If this were miles on the odometer, it would be impossible, so, I suspect that it has to do with the one different between a tripmeter and an odometer... the fact that it can be reset.
If this is the case, then it must be 64 miles to grandmas house, the car that needed fuel drove half way, 32 miles, turned off and drove 2 miles to the fuel station, filled up, reset the tripmeter, then drove 2 miles back to the highway and 32 more miles to grandmas house, making 34 miles since the reset.
The other car has 64 miles on the clock, the fuel car has 34 miles on the clock. Difference, 30 miles.