The trip odometer quit working on my truck about mid summer last year. Does you car still have a functioning odometer? Record the miles when you fill up, record the miles when you fill up the next time, and subtract the little number from the big number. Then divide that by the amount of gas you have added since the last fill up. the only way that is not accurate is if you didn't stop filling it at the same point each time. If you don't add any gas after the automatic shut off on the pump shuts off (buy the gas from the same pump from the same direction, at the same angle), you will be very close to getting the same fill each time.

I keep a little note book and a pen in the glove box to record the miles from the fast fill up. I can tell you pretty close how much gas its going to take to fill my truck up each time, based on the miles, and if it was all city, 1/2 city, or all highway driven. Any big block Mopar I ever had was between 8 mpg and 12mpg (that 12 was all highway).