Posted by: rbmcarriers | May 20, 2008

The Speed Limiter Debate

Today’s Trucking posted an interesting article regarding the proposed Speed Limiter on the trucking industry.  They make a very valid point on the “sweet spot” of running your truck.  Here’s the article in full, your views and comments are appreciated.

Road Speed Trap: Can slowing down hit you in the ‘sweet spot?’

TORONTO — Slow down, save fuel everybody says. To a point, they’re correct, but there’s another wrinkle to consider when implementing a road-speed reduction strategy (or having one pushed on you by law).

Reducing road speed runs the risk lowering engine speed to a point where drivability suffers. Reducing road speed by 10 mph today could cost you more than leaving it alone. Today’s engines are much more sensitive to speed, and don’t deliver optimum performance or fuel economy if you stray a couple of hundred rpm above or below the so-called “sweet spot.”

Consider the following example, calculated with a Mack drivetrain (other OE drivetrain combinations would yield similar results with similar componentry):

A truck spec’d to run at 70 mph has a 10-speed transmission a 0.74 overdrive, 11R22.5 tires with 3.36 rear-axle ratio. Running at 70 mph gives us an engine speed of 1,425 rpm. Performance is good, as are power, gradability, and fuel economy.

Speed limiters could interfere with a
truck spec’s optimum efficiency

Reduce road speed to 65 mph, and you lower the engine speed to 1,323 rpm — below the engine’s sweet spot. Performance and gradability are marginal because engine speed is too low. If you run one gear down to improve performance, fuel economy will suffer. In 9th gear at 1,813 rpm, you’re well above optimal fuel economy.

Reducing road speed to 60 mph gives us an engine speed of 1,221 rpm — well below the engine’s sweet spot. Top-gear performance will be extremely poor. You’d really have to run one gear lower, but performance would remain poor as the engine speed is 1,673 rpm, still well above optimal fuel economy, plus you’re final drive ratio is now less the direct.

“Multi-speed transmissions such as 13- and 18-speeds tend not to be impacted as severely as a 9- or a 10-speed,” says Mack’s powertrain marketing manager, Dave McKenna. “The top gear step in a 10-speed is 26 to 27 percent. It’s closer to 16 percent with the multispeed transmissions.”



  1. There are some road speed limiters out there that do not limit the engine output thus solving this issue! They limit the vehicle speed but not the engine output if you would like more information on this let me know!

  2. Tom, there have been a number of articles written on this very topic of engine output and the damage that speed limiters could have on the engines of our very expensive machines!! If you can provide any links that would be useful to our readers – thank you!

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