, 13sp, 3.25 rears on lowpro 22.5. 80 mph is a little over 1600 rpm, and it will do it and then some. On the other hand, I don't trust my tires past 75 for short bursts and am allergic to fuel pumps. So I don't do that very much.

Besides that, I set my speed limiter to 77/79 foot/cruise just for the sake of liability reasons. So I'd have to get my ECM reset or be going downhill to hit 80 if I wanted to these days anyway.

Even the guys with power upgrades don't pull 6% grades at 80. Unless they have some magic tuning and a seriously upgraded cooling system, they'd be in meltdown before reaching the top if they even could do it.

 

Heck my old company spec Prostar with a 475 Cummins ISX and a 10 speed would run out of gear at 99 to 100 mph...
I could cruise 75 to 80 all day and get 5.9 to 6.5..
Not a large car by any means but would get down the road.

 

I have ran from OKC to the NM line with some fuel haulers and we ran 90-110 for MILES !

Back in the day (or night I should say) ran I20 across Louisiana dozens of times at 90-95. But with age and $$$ you learn to slow down

 

I don't go to the low side of transmission for a 6% grade but I don't pull it in high gear by no means, it takes a big winding grade to get me below 8th and 9th with a 80000 pound gross. I usually don't like to run at it and downshift but rather try to pick my gear at the bottom gonna end up at that speed anyway I think its easier on the driveline. I had 3.70 when I first built the engine but after a learning experience they are 3.55 now, can't tell any difference in the two but I had both 3.55 when I broke the rear diff. As Red said the faster you go the more wind resistance the harder on fuel it gets.

 

There is thread in here explaining why its preferable to use high rear gear ratios than going double over on a transmission to gain speed and increase fuel economy. I an old school and believe the double over 13 speed was the way to go and stay with 3:90's or 4:11's for hill climbing and the legs on the overdrive for the flats. But after reading the threads, there saying to go with 2:64's and stay with a direct drive transmission. Also have read similar articles in performance forums, magazines. SO WHO'S RIGHT ?

 

You can calculate the speed easily, but the amount of power it takes to overcome the forces at those speeds is high.

Take our T600 for example, 13spd (0.73 OD), 3.36 rears, 11R22.5s, c13 430hp. Theoretically this truck could do 110mph at 2100rpm, but I think it would take a few miles and a very light load to hit that. Although when we bought it the max registered speed was 89mph, so somebody shook the bugs out of it before us. We run 70-72mph with it doing shorter hauls with decent hills (7% and 9% in some spots) hauling milk and average between 5-6mpg. Its nothing fantastic but if we had a dedicated driver we would probably be able to afford dropping to 65mph or so.

 

I believe that high ratio and direct will save fuel but as stated 1 or 2 percent. How much is 1 or 2 percent of lets say 6 MPG not a lot. When you go really high in the rear with a direct if you don't have a deep it kills you moving a load up to speed and hills will work you. To much overdrive on a low gear set will spin the ring and pinion to fast and create to much heat in the differential in my opinion. So I always run 3.36 to 3.70 or so out back to try split the difference. With an overdrive you can always come down for the 2 lane roads which we run a lot of. Direct usually works good for those speeds with the ratio I run. The only way I can truly see the high ratio with direct real advantageous is running long distances at constant speed which once loaded quite a few do. When I was coming up I remember everyone running tall rubber for fuel mileage, now I hear a lot of lo pro gets the mileage.