# How to calculate fuel consumption?

If i know the rpm at which the (tachometer indicated) engine is running and the current speed and the engine capacity is there a where to calculate the fuel consumption of the engine? Need not be a precise one also. It can be simple thumb rule also.

• Are you looking for a current look at fuel consumption (moment by moment), or fuel consumption over time? – Pᴀᴜʟsᴛᴇʀ2 Sep 28 '15 at 11:36
• If you want an instantaneous fuel consumption approximation, get ahold of a vacuum gauge graduated in MPG. It won't be very accurate but is the basis for which most older vehicles display instant MPG. – Steve Matthews Sep 28 '15 at 14:18
• It sounds like you want to do this theoretically based on engine operating parameters (load and speed). The problem is that this is very tricky because, while getting engine speed is easy, getting engine load isn't. You can do that in practice with a manifold gauge to get manifold pressure, but in theory, not so much. If you had both pieces of info, you could look up that point on an engine map to find the BSFC with which you basically have your fuel consumption. Consumption = BSFC * Speed * Load. – Hari Ganti Mar 24 '17 at 23:47

Though There are many geeky ways to calculate fuel economy this one is one of the easiest ways to calculate.

• Fill up your car's tank all the way.
• If your car has a trip odometer, reset it, or record the master odometer mileage.
• Drive your car as you normally would, and let your tank deplete to at least a half of a tank of fuel.
• Get to the fuel station and fill your tank again.
• Record the amount of fuel it took to refill the tank.
• Record the elapsed trip miles or new odometer mileage.
• Calculate the kilometres travelled from the trip odometer, or subtract the original odometer reading from the new one.
• Divide the kilometres travelled by the litres of fuel it took to refill the tank. The result will be your car's economy.

For example if your car has done 200 km for half tank and to refill it takes around 15 liters then 200/15=14kmpl is your mileage.

• Make sure you go to the same gas station and use the same pump. This is to keep the fill up level consistent. – rana Sep 29 '15 at 15:02
• Yes, Good point.! – Shobin P Sep 29 '15 at 15:03

There are many other factors that need taken into account. Engine temperature, coolant temperature, air temperature, barometric pressure, etc... Some vehicles do the calculation and the numbers can be looked at with a scan tool.

If you are looking for general fuel consumption. Fill the gas tank to full. Reset the trip odometer. Drive the car till the tank is empty. Fill the tank again to full and record the amount filled. Divide the trip odometer by the amount filled.

You won't be able to calculate it without monitoring the OBD2 port. You can't just use RPM and engine displacement. You'll also need to know volumetric efficiency, engine load and air/fuel ratio. These things change dynamically, so you'll need to monitor them in real time.