Many (most?) motorcycles don't have fuel gauges. Mine doesn't, in spite of being a late-model bike. (Honda Goldwings do, but then again, they also have stereos, GPS, and air conditioning.) I asked the salesman about this when I bought the bike. His answer, basically, was that motorcycle fuel gauges aren't very accurate. The tank sits high, and as such, the level of liquid in the tank varies, sometimes considerably, as you go up and down hills and around curves. This seems to be what they tell the folks who buy sport bikes, anyway. Given that high-end bikes have them, there might be other factors. On bikes that have carburetors, there is a workaround for not having a gas gauge — there's a petcock valve:
Before you start the bike, you have to turn the petcock valve from OFF to ON so that fuel will flow to the carb. If you run low, the fuel supply stops, but you can then turn the valve to RES, which drains the the last little bit (aka the reserve) from the tank while you desperately look for a gas station. Here's picture of how that works:
When I was taking the motorcycle class, one of the instructors advised that you should practice turning the valve from ON to RES while you were riding and without looking, in case you needed to do this while you were on the freeway or someplace else inconvenient.
As noted before, tho, I don't have carbs on my motorcycle, I have fuel injection. This means no petcock valve, yay, but it also means no RES setting to turn to when the fuel runs low. What I do have is a low-fuel indicator light, the way most cars have nowadays. This is supposed to function as a more convenient version of the reserve tank — IOW, when the light comes on, get yourself to a gas station pronto.
This can be nerve-racking. So I've turned to a time-honored method, which is to keep an eye on the odometer. What I've discovered is that I can go about 150 miles before the low-fuel light starts flickering on. At (or before) 150 miles, I fill up, and I have been very diligent about resetting the trip odometer each time I refuel. (It is true that the light will come on and go off according to the immediate vagaries of the road.)
Whether as a result of this diligence or as an incentive to it, I've also been tracking my fill-ups and miles in a (what else?) spreadsheet. I've done this for the last 3500 miles or so, which has allowed me to track with some precision my overall MPG. (50.8, it says.) This has all worked well enough that I can glance at the odometer when I get on the bike for the commute to work, and I'll know whether I have to tank up before hitting the freeway, on the way home, or later in the week.
However, this is not a foolproof system. On his Scooter in the Sticks blog, Steve Williams recounts a time when his scooter's gauge was on the fritz and he was relying on the odometer, only ... oops.