So here’s a question: which motorcycle engines are the most efficient? And before you all jump up and tell me that the smaller the engine, the more economical it is, that’s not actually quite what I asked, although many perceive fuel economy and efficiency to mean the same thing. Conventional wisdom holds that because motorcycles are generally not very aerodynamic, it all comes down to power; the more you make, the more fuel you use. And in general, the bigger the displacement, the more power you produce, the more weight you’re carrying, and therefore the more power you use to get up to speed in the first place. But, as usual, things aren’t quite as cut and dried as all that…

Let’s look at a few examples, shall we?

First up, my old Peugeot Trekker 50. A 50cc two-stroke engine maxing out at 40mph. Small? Yes. Slow? Certainly. Fuel-efficient? Well, no. It can’t have been making much power, a rough calculation suggesting that such engines output around 5bhp, and it certainly didn’t weigh much more than 90kg fully fuelled. Despite this, I averaged 65mpg over the year I owned it, which isn’t far off what my 250kg 90bhp Suzuki Bandit is getting.

My high-tech modern Honda PCX125 may well be the record holder on my books, averaging 120mpg over the meagre 6 months I owned it, and that was putting down 11bhp while hauling a titanic 124kg. Ignoring weight for the moment, and using the more sensible litres-per-100km measurement, that means my Peugeot was getting 1.18 bhp per l/100km, while my PCX managed 4.23 bhp per l/100km.

My Bandit only averages around 53 MPG (5.36l/100km) across all kinds of riding, its more powerful 656cc four-cylinder engine drinking much more fuel than the tiny little single in the PCX, yet it manages a bhp/l/100km score of 16.79.

Power!!!

Bear with me while I invent an all-new measurement of efficiency here. Horsepower per litres-per-100km. Let’s call it hp/litre/km or HPLK for short.

Now, those of you who have managed to follow my tangled train of thought will be furrowing your brows at this point, having realised quite rightly that my Bandit manages to accomplish a heck of a lot more with each litre of fuel than the PCX did. Sure, it can’t travel as many miles per gallon of fuel, but it makes 8x as much power whilst only using just over 2x as much fuel. It also managed to make 18x as much power for 15mpg less than the two-stroke. Not a bad trade-up, if you ask me.

A quick look on fuelly.com shows us that the BMW S1000RR, a 200bhp race-ready monster, drinks on average 6.5 litres of fuel per 100km across all the submitted results. To make this a really fair test I’d need to own one, tour and commute on it for several months, and check my own averages, but sadly I don’t have a spare £11k. Donations are always welcome, in the name of science, of course…

One more reason to buy this motorcycle.

So, while a 200bhp monster sounds like it’s the antithesis of efficiency, the BMW scores a 30.77 on the HPLK scale. Although the price-per-mile might be eye-watering as a daily commuter, you can’t argue that you’re getting your money’s worth from that engine. It trounces my Bandit, and destroys the PCX utterly in the power vs consumption efficiency stakes.

The answer lies in economies of scale – the BMW’s engine displaces 8x the capacity of the PCX, yet produces 18x as much power for just under 3x as much fuel. The smaller you make an engine, the more energy you lose (relatively speaking) in heat, the less efficient the combustion, and therefore the less energy extracted per unit of fuel. Naturally, these are peak-power numbers, and no engine spends its entire time making 200bhp, but the numbers during general riding will certainly be relative to one another. Let’s take a look at a broader picture:

I’ve tried to stick to bikes I have owned, but I’ve thrown in my Dad’s Triumph as I know he rides in a similar manner to myself, and I’ve used the NC700X and S1000RR from fuelly.com for additional comparison. In general, the more powerful an engine is, the more it gets from each unit of fuel, but there are interesting outliers already in even the limited data here.

The really interesting question is this: does power = fun? The poor HPLK score on the NC700 certainly goes some way to explaining my underlying uncertainty regarding Honda’s new engine, and reinforces the idea that perhaps it’s a bridge too far in the name of pure miles-per-gallon fuel economy. The frugal PCX scores worst, prompting again the question of whether the fuel savings are worth putting up with a power output barely in the double digits. And these calculations seem to justify something I have always gritted my teeth about, which is the apparently terrible fuel economy of the latest top-power sports bikes.

Turns out they’re not inefficient at all. They’re achieving an incredible amount with the fuel they burn. Car drivers are often heard to wonder why cars like the new C7 Corvette can get better than 30mpg from a 6.2litre V8, yet a scooter engine displacing 0.125litres can only quadruple that score. They instinctively feel that it should be getting 10x the economy it ends up achieving, and in a way they have a point.

It’s such a shame that there’s apparently no way to get the best of both worlds – power when you want it, and great mileage when you don’t.