Gear fast, run slow. It’s a universally accepted principle of linking engine with drivetrain to give you a balance of performance and fuel economy. But how about spec’ing to run 105 km/h at 1,325 rpm?

Caterpillar suggests exactly that with some of its new ACERT (Advanced Combustion Emissions Reduction Technology) engines. No, such a spec won’t help you battle mountain passes or haul frozen fish around the Maritimes. But if you want top fuel economy-with surprisingly good performance and drivability-in a rig that spends most of its life on open highways and at 80,000 gross weight, Cat has some intriguing ideas.

For example, you can mate a C15 ACERT Multi-Torque 435 (435 hp with 1,550 lb ft of torque, rising to 1,750 in the top two gears) with an Eaton Fuller RTOC-16909A Top 2 transmission. That’s a nine-speed that’s essentially a 13-holer with the splitter mechanism removed (though convertible to a normal 13) with fairly close steps and two overdriven top ratios of .86 and .73. Out back you’d need a very fast axle ratio giving you 90 km/h at 1,121 rpm and 105 at 1,325. I recently drove that spec in a Kenworth T2000 grossing 80,000 lb out of Cat’s Technical Center in Mossville, Ill.

The key to Cat’s recommendation of the convertible transmission-conceived partly to answer the American fleet manager’s penchant for 9-speed transmissions-is the very close 17-per-cent step between those two overdriven top ratios of .86 and .73. You can get those same top ratios in the Eaton Fuller 13-speed box (a far more common sight here) or the 18-speed, for that matter. While ZF Meritor has yet to mate its FreedomLine transmission with the C15, the 16-speed model would offer an 18-per-cent step between 14th and 15th gears, and 20 per cent between 15th and 16th-making it a promising option for a low-rpm spec. There’s 29 per cent between the top cogs on the 12-speed version.

In my drive of the 1,325-rpm Kenworth, I was frankly surprised at how well it worked. In three hours of cruising, including several long grades ranging from 3 to 5 per cent in and out of the Illinois River valley, the transmission made just two downshifts from top gear.

Being a Top 2 transmission, shifts between the highest two gears are automated, and Cat lowers the torque peak from 1,250 rpm to 1,150, so the driver has a little room to maneuver. The electronics also limit vehicle speed and activate gear-down protection so that drivers aren’t tempted to run on a lower rung.

Cat knows it’ll be a challenge getting drivers to buy into this low-revs thing, but I found no driveability issues at all.

The test rig’s full-load upshift point into top gear was 1,400 rpm, and it shifted down from the top at 1,150. That downshift point is programmed to be 1,225 rpm if the engine load is 100 per cent and the deceleration rate is more than 10 rpm per second. Heading into a long grade at cruising speed, and with cruise control on, the revs would fall but I could feel that extra 200 lb ft of grunt.

The two times where a downshift happened, I didn’t have to make another as the rig crested the hill with the tach reading 1,100 or so. The engine didn’t mind, and in fact that’s now the lower end of its operating range.

Many owner-operators cruise at low rpm anyway to cut fuel costs. It’s restrictive in theory, even if you only pull 80K, and I was skeptical going into this test. After all, fuel economy-and driver satisfaction-is largely a matter of the number of times you shift. And with such a narrow gap between cruise speed and maximum torque output, I had visions of using that gear lever a lot. In practice, no sweat.

There’s another C15 MT model, the 475, with 1,650 and 1,850 lb ft, and two C13 MTs: a 410-horse version (1,450/1,550 lb ft) and a 430 (1,550/1,750 lb ft). With non-Multi-Torque C15 models pulling 80,000 pounds or less, you can still spec for 1,325 rpm at 105 km/h provided that you have an engine producing 1,750 pounds-feet of torque or more. Less than 1,750, Cat suggests you gear it to cruise at 1,400 rpm. Non-MT C13s should be spec’d for 1,450 rpm at 105 km/h. For higher gross weights, you should spec a C15 of any type to cruise at between 1,500 and 1,650 rpm as you’ve been doing all along.

In every case, you’ll have to look carefully at your specific startability and gradeability needs.

So what about ACERT? Cat is the only engine maker to use something other than exhaust-gas recirculation (EGR) to meet last fall’s stringent new emissions regulations. They did it with a combination of new electronics, variable valve actuation, combustion-chamber changes like higher cylinder pressure and compression ratio, higher fuel-injection pressure and multiple injections per combustion stroke, twin turbochargers linked in series with electronic wastegates, and an oxidation catalyst incorporated in the exhaust system.

Two other key changes are in bore and stroke: the C15’s displacement is now 15.2 litres, up from 14.6, and the C13 is at 12.5 litres, up from 11.9 on its predecessor, the C-12 (note that the hyphen has been dropped on ACERT-engine nomenclature). The twin-turbos on both the C15 and the C13 meet ACERT’s demand for high air flow, and give a lot more overall boost (up to a whopping 42 psi) although the gauge in the Cat engineering test truck hadn’t been calibrated to reflect that higher value. The two turbos actually loaf along, the one on the high-pressure side turning shaft speeds about 80 per cent of normal, and its mate turning slower still at 60 per cent.

Aside from the unique cruise rpm, this C15 delivered the solid throttle response and smooth, pleasing torque flow I expect from a Cat engine. I like multiple or changing torque output-the more, the merrier, I say-partly because they let you spec a less expensive drivetrain. You spec for the lower torque figure because the higher output only happens in higher gears where its stress on driveshaft and U-joints and axle is light.

C15s are available in horsepower ranges from 435 to 550, with up to 1,850 lb ftof torque. Next year Caterpillar will offer 600 horses with models offering both 1,850 and 2,050 lb ft of torque. Displacement is not yet decided, but my own horsepower preference is. Forgetting the health of my bank account, and failing to be a responsible commentator, I’ll take 600 ponies any day.