One of the most common complaints heard from drivers who have just converted their vehicles to Autogas is: "My car is burning through fuel so much faster! I thought LPG was supposed to be efficient?"
This confusion stems from a fundamental misunderstanding of physics and fuel chemistry. When a car switches from gasoline to LPG, it is entirely normal—and mathematically required—for the vehicle to consume a higher volume of fuel to cover the exact same distance.
In this article, we will explain the thermodynamics behind the famous "20% rule" of LPG consumption and show you how to properly account for it when estimating your savings. To see how this volume difference impacts your wallet, you can use our LPG Savings Calculator.
The Science: Volumetric Energy Density
Internal combustion engines do not run on "liters" or "gallons"; they run on energy. The volume of the liquid is just how we measure its storage.
Different fuels contain different amounts of energy packed into the same physical space. This is known as volumetric energy density, usually measured in Megajoules per Liter (MJ/L).
Let's look at the scientific averages:
- Gasoline (Petrol): Contains approximately 34.2 MJ/L of energy.
- Autogas (LPG): Contains approximately 25.3 MJ/L of energy.
As you can see, a single liter of LPG contains about 25% to 26% less raw energy than a single liter of gasoline.
When you drive down the highway at 100 km/h, your car requires a specific, fixed amount of energy to push through the wind resistance and maintain speed. Because LPG has less energy per liter, the engine’s computer must inject more liters of LPG into the cylinders to match the energy output it would normally get from gasoline.
The "20% Rule" in the Real World
In theory, based on raw energy density, a car should burn about 26% more LPG than gasoline. However, LPG has a secret weapon: Octane.
LPG has a much higher octane rating than standard unleaded gasoline (typically around 105 to 110 RON, compared to gasoline's 91 to 95 RON). This high octane rating means the fuel resists "knocking," allowing modern engine management systems (especially those with variable valve timing) to burn the LPG slightly more efficiently than gasoline.
Because of this efficiency boost, the real-world volume increase is usually lower than the chemical 26% gap.
- The Industry Standard: A properly calibrated, modern sequential injection LPG system will consume 15% to 20% more fuel by volume than gasoline.
Calculation Example:
If your dashboard computer says you consume 8.0 L/100km on gasoline, you should expect to consume:
- 8.0 × 1.20 = 9.6 L/100km on LPG.
If your mechanic tells you that your car will burn the exact same amount of LPG as gasoline, they are either misinformed or they are tuning the system to run dangerously "lean" (starving the engine of fuel), which can cause catastrophic valve damage.
How This Affects Your Financial Savings
The fact that you are burning 20% more fuel sounds terrible until you look at the price signs at the gas station.
The entire economic model of Autogas relies on the fact that LPG is taxed differently and refined cheaply, making its pump price significantly lower than gasoline—often 40% to 50% cheaper.
The Mathematical Trade-off:
You lose 20% efficiency in volume, but you gain 50% efficiency in price.
The net result is still a massive financial saving.
When you use the LPG Savings Calculator, it is vital that you do not enter the same consumption number for both fuels. If your car uses 10 liters of gas, you must enter 12 liters for the LPG input to get a mathematically sound and honest projection of your financial savings.
Does Driving Style Affect the Gap?
Yes. While the 20% rule is a great baseline, extreme driving conditions can widen the gap between gasoline and LPG consumption:
- Heavy City Traffic: Stop-and-go traffic requires moving a heavy vehicle from a dead stop repeatedly, demanding a rich fuel mixture. LPG consumption can spike noticeably higher than gasoline in heavy congestion.
- High-Speed Highway Driving: As you exceed 120 km/h (75 mph), aerodynamic drag increases exponentially. The engine requires massive amounts of energy to maintain speed, forcing the LPG injectors to work overtime to compensate for the lower energy density.
For the most accurate savings, drive smoothly, adhere to speed limits, and accept that the slightly higher fuel volume is just the small price you pay for massive financial savings at the pump.
Frequently Asked Questions (FAQ)
Can I trust my car's trip computer for LPG consumption?
No. The trip computer on your dashboard calculates fuel consumption by measuring the injection times of the gasoline injectors. When running on LPG, the car's computer is being "tricked," so the L/100km or MPG displayed on your dashboard will usually be inaccurate (often displaying the lower gasoline equivalent). Always calculate LPG consumption manually at the pump.
Does LPG burn hotter than gasoline?
This is a common myth. The actual combustion temperature of LPG is virtually identical to gasoline. However, because LPG enters the cylinder as a dry gas rather than a wet liquid vapor, it lacks the slight cooling effect that liquid gasoline provides to the intake valves.
Why does my tank not fill to 100% capacity?
For safety reasons, LPG tanks are equipped with an automatic shut-off valve that prevents them from being filled beyond 80% of their total volume. This 20% empty space allows the pressurized liquid gas to safely expand if the tank gets hot during summer months.
