FUMES 73 Plymouth Valiant

1...You cannot figure fuel mileage by watching the gauge, if that is what you are doing
2...You can not figure gas mileage by the odometer, either, on a car this old Get a used Garmin Nuvi and use it for a speedo/ odometer
3....Pull plugs and look at them. What color are they?
4....What size tires, and figure out your rear axle ratio. To do that jack up one tire with front safely blocked. Put transmission in neutral, brake released. Can you turn the tire? If so, watch the driveshaft, and count shaft turns while turning the tire TWO exact turns. Your ratio will be a direct result of the shaft count, IE 2.73 the shaft will rotate 2 3/4 turns with 2 turns of 1 tire

If the one tire will NOT turn (in neutral!! / and brake released!!) then jack up BOTH rear wheels and see if they turn together. If so, you have a limited slip rear axle. Now turn one tire ONE turn, exactly, while counting shaft turns.

SMELL IN CAR This can be DANGEROUS. Carbon monoxide. Do you have full tail pipes to the rear? Is there ANY rust holes in the body into the interior OR TRUNK?