[ProDigit]

For future bike owners that are looking at fuel sippers,
On number 1 is Honda CBR250, with fuel injection gets between 80-100mpg.
If you're not into sport bikes, know that less efficient body frames eat mpg.
I would say bike number 2 would be a Honda Rebel 250.
It gets between 66 and 80mpg (us, not imp) stock , and it can be raised to 100mpg with a sprocket swap.
Stock a Rebel has a 14/33t sprocket setup, hopelessly undergeared, and only good for either a mointain climber, or a hooligan wanting to look old school?
Pay $75 to buy a 15t sprocket front, and anywhere between a 30 to a 26t rear.
I tried all combinations, and found:
15/30t on a rebel is pretty neutral, boring gearing
15/28t for fastest acceleration (allows you to shift from the top of the powerband to the bottom of it in next gear, basically allowing you to constantly accelerate at the powerband), and highest top speed sitting upright of 80mph
15/27t highest top speed tucked of 87mph
15/26t highest top speed if you're small, and light, and have feet on the passenger pegs and tucked forward, 90mph.

With a Honda Rebel, you have 65mph guaranteed (headwind of below 20-30mph), and 75mph wind still stock.

If you want a tad more power, a VStar 250 will do +3mph, but consumes more fuel.

If you want better fuel mileage, and less top speed, a Suzuki TU250X does 70mph windstill, and upto 80mph top speed with a sprocket mod.
It also sips 80mpg stock, and 100mpg with the sprocket mod.
I personally would never take a Suzuki TU250X on the interstates, but it's great for town, suburban, and highway.

A motorcycle's mpg will drop to 90mpg tops at 60mph, 80mpg at 65mph, and can drop to 60mpg at 80-85mph. A 3/4 sized bike gets the best mpg.
Bikes in this class are:
Honda cbr250r/300r, CB300f, CBR300R, Kawasaki Ninja 250/300, Suzuki Boulevard S40, TU250X, Yamaha MT03, Vstar 250, KTM Duke 390, Rc390, and more....

Larger than 3/4 sized bikes, is linked to added wind resistance, thus lower mpg.
Sport bike fairing may reduce wind drag, and increase mpg by a few over cruiser/standard style bikes.

The most aerodynamic bikes are the 3/4 sports bikes.
Then the naked bikes
Then the standard bikes
The cruiser bikes, touring bikes, and dual sport bikes are the least aerodynamic.

On average,
Honda focuses on best mpg, and has smallest cc in category. They're usually also the most reliable and most efficient engines around.
Yamaha usually beats the competition by upping the ante in the cc department.
Their bikes are good and reliable, almost honda quality, and in some ways even better.
Kawasaki is usually right in between Honda and Yamaha. It builds it's engines around numbers. 300cc for kawasaki means 299cc. Not 286 like Honda, nor 324 like Yamaha.
Suzuki usually has the worst performing engines of them all.
They're like the "Nissans" in cars of the motorcycles.

On the other hand, Honda makes the worst transmissions. They're usually clunky and shift out of gear. Yamaha and Suzuki produce very smooth shifting transmissions.

Qua bike design, Honda bikes are lightest in weight with no frills.
Yamaha would be second in lightweight, and come with frills.
Kawasaki is a nice compromise.
Suzuki bottoms out usually with top heavy designs, as well as being no frill. Add that to a less good engine design, and which makes them hopelessly overpriced for what you get...

KTM doesn't have a lot of beginner bikes, but the RC390/Duke 390 is right in the sweet spot power wise, and the weight is great too.
Body design is sublimal. KTM just has an older, ugly looking dash, a vibrating engine that together with the hard seat make the bike unsuited for the longer rides.
The stock brakes are also pretty bad, so not meant for track racing either...

A 250 is most at home at speeds of between 35 and 75mph, aka city and highway, or, the slower lanes on the interstates.

If you need to do frequent rides of 75mph plus, you'll need to get larger ccs, starting from the "holy grail of motorcycles", a 350cc.
A Honda CB300F with a 50cc bump would be it. Many people are asking for it.
Yamaha R3, and MT-03, and Kawasaki Ninja 300/Z300 may do 100mph, but only at peak engine rpm.
Personally, I'm not so much for hese type of engines (short stroke engines), and much more for a CB300F (which unfortunately has a tad too little power for interstates).

As far as the Suzuki Boulevard S40, and Yamaha SR400, both their top speed is low (85mph), and vibrate like crazy, they both are air cooled, which means lower compression, resulting in lower performance and worse mpg.
What's worse, is the Boulevard S40 is a belt drive, so you can't modify the gear ratios, and the conversion kits for sale on the S40 look mightily ugly!

The SR400 doesn't have a starter engine, and costs way too much!

So if you're still looking for a bike, to get high top speeds from, and good mpg,
A bike that doesn't cost an arm and a leg,
Honda's CB300F comes closest, with a CBR250R second, and a Honda Rebel 250 third.
I'm not a Honda guy, but Honda specializes in mpg, so it would be the no-brainer to get.
If you'll never find yourself on the interstate, and wont surpass 60mph, Suzuki TU250X is the right one for you.

If speeds of over 100mph are necessary, then you'd have to step up to a 500cc class.

120mph, 650+cc sport bikes, or 900+cc cruisers