Gearing has always been important for all types of trucks, but it's even more important nowadays for those powered by EPA '07-spec diesels. Builders have extensively re-engineered them to meet the new exhaust emissions limits, and everything needs to be pumping and spinning at just the right speeds to work most efficiently.

Some engine makes and models run a little slower than before, while others work the same. All have good operating characteristics and driveability, and should deliver decent fuel economy. But the right combination of transmission and axle ratios, along with the desired tire/wheel size, is a must.

Unless he or she is an expert – and some fleet managers are – anyone spec'ing out a truck or tractor needs to consult closely with a knowledgeable sales person or manufacturer's representative to make sure the new vehicle will deliver the desired performance and fuel economy. If there are any questions at all, the sales guy or rep who can't answer them on the spot can get the right info from the manufacturers, and so can the buyer. Go-to people include not only the engine makers, but also the suppliers of driveline components that include gearboxes.

Many '07-spec diesels run 50 to 150 rpm slower than '02 to '06 diesels, which in turn were as much as 200 rpm slower than pre- '02 engines, notes ArvinMeritor, a major axle supplier. Most fleets have stayed with the same size wheels and tires, but have made other changes. For instance, some linehaul operators switched from overdrive transmissions to more efficient direct-drive models, which require "faster" (lower numerical) ratios to keep engine speed down. In view of this trend, ArvinMeritor developed and released a 2.50 to 1 ratio for its RS23-160 axle family in late 2005.

However, with both engines and drivelines turning more slowly, torsional vibrations can become a problem. The engine produces more vibrations at lower rpm, and constant operation at low revs also sends higher amounts of torque down the driveline. Is the driveline hefty enough to take the greater vibration and torque loads? A higher-rated driveline might be in order, as is a fresh look at angularity – the angles formed by U-joints as they link the transmission, driveshaft and axles. Those angles should be as low as possible, especially during acceleration, when repeated torque jolts cause the axle suspension to rise and fall and U-joint angles to change. Such conditions can affect the choice of componentry.

Linehaul tractors and trucks that spend a lot of time on freeways and Interstate highways might be set up to cruise at 65 mph. This, in fact, is the speed often cited by many engine makers, who state what rpm their engines should be turning at 65. The desired cruising speed might actually be 75 or 55. Gearing can accommodate anything, but on real roads, a truck will move according to changing speed limits and traffic conditions and undulating terrain.

Eaton, which makes most transmissions that go into heavy trucks, points out that a multi-speed transmission can better keep the engine in its best operating range than one with fewer gears. That's why Eaton engineers developed the 13-speed UltraShift LEP (Linehaul Efficient Performance) automated mechanical transmission, whose smaller steps compared to the usual 10-speed can better cope with varied conditions. And it works well for all driver skill levels.

Of course, it all begins with the engine, and the chosen make and model's characteristics should be studied so the match with the rest of the drivetrain will be a good one. Sweet spots and operating ranges vary by engine make and model, and we list them below. Another number often cited by engine makers is desired rpm at 65 mph, though that engine rpm might also be used at another road speed for other operations. Read and heed:

• Caterpillar – '07 Cat C15s, including Multi-Torque models rated at 1,750 pounds-feet or more, should be geared for 1,325 rpm at 65 mph. C15s making less than 1,750 pounds-feet should run slightly faster – 1,400 rpm at 65 mph. And the C15 Heavy Haul, for rigs grossing 90,000 pounds or more, should spin at 1,500 to 1,650 rpm at 65 mph. As with the '04 ACERT engines, operators of '07 Cats have a wide operating range, with peak torque at 1,200 rpm but luggable to as low as 1,100 rpm. In high-range gears, transmissions should be upshifted at 1,500 rpm to get maximum fuel economy.

Cat's other heavy-duty models should turn at these rpm at 65 mph: C13 Multi-Torque with 1,750 pounds-feet, 1,325 rpm; C13 with 1,650 pounds-feet, 1,400 rpm; C9 with 1250 pounds-feet or more, 1,550 rpm; C9 with 1,150 pounds-feet or less, 1,650 rpm. Cat's medium-duty C7 should turn 2,000 rpm at 60 mph, or 2,200 rpm at 60 mph if better low-end startability and performance is needed.

• Cummins – The ISX's sweet spot is 1,400 to 1,500 rpm at 65 mph for linehaul applications. In vocational trucks, the ISX should run between 1,600 to 1,900 rpm at intended cruising speed. The ISM should run 1,500 to 1,600 rpm in linehaul and 1,700 to 2,000 rpm in vocational trucks. The ISL should operate between 1,600 and 1,900 rpm in linehaul and vocational trucks. For the higher-speed midrange ISC, it's 2,000 to 2,150 for steady 65-mph cruising and 2,150 to 2,300 for P&D and other urban applications. The even faster-running ISB, used in medium and light trucks and as the Dodge-Cummins Turbo Diesel, should cruise at 2,100 to 2,400 rpm at 65 mph.

