[dcb]

If it does indeed not have vss, then it is inconvenient but still not a lost cause. Many times we have regular destinations (i.e. home>work), and since you are still tracking fuel used, you can still "beat your high" score to those destinations without any extra thought.

OF course cars have speedometers and odometers built in, so you can look at the gph and divide it into the mph in your head to get instant mpg, likewise note the change in odo and fuel used for a longer term sample.

Another option, if you are really convinced you don't have a speed pid, but your car has a speed sensor is to make a hybrid obduino that monitors the speed signal directly (advanced topic), or just go with a mpguino.

You don't have any "check engine light" or stored trouble codes do you?