That is the gear you should be in when pulling mountains. Now, for those of you who have a double overdrive transmission, did you know in order to run in direct gear you have to drop down two gears? Therefore, a 13-speed double over has to run in 11th gear and an 18-speed has to be in 16th gear to be in direct gear. The transmission only makes it possible to get to the predetermined cruising speed, it is NOT intended to determine the cruising speed. The differential gear ratio is what determines the cruising speed of a vehicle, NOT the transmission. Overdrive is NOT the place to make up for speed, the differential is. If you need the ability to start on steep grades, just make your first and second gears lower. However, I do run in direct gear when I’m pulling long grades, bucking head or side winds or driving slow. My Kenworth has a single over 18-speed that I would not give up for the world. I know many of you are thinking you don’t want a 10-speed transmission – you love your 13-speed or 18-speed and that’s okay (I do too). Many fleets out there are running this gearing setup with 10-speed direct transmissions. With this gearing setup, everybody seems to think they will not be able to start out on a hill, but this is just not true. However, you can run your truck in direct gear if you install 2.65 (or better yet 2.79) rear gears. So, now you’re thinking, “If I drive in direct gear my speed will be in the low 50 mph range – I can’t do that!” Well, if you have 3.36 or 3.55 gears you’re right – you can’t drive in direct gear. Instead of power going to the rear wheels, it’s being lost through heat in the transmission. Overdrive gears are where most of these losses in power come from. We always knew that the direct gear was the best gear to be in when pulling a hill, but did you know that it is also the most efficient gear to run in when not pulling a hill? In this case “most efficient” means less power lost to the drivetrain, more power to the drive tires and better fuel mileage. This month we will look at the best transmission and gearing setups for taking advantage of using direct drive. These trucks are custom built to your specifications and you get to pick every option you want. In last month’s article we talked about building our own custom 2009 Peterbilt 389 glider kits with our engine and choice of transmission out of our shop in Saxonburg, PA. In this Signature Series truck, both Kevin and I have combined our knowledge and built the ultimate truck for fuel mileage and longevity – something we feel the readers of 10-4 Magazine are particularly interested in. This truck is a combination of features that Kevin Rutherford and I have come up with over the years of our experience in the trucking industry. As a 10-4 Magazine reader, you know from previous articles that we are currently building what we call the “Signature Series” truck.
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