When I was in college we used a flow meter like this for a lab once.
The fluid pushes up on a ball that is more dense than the fluid you are working with. You can calibrate it to take eye readings (with a glass tube), or use some sort of dual or triple coil system to determine the location of the (usually ferrous) ball by determining magnetic field in the coils. It may sound hard, but I remember it being pretty easy. We were getting fantastic results with water, but it should work for gasoline as long as your materials can withstand the environment.
EDIT:
These are called rotameters or area meters. I haven't found any that will just give a quote that have a reasonable cost, but most are made from stainless and glass. I did find one that was about right, but I guarantee it is way to expensive. It has low flow readings (one model as low as 0.02 gph; my 1.3L idles at ~0.2 gph). They also have ouptuts from 4 to 20 mA, which should make them usable without modification.
ROTAMETERS with electric output.
DATA SHEET.