Recent higher MPG

[embee]

Originally Posted by Harry_Mc

....doing 55 everywhere... i heard somewhere thats the optimum speed for fuel efficiency...

That's a common myth (or misunderstanding anyway). It comes from the old official fuel economy figures required of manufacturers, which were "urban, 56mph, 75mph". The numbers actually came from a specified drive cycle (done on a chassis dyno) simulating a typical urban journey, and then a constant 90kph (56mph) and 120kph (75mph). The fuel economy at each condition was then weighted (40/50/10) to give a single "combined" figure.

The USA used to use a similar (but different) method of city/highway tests (weighted 45/55 IIRC) to give the "gas guzzler" number.

The European test has been superceded by new ECE tests (** see edit) which give rise to the official "gm/km" rating used for taxation purposes.

Because under the old system the mpg at the "56mph" condition was inevitably the best of the 3 published numbers, people assumed this was the "best" speed to drive at, whereas it was just the best of the 3 conditions tested.

In most (petrol) cars the best fuel economy is typically around 2000rpm in top gear, often in the ballpark of 45-50mph, but of course it actually depends on a lot of variables. Typically petrol engine thermal efficiency drops quite dramatically as you reduce engine speeds much below around 1750rpm due to heat loss during the longer cycle times, so there is a trade off between keeping vehicle speed down (and thus road load power requirement which is more or less speed cubed) but not letting engine speed get too low.

Car sized diesels usually give best economy at somewhat lower engine speed, say down as low as 1500rpm, but that depends too. Bigger truck diesels typically have "green bands" on the rev counter in the 1250-1500 range.

Bikes are tricky because of the ridiculously high power/weight ratio typically and the generally higher engine speeds. Bike aerodynamics are truly awful so speed is very significant, but at lower speeds the engine is usually running at such light load that the specific fuel consumption is dreadful. The only answer for a bike is smaller engine and lower speed.

**(EDIT - sorry that's a little misleading, the constant speed "56" and "75" figures are still quoted by the manufacturers).

I get about 120-130 per tank on the zx6 around town and motorway i've got 160.

I just don't worry about mpg and just make sure I'm not going to run out of fuel.

I bought the bike for fun,not interested in mpg

Originally Posted by embee

Bikes are tricky because of the ridiculously high power/weight ratio typically and the generally higher engine speeds. Bike aerodynamics are truly awful so speed is very significant, but at lower speeds the engine is usually running at such light load that the specific fuel consumption is dreadful. The only answer for a bike is smaller engine and lower speed.

It's a less aerodynamic shape than a car, but of course there is still far less drag than you get from a car because of a relatively very small frontal area.

Imo bikes need lower revving engines which make far less power per litre than is currently "acceptable". (Who's going to buy a "big bike" with a heavy engine that makes less than 100bhp per litre?). Heavy, big-fronted cars can manage 60mpg from 1.0 litre engines offering 60bhp, there's no reason why a "feather" weight motorcycle shouldn't exceed that significantly.

I generally get 130ish from a tank (to the light, K4 with just a solid light), less if I'm doing short blast type travelling, more if I'm using it all in one trip.

[embee]

Actually the aerodynamic drag isn't a different as you might think, a typical bike+rider is around 0.75m*2 and a small car (as you indicate) is around 2.0m*2 , or around 3 times the frontal area, but the bike drag coefficient is at least double that of a good car.

See table in Wiki for example (so it must be right!!), it shows a cyclist but a motorcycle+rider will be pretty much similar (probably worse if anything due to the bike's cooling system, mirrors etc.)

A small bike producing 10kW will just about do 70mph, and this isn't all that different to the road load for a small car, believe it or not, somewhere around 15kW typically.

I'm not suggesting that a bike in terms of coefficient is as slippery as a car, but whilst you can't only look at area, you can't only look at Cd, either. CdA is the figure that counts, as you said.

Bikes clearly are held back by less air, hence 190bhp being good for near 200mph from a sports bike.

So whilst I shan't argue that motorcycles are usually fuel-efficiency friendly shapes, I'd definitely say that aerodynamics are not the principal cause of their typically lack luster MPG figures against automobiles, I'd also say that 10kW vs. 15kW at just 70mph is a fairly large % difference, if those figures are accurate.

[yorkie_chris]

Turns out that snorting great carbs, cams with big pointy bits on and ports big enough to lose a gibbon down aren't very good for economy. Wow!

There's a guy put a smart car diesel engine in a triumph tiger, he's riding to australia on it and averaging 100mpg with a 100mph top speed. I want one of those!

i have sk6 fuel injected and im getting 130miles between fuel lights. Its consistently very very close everytime, however I do ride it the same way, speed limit in high gear with the occasional scenic route for some twisties. but its working out at £13 = 130 miles. I know it took £17 from empty so by logical equation there is 40 miles left in her from when the light comes on/starts flicking on. I doubt I could get much more out of it at all though.

Originally Posted by yorkie_chris

Turns out that snorting great carbs, cams with big pointy bits on and ports big enough to lose a gibbon down aren't very good for economy. Wow!

who would have thought it eh!