Ask Kevin: Why Does My Kawasaki KLR650 Get The Same MPG As My Triumph Tiger 1050?

Question: I have two motorcycles: a 2009 Kawasaki KLR650 and a 2007 Triumph Tiger 1050. The Triumph has nearly three times the power and twice the displacement of the KLR, but they both get about the same gas mileage. How is it that the Triumph, with more power, more gross weight, and more moving parts, uses the same amount of fuel per given distance? I love my KLR, but where is all that fuel going that isn't being converted to power?

John Timoti

San Clemente, CA

Answer: Let's assume both bikes are operated on the freeway, at freeway speeds. Yes, the Triumph is 60 percent larger in displacement and weighs considerably more. But it has a fuel-injected engine with a high (and fuel-efficient) 12.0:1 compression ratio, versus the carburetor and 9.8:1 compression ratio of the KLR. Carburetors have to be jetted rich enough to be safe from lean mixture on the coldest anticipated riding day, while electronic injection continually "re-jets" itself to deliver correct mixture as air temperature and pressure vary. For this reason, the Tiger will run at all times on a correct mixture while the KLR runs rich at all times except the coldest days.

Torque is not the only variable that increases with increasing compression ratio; fuel economy also increases (half of the reason for the extreme fuel economy of diesels is the high compression they run). But probably the biggest effect on mileage is engine revolutions per mile. The Triumph Tiger, a purely highway bike, is geared for comfortable and economical cruise at moderate engine revs. The lower the revs, the lower the engine friction. The KLR, which has some off-road capability, must have a lower first gear even though it has only five transmission speeds to the Tiger’s six, so in fifth I’d bet the KLR is turning fairly high rpm and generating a fair amount of extra friction loss. Normally, you can expect a bike with fewer cylinders to have lower heat loss (because of lower total combustion-chamber surface area) than a bike with four cylinders. You could see this in Superbike racing, when 1,000cc fours needed visibly bigger radiators than did 1,000cc twins. This effect, however, seems unable to push the KLR’s mpg far toward that of the Tiger.

While this is not, I admit, a rigorous answer quoting specific numbers, it does indicate important differences between the two bikes’ fuel usage.

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