Don't pull out calculators, google, or scratch your noggin. Here are some quick conversion hacks to make your life easier.
I read reviews about cars and motorcycles from all over the world. I read forum posts and opinions in Facebook groups about power and torque figures, and I always see people complaining (usually) or sometimes bragging about mileage.
The figures are always in different units though. So I have learned to make these quick and easy calculations to avoid having to use a calculator all the time.
These aren't accurate, but they're ballpark. Within a couple of percentage points. You're doing the math in your head anyway, so you're likely to take shortcuts.
And besides, there's so much more that goes into how fast a car or motorbike feels — weight, gearing, unsprung and sprung mass, traction — so this is just a small part of it.
Here they are!
In this guide... quick and easy ways to convert between
Power: Kilowatts (kW) to Horsepower, and Horsepower (hp) to Kilowatts (kW)
The key to remembering this trick is to remember that horsepower is smaller; a motorcycle or car always has more horsepower than kW.
I.e. the imperial number is bigger as the unit is smaller.
I always remember this as I think of a Yamaha R1 and its massive-sounding 200 hp. That's already crazy... but I know that 200 kW would be ridiculous.
The exact conversion is one horsepower = 0.7457 kilowatts, and one kilowatt = 1.34102 horsepower. But the astute observer will notice that the proportions are roughly 3/4, or 4/3.
To convert horsepower to kilowatts, take off a quarter (multiply it by 0.75).
- 200 hp (Yamaha R1 territory) ~= 150 kW
- 100hp (CBR600F4i territory) ~= 75 kW
- 50 hp (CBR500R territory) ~= 37.5 kW;
To go the other way, to convert kilowatts to horsepower, add a third (multiply by 1.33).
- 60 kW hp ~= 80 hp
- 100 kW ~= 133 hp
And so on.
Torque: Newton Metres to Pound-Feet, and Pound-Feet to Newton Metres
There's a similarly easy trick to convert between torque units for different motorcycles. It's actually the opposite to the metric system. A motorcycle or car always has fewer Nm of torque than lb-ft.
I.e. the imperial unit is smaller, as the unit is bigger.
I always think of the fact that a big Harley or cruiser regularly exceeds 100 lb-ft of torque. That's impressive, especially when it's down low. It's less impressive if something cracks 100 Nm of torque.
The exact conversion is one pound foot = 1.35582 newton metres, or one newton metre = 0.73756 pound feet.
But the astute observer will note that that's roughly multiplying by 4/3, or 3/4 going the other way.
From newton metres (Nm) to pound feet (ft-lb): Take off a quarter (multiply by 0.75).
- 100 Nm ~= 75 lb-ft
- 80 Nm ~= 60 lb-ft
From pound feet (ft-lb) to newton metres (Nm): Add a third (multiply by 1.33).
- 90 lb-ft = ~120 Nm
- 30 lb-ft ~= 40 Nm
Mileage: Litres/100km to MPG, and MPG to Litres/100km
This one gets interesting because not only are the metrics inverted — the UK has a different definition of a gallon to the US! C'mon guys, can't we all get along??
In Europe, people often use the term "kilometrage", but it doesn't sound natural in English.
There's no really simple one for this one — you need a calculator, because it involves an awkward division. But I did all the algebra, so here is how it works.
US MPG to Metric L/100KM — or vise-versa — use the calculation 235 divided by the number (either way).
UK MPG to Metric L/100KM — or vise-versa — use the calculation 282 divided by the number (either way).
E.g. (all examples for US mpg)
- 5 l/100km (pretty good) = (235/5) mpg = 47 mpg
- 10 l/100KM (ok for a big car) = (235/10) mpg = 23.5 mpg
Or the other way
- 50 mpg (pretty good!) = (235/50) l/100km = 4.7 l/100km
- 30 mpg (like a corolla) = (235/30) l/100km = 7.8 l/100km
By the "gut feeling" test, those numbers get the same feelings out of me. If someone told me a vehicle got 50mpg I'd be impressed; same if they told me it used 5 l/100km.
If you want to convert from US to UK mpg, just remember that a UK gallon has 1.2 US gallons. Why, I don't care to find out. You guys sort it out, which I thought you did hundreds of years ago...