I have two pieces of 3/4x1/2" pine about 26" long, one has at each end a builders tape measuer secured to it (the end of the tape with the metal hook), I then use a bungee cord to secure this as high as I can to clear the underside of the car, to one side of the car's outer tire edge on the flat sidewall of the tire.
The other piece of pine is attached to the other side outer tire at teh same height, (use bungee cord through the rim etc) then I simply pass both tape measures und the car and read front and back by hanging the tape measure over the pine. cost me like $15 in materials and very easy to use.