Mate had an old old VW beetle where he'd stuck a calculator on the dash.
Apparently that model didnt have a fuel guage, just a 'reserve' tank activated by a lever. When he was running out of fuel you'd get a massive wiff of petrol, and then had about 50yds to twist the lever (to stop it conking out), which would then give him an extra 1-2 miles to find a petrol station.
After a fair few walks petrol can in hand, he added the calculator so he could work out of how many miles a fiver of petrol would give him.