I run it to the light usually, i know, its very bad for it, but it runs in the family i swear. To my dad if the light comes on, the tank is damn near full and your good to go! Changing the fuel filter once a year helps as well! I put Superstore Premium 91 in my car, has made 220 kms and has just burnt over a half tank, usually its a 1/4 tank by now, the gage is off by a bit though, the needle goes past the F by a bit when i fill it.

this is very true... ran my mustang with 89 while on the bottle... sounded like a harley backfiring on acceleration... i noticed a dramatic difference in mileage between the 94 octane i got in America compared to the 91 in Canada, and the car ran much better on Uncle Sam'* gas as well...

I worked at a gas station before joining the Navy. Gas pumps do keep fuel in the hoses and as a firefighter I 've been to calls of fuel spills because people pulling off with the nozzle still in the tank.
If you pull up behind some one who just used 87 Octane gas and you need Premium and its rated at 93 at this station.
Assuming a 10-foot filling hose with an internal diameter of 3/4 inch, the amount of regular gasoline would be 0.229 gallons (V = pr2h, or 3.14 x (0.375 in.)2 x 120 in. x 0.00433 gal/in.3). Assuming a 20-gallon tank and a fill up with a premium of 93 octane (with the hose filled with 87 octane at the start of the fillup), the effect is best described by the equations below:

Octane number =
(0.229 gal/20 gal) (87) + (19.771 gal/20 gal) (93)

87 octane contribution 93 octane contribution

Octane number = 0.996 + 91.943

Octane number = 92.9

Considering the precision of the octane test procedure is 0.2 octane numbers and the dilution with 87 octane reduces the octane in the tank from 93 to 92.9