What you're asking has limited utility because people can't measure, and have ego attached to their builds and feel the pressure of shame from their low range so they often flat out lie about it (as in, a large amount of the numbers given are physically impossible). I'd say maybe 10% of people aren't massively inflating their range.
But to answer your question:
If you go to the Evil Bum website, scroll down, and select "By Type/Make" you can select motorbikes.
http://evalbum.com/type/MTCY <-- Like so.
Then just scroll through their stats and either:
A - Check to see if they list it directly (they'll list watt-hours per mile, not miles per kwh, but you can figure that out).
B - Take their kWh size (if mentioned) and their range (if mentioned) and calculate it yourself. Also keep speed they claim in mind, double the speed is 1/4 the range, lots brag about their range at "45 mph" which is moped speed and power draw.
C - Try to decypher whatever random smatterings they typed in for battery size and amounts, into a total kWh yourself, and divide by range.
... Alternatively... I didn't write anything down, but
I've already done this, and people generally report 100-200 watt-hours per mile. So, for what you're asking,
5-10 miles per kWh.
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This is one of my biggest pet peeves, the non-standardized way of reporting range from EV builders. Is it too much to ask for someone to dedicate 1 hour after their build to accurate measure their range on flat ground, windless day, consistent speed?
Going by physics:
http://www.enginuitysystems.com/EVCalculator.htm
1 - You can only change stuff in black, not blue. It's a bit clumsy if you want imperial units but you can just guess at Metric ones until the conversion matches.
2 - Set the weight of the bike + rider. 500 kg for a huge bike (Goldwing), 330 kg for a medium-sized bike (400-750cc), 200 kg for a little bike (250cc).
3 - Change "acceleration" to 0, because you probably want to know the power consumption at a given speed. For some reason it defaults to something non-zero, 0.1g, which is continuous acceleration forever and not useful unless you're curious about drag racing power requirements. I keep forgetting to do this every time I refresh the page and the numbers come out way wrong. Zero it out.
4 - Set "velocity" to whatever target speed you want to find out. Note that 2x the speed requires 8x as much power, and you only arrive at your destination in half the time, so, 2x speed means 1/4 the range.
5 - If you want to tweak your "frontal area", "air density (thinner and less resistive at higher altitudes)", coefficient of friction (tires), etc go ahead, but you'll get accurate results by leaving it as is.
6 - Read the "Power" result and the "Watt-hours per mile" result to find out what size of motor you need and what size of battery you need.
7 - Read the "Inertia" in watt-hours to find out how much energy you waste every time you have to accelerate from a stop (traffic light/stop sign). This is entirely based on weight and speed. Half the weight, half as much energy to get it going to a target speed. Half the speed, 1/4 as much energy to get the object going that fast. For example, a 700lb bike and rider use up 34 watt-hours every time they accelerate to highway speed. Your regen (if any) can only recover this amount per stop. You'll note that for low-speed stops around 30mph in city driving, it's pretty trivial (8 Wh... about a single 18650).
TL;DR - Change Weight, Change Velocity, Set Acceleration to Zero, leave the rest alone.
In my opinion, the calculator is optimistic and presumes no losses. After battery/controller/motor/mechanical losses, and accounting for the un-aerodynamic shape of a motorbike that's hard to figure into these results, I'd expect nearly double what it claims.