How to track LPG consumption in a bi-fuel car
Track LPG (or CNG) and petrol separately in a bi-fuel car: log each tank on its own, calculate l/100 km per fuel and compare the real cost per kilometre.
Treat the car as having two tanks and log each one separately. Every LPG fill-up goes against the LPG tank, every petrol fill-up against the petrol tank, both with the odometer reading. Then work out consumption for each fuel on its own and compare the cost per kilometre, because LPG cars burn more litres than they do on petrol.
Why ordinary fuel logs get it wrong
A standard mileage log assumes one tank. In a bi-fuel car you might fill LPG on Monday and petrol on Friday, and the distance between those two entries was driven on a mix of both. Divide the litres of either fuel by that distance and the result is wrong.
Owners on car forums often fall back to a paper log of every switch between fuels, or create two separate "vehicles" in an app that doesn't understand two tanks. Both work, but both are tedious and make total running costs hard to see.
Setting it up properly
In Fuelio, open your vehicle's settings and switch on Bi-fuel vehicle (2 tanks). Pick the fuel for each tank, for example petrol for the first and LPG or CNG for the second. When you add a fill-up, you choose which tank it goes to. Each tank has its own unit, so you can use litres for LPG and gallons for petrol, or kilograms or cubic metres for CNG. Each fuel gets its own consumption figure, worked out from the distance between that fuel's own fill-ups.
If your car is a flex-fuel model that takes petrol and ethanol in the same tank, don't use bi-fuel. Choose Flex as the fuel type, which lets each fill-up be petrol or ethanol in one tank.
A worked comparison
Suppose a car does 410 km on LPG and the refill takes 38 litres at £0.80 a litre. LPG consumption is 38 ÷ 410 × 100 = 9.3 l/100 km, and the cost is £30.40, or 7.4p per km.
On petrol the same car averages 7.4 l/100 km at £1.50 a litre, which is 7.4 × 1.50 ÷ 100 = 11.1p per km.
The car uses about 25% more litres on LPG, yet each kilometre costs roughly a third less. Your numbers will differ with the engine, the conversion kit and local prices, which is exactly why it's worth measuring them.
Tips for accurate LPG figures
- Fill LPG to the automatic stop. LPG tanks are only filled to about 80% by design, and the valve makes that level repeatable.
- Don't mix up the odometer. Record it at every fill-up of either fuel.
- Expect a little petrol use. Engines usually start and warm up on petrol.
- Give it a few tanks. As with any fuel, one tank is a sample; the average over a month is the figure to trust.
The general method is the same as for any car, so the l/100 km guide applies to each tank. To put the savings in context, add LPG servicing and the conversion cost to your cost per kilometre.
Track a bi-fuel car
- Set the vehicle as bi-fuel. Give the car two tanks, for example petrol and LPG, each with its own unit.
- Log every fill-up against its tank. Record LPG and petrol fill-ups separately, with the odometer reading each time.
- Fill each tank to full when you can. Full-tank fill-ups give the most accurate consumption for that fuel.
- Compare cost per kilometre. Compare what each fuel costs per kilometre, not the litres used.
Questions about this
Why does my car still use some petrol when running on LPG?
Most converted engines start on petrol and switch to gas once the engine warms up, and some switch back under heavy load. A small petrol consumption alongside LPG is normal.
Is LPG really cheaper if the car uses more litres?
Often, yes. LPG usually needs more litres per 100 km than petrol, but each litre is much cheaper in many countries. Compare cost per kilometre for your own car and local prices.
Learn more: Fuel Log
