Calculating fuel efficiency on a 1970s car

[Hutkikz]

@johnwasser Has your answer in the first reply.

@johnwasser

You need to know how fast fuel is being burned and how fast the vehicle is moving.

@johnwasser

To get the fuel flow you could put a flow meter in the fuel line. Say you get 20 MPG at 30 MPH. That's 1.5 gallon per hour = 192 fl oz per hour = 3.2 fl oz per minute. If you want data every 10 seconds or so, you will need a flow meter that can measure in increments of about half an ounce (about 15 ml).

This Flow Meter here: https://www.amazon.com/DIGITEN-Sensor-Gasoline-Diesel-Counter/dp/B07QNN9V3R?th=1 seems ideal. It outputs one pulse for every 2.5mL and works with anything from 3 - 12v. However if there is a fuel return line that complicates things. you may have to put a 2nd flow meter in to measure how much fuel is returned to the tank and subtract it from the results of the first.

@johnwasser

You also need a way to measure rate of travel. Older systems put a magnet or two (or 8) on the driveshaft and used a reed switch (or, morte recently, a Hall Effect switch) to sense the rate at which the driveshaft turns. Knowing the differential gear ratio and the circumference of the tire you can get distance per magnet passing.

Having installed Many aftermarket speedometers that use magnets on the driveshaft. They all used an Inductive Proximity Sensor such as this one: https://cdn.automationdirect.com/static/specs/prox12mmpnm.pdf
While I'm sure it can be interfaced with the arduino I am not sure how to do it. Maybe one of the experts here can enlighten us both