Caining my ST170 about 15mpg.
If I wanted economy I'd go out on my Cagiva Mito. 1 week on £12. Now that's economy.
Not sure what I'd get if I thrashed the MX5. In normal driving with a bit of thrashing the least I've got is about 28mpg in UK gallons, which isn't shabby at all. I have heard stories of people with MX5s on throttle bodies getting 10mpg though
I seem to remember from when I used to frequent bike forums that most people on Mitos, or indeed any other 125cc smokers, used to get around 50-55mpg.
So it's good considering the performance, but also pretty pathetic at the same time for something that only carries yourself, and perhaps a pillion at an absolute push. 4-stroke 125s can get a fair way north of 100mpg, though of course 4-strokes don't have the power of 2-strokes.
Most bikes aren't really economy machines though. They aren't particularly aerodynamically efficient and they're generally tuned much more towards performance. And ridden in a style that makes use of that performance. The bare economy figures don't sound too bad but then you have to consider that they're very light and
still only manage the figures they manage, usually with only one person on board too.
The economy sweet spot with bikes seems to be 125cc four-strokes, either geared like a CG125 or something, or CVT like all the scooters. They're easily the least polluting too, apart from the very few 50cc 4-strokes on the market.