Has anybody heard of some sort of a gas cap that lets you know how much gas you have left in the tank, or just if you need to fill up?  
It sure would be handy to have.

i normally just open up the cap and have a look

I don't know of any gas gage to fit the tank.  Maybe with some creativity you can modify some other bikes gage.

I simply ride 100 miles and fill up.  If I loose track, I ride until it sputters, switch on reserve and get more gas.  If I plan to ride where there is a long distance between gas stations, I just fill up ahead of time.

Been there, done that  

Me, I wait for the sputter.  Guess that I'm just an old fart and can't handle things that aren't "made for a motorcycle" being attached to it.  So I wait for my bike to tell me that it's hungry.

It was a little sluggish starting after work, so I used the choke to get going (that was a warning flag).  Half way home I heard a couple pops and felt a misfire (flag #2).  I didn't wait for the next warning.  I reached down, flipped it forward and it smoothed right out....

Since I know that I can get 25 miles on the reserve, I'll fill it on the way tomorrow morning.

Yeah, we've all gotten used to fuel gages....when most of them are only a reference and don't really indicate the true level anyway.  And then most newer cars have a low fuel warning light, and most of us know they come on a long time before a fillup is really needed.  Like when you go up a really long grade with a 1/4 tank full.

So, with all that in mind, the old p_e_t_c_o_c_k works pretty well.  Just about as fool proof as an hour glass.

I got an idea...How about a p_e_t_c_o_c_k position light?  Call it a PPL.  That way you'd know why you ran out of gas, when you left it in reserve after the last fillup.  

Yep, it ain't rocket science.  Pop, pop, sputter...get gas.

I also gas up after 100 miles - at least usually.  

It's fun going home after taking that long trip - running down the highway - catching up to a semi - all of a sudden you are in the slip stream and the bike seems to be having a problem....you just think it's the wind causing a problem.  Then you don't have any power...what the heck is wrong...you check real quick left and right and pull over to the side of the road.  The bike won't start.  And after having problems with the battery, you think it hosed.  I'm in the middle of nowwhere at midnight and can't start.  Then for some reason, you remember the milage from the last stop for gas.  Duh!  You switch to reserve, starts right up, and you're off to find a gas station!

It actually scared my wife more than me - I pulled to the left side of the highway since I had cars coming up on my right.  She was driving behind me at the time (so at least I could get a ride if needed - but who wants to leave their 'precious' out on the highway all alone?).

I think its all about the old "romantic" phrase of "Man and Machine"(....figuritively, not forgetting the women riders)

It's all about getting to know the sounds, smells, and the feel of the motorcycle.  We've gotten so used to modern conveniences like in a modern auto, where there are devices of all kinds to feed us "vital" information, that we panic when the information flow stops.  We've become the "Machine and Man" instead.

Who really rides for 2 hours straight without stopping?  If nothing else that can lead to fatigue anyway.  At 55-60 miles per gallon at a moderate speed that is a tank of gas.  So you pull into a station and stop at the pump.  You fill up, stretch your legs and take a break.

If you are just a "weekend warrior" and only ride occassionally, then fill up first thing on the way out.  It's a no brainer.  All our modern conveniences have made us lazy, too.  We want to just go, go, go and we don't want to have to think about it.  It's like doing "long math", who can do it without a calculator anymore?

Motorcycling is about the wind in your hair, the scenery going by, and your machine.  It's about feeling the lean going around a curve.  Its about listening to the engine as it pulls a hill...I often think about that piston going up and down, or what area of my tires is in contact with the pavement, while I'm riding.  Motorcycling is very much a thinking activity.

Leave the cell phone, bells and whistles, and gadgets behind.  We don't need little things with numbers, grease pencils, or other devices to tell us what to do.  They just slow down your mind.  Learn your motorcycle, and you'll know how much gas is in the tank.

Just my opinion...