1987 325Ic With 220,000 Miles
When the low fuel light comes on, the computer tells me that there is around 35 miles left. According to the computer's estimated mpg (I let it sample the mileage for 2000 miles at a time), the car gets 20.5. However, according to my calculator, compiling evidence from adding around 1500 gallons in the last 18 months, the car is getting closer to 23 mpg. So, using to my figures, the reserve tank should go approximately 39 miles.
Caution: Don't depend on the computer. Twice (yes, I'm a slow learner) I ran out of gas when the trip computer showed more than 25 miles left. Before that I had run it as close as 5 miles!
Aside from the issue of AC, you should NEVER allow your dog to ride with his/her head hnaigng out the window! Would you allow your child to ride that way? Dogs should always be restrained in a seat belt harness for their safety and yours.