Not sure if optical triangulation will work as the petrol is almost clear (pale to dark straw colour) and the inside of the tank is white/cream. The colour of the tank inside is useful as it will reflect light but only in a diffuse way. The tank is made of nylon and is painted dark red. I'm not sure if light can shine through it as I've not tried it.
There is an air vent under the tank, I wonder if air displacement could be measured in some way, just thinking out loud.
Or an air bubbler measurement system
Air bubbler (from wikipedia)
An air bubbler system uses a tube with an opening below the surface of the liquid level. A fixed flow of air is passed through the tube. Pressure in the tube is proportional to the depth (and density) of the liquid over the outlet of the tube.
Air bubbler systems contain no moving parts, making them suitable for measuring the level of sewage, drainage water, sewage sludge, night soil, or water with large quantities of suspended solids. The only part of the sensor that contacts the liquid is a bubble tube which is chemically compatible with the material whose level is to be measured. Since the point of measurement has no electrical components, the technique is a good choice for classified “Hazardous Areas”. The control portion of the system can be located safely away, with the pneumatic plumbing isolating the hazardous from the safe area.
Air bubbler systems are a good choice for open tanks at atmospheric pressure and can be built so that high-pressure air is routed through a bypass valve to dislodge solids that may clog the bubble tube. The technique is inherently “self-cleaning”. It is highly recommended for liquid level measurement applications where ultrasonic, float or microwave techniques have proved undependable.
David
P.S A picture of a similar tank
http://cgi.ebay.co.uk/MZ-MUZ-Skorpion-Sport-Cup-660-01-Fuel-Petrol-Tank-/390187062108?pt=UK_Motorcycle_Parts