I tried a magnetic thermometer, designed for "gas" (LPG) bottles
The idea is that in the morning when you take your moped out of the garage in the morning and the sunlight hits it, the tank heats up quicker than the liquid inside and you can see the level of fuel inside
It works when the sun is shining...
but with the RM4 i try to just fill it up once a fortnight (luckily I can visually check inside)
I like the idea of the string!