Ethanol is commonly blended into petrol to reduce emissions and increase renewable fuel use. However, ethanol contains less energy than petrol, so when it is added to fuel, vehicles typically travel fewer miles per litre. The reduction in mileage is small for low blends like E10 but becomes more noticeable as ethanol content increases.
Does ethanol in petrol reduce mileage?
Yes. Ethanol blended into petrol does reduce fuel mileage, and the reason comes down mainly to energy content, plus a few secondary effects.
How Ethanol in petrol reduce mileage?
1. Energy content: the main reason mileage drops
Fuel mileage is largely determined by how much chemical energy is in each litre (or gallon) of fuel.
Energy comparison (approximate):
- Pure petrol (gasoline): ~34.2 MJ per litre
- Ethanol: ~24.0 MJ per litre
Ethanol contains about 30% less energy per litre than petrol.
When ethanol is blended into petrol, the average energy per litre of the fuel decreases, so your engine must burn more fuel to produce the same power, which reduces mileage.
Typical real-world impact:
- E10 (10% ethanol): ~2–4% reduction in mileage
- E15 (15% ethanol): ~3–6% reduction
- E85 (85% ethanol): ~20–30% reduction
These figures match what most drivers and fleet tests observe.
2. Stoichiometric air–fuel ratio changes
Engines are designed to run at a specific air–fuel ratio for efficient combustion.
- Petrol