• Detroit Diesel – For ‘07 the Series 60 is only offered in a 14-liter size, and its sweet spot is 1,450 rpm. For vehicles grossing up to 80,000 pounds, the suggested engine cruising speed is 1,450 to 1,550 rpm, which would require a lower numerical axle ratio compared to an ‘06 spec with a cruise rpm of 1,550 or more. For gross weights greater than 80,000 pounds or for vocational applications, add 50 rpm. The ‘07 variable-geometry turbocharger and a torque curve extending down to 1,100 rpm make the S60s very driveable at lower engine speeds. Progressive shifting is greatly encouraged; upshift at no more than 1,600 rpm, and pull the engine down to 1,150 rpm before downshifting.

The '07 MBE 4000 engines should be geared like the '07 Series 60s, because the M-B engines have always had an 1,100-rpm peak torque speed and their sweet spot is also 1,450 rpm. The medium-duty MBE 900, which now displaces 7.2 liters, has a sweet spot of 1,400 to 1,600 rpm, but it should spin faster at cruising – generally between 1,800 and 2,000 rpm. Vocational applications should be discussed with dealer and DDC representatives.

• General Motors – Though optimum fuel economy theoretically might be obtained at a diesel's peak torque – 1,600 rpm for the Duramax 6600 and 1,450 rpm for the Isuzu 6H and the Cat C7 – gearing a truck to cruise at those revs would result in sluggish performance. Medium-and light-duty engines need to rev faster, so GM trucks with inline sixes are generally geared to cruise at 65 to 70 mph at 2,000 to 2,200 rpm. Geared speed might be 80 to 85 mph, but trucks rated over 19,500 GVW have tires that shouldn't go over 75 mph. Class 5 and 6 trucks with the Duramax 6600 V-8 have higher power for given weights, so a 4.33-to-1 axle ratio would provide the best economy.

The gasoline Vortec 8100, which is available in GM's medium-duty and some Baby 8 trucks, spins faster than diesels. So Class 6 and higher trucks should cruise at 2,500 to 3,000 rpm at 65 mph (or whatever the desired cruising road speed is), while Class 5 and lower trucks, including light trucks with the Vortec 6000, can cruise at slower rpm. Aftermarket propane conversions can be done with GM gasoline engines.

To help achieve the best economy, GM is using Allison six-speed double-overdrive automatics in Class 2 through 5 trucks. Allison five-speed single-overdrive automatics go in Class 6 and higher trucks, where heavier gross weights require more power that's obtained at higher engine speeds.

• International – All diesels are now called MaxxForce with numbers approximating displacement in liters. Although all have been upgraded for '07, their sweet spots and operating ranges are generally the same as before. Thus the best cruising speed of the MaxxForce 9 inline-six (formerly the DT 570 and HT 570) is 1,600 to 1,800 rpm. The MaxxForce DT inline-six (formerly DT 466) and MaxxForce 7 V-8 (formerly the VT 365) best cruises at 1,800 to 2,000 rpm (as does Ford's Power Stroke V-8). The MaxxForce 5 V-6 (formerly the VT 275, and also used by Ford) best cruises at 2,100 to 2,300 rpm.

• Mack – The 12-liter ASET (application-specific engine technology) engines have been phased out in favor of new Mack Power (MP) diesels, built in Maryland by Volvo Powertrain (which also supplies engines to Mack's sister company, Volvo). All MP engines generally run at 200 to 250 rpm less than the previous ASET vocational engines and about 100 to 150 rpm less than ASET highway engines.

Mack's 11-liter MP7 and 13-liter MP8 come in three versions: Econodyne, for highway trucks and tractors, with a sweet spot of 1,500 rpm; MaxiCruise, for local and regional vehicles and for use with Allison automatics, with a sweet spot of 1,400 to 1,500; and Maxidyne, for vocational trucks, with more torque than before and a 1,500- to 1,600-rpm sweet spot. Next year there'll be a 16-liter MP10, based on Volvo's D16.

• Volvo – The '07-spec D11, D13 and D16 diesels (similar to sister company Mack's MP series engines) have sweet spots of 1,300 to 1,500 rpm (1,350 to 1,550 rpm for the lowest ratings of the D11), and should be geared to cruise there. They have torque peaks of 1,100 rpm (1,200 for the D16-600) and can be lugged down to there. Governed speed for all is 2,100 rpm, but they should be upshifted at no more than 1,900, and preferably lower, for best economy.

For vocational applications (which usually means the D13 in the VHD truck), the engine's sweet spot is 1,400 to 1,600 rpm.