Highway Finance
Spec An Efficient Truck
Last Updated: Jul 8, 2007 - 9:52:25 PM
By Kevin Rutherford
Mar 5, 2007 - 8:05:58 PM
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Transmission
When spec’ing transmissions, there are many factors to consider:
• Manual or “Auto-shift”
• Number of gears
• Direct or overdrive
• Weight
• Torque rating
The ultimate outcome of spec’ing your drive-train is to get the engine RPM in the “sweet spot” at our ideal cruising speed. We have to make sure that we take into account all of the factors that will affect this outcome, starting with the engine. The sweet spot on all engines is different, even from year to year and model to model, and especially now with emission-related engine changes. Check with your dealer as you are ordering the truck to find out at what RPM the engine will get the optimum fuel mileage. Once we have a target RPM then we can begin to choose the other components that will have an impact on the operating RPM.
For most operations we recommend starting with a 10-speed direct auto-shift transmission. There are many reasons to use this set-up, including reliability, weight, and torque ratings. Another reason is that direct drive transmissions have lower operating resistance than overdrive transmissions because of less fluid movement in the gearbox.
Gear Ratios
Gear ratios in the transmission and rear axle(s), along with the tire/wheel size, determine how fast the engine will spin at a given road speed. Each must be chosen with the other in mind. The primary benefit of proper gear ratios is better fuel mileage. Secondary benefits include longer engine life, a smoother, quieter ride, and less stress on the drive-train components, which will lower overall maintenance costs. The best way to gear for fuel economy is to “gear fast, run slow.” Spec the truck to run fairly fast, but drive it slowly so the engine loafs. Most trucks are set up so they can run as fast as 75 or 80 mph but cruise at about 65 mph, at which point the engine will turn over at 1,500 to 1,600 rpm. This is the engine’s sweet spot, where it uses the least fuel. Exactly where that spot is will vary with the engine make and model, so check out its specifications before choosing or approving the gear specs. Of course, engine controls can be set to limit top road speed no matter what mechanical gearing allows.
Axle ratios are said to be “fast” or “slow.” All other specs being the same, the higher the axle’s numerical ratio (for instance, a 4.11), the slower the truck can run. The lower the number (like a 2.90), the faster the truck can run. You can pick either ratio type, or something in between. Just be sure the transmission’s topgear ratio is something that works well with the axle. An overdrive top-gear ratio is usually paired with a slow (numerically high) axle ratio, and a direct-drive transmission is used with a fast (numerically low) axle. A direct-drive top gear can save fuel. That’s because it involves fewer gears in the transmission and less churning of lubricant. Controlled tests show the reduced friction of a direct-drive transmission saves up to 2% in fuel at cruising speed. Some fleet managers have measured savings in tank mileage, and you may or may not see it at the pump. The more varied your cruising speed and operating conditions, the less actual savings you’d get from this or any other component that is most effective at relatively high speeds. For optimum fuel mileage you should spec a smaller tire such as low profile 22.5 matched with the proper gear ratios to get you to the sweet spot. Smaller tires have less rolling resistance.
Check back next month and I’ll continue this series with some basics on financing, insurance and contracts. If you have any questions call my office at 888-262-8585 Remember be safe be profitable and “Master the Journey”.
Kevin Rutherford
The Rutherford Group
www.cmcseminars.com
trucktax@mac.com
888-CMC-8585
888-262-8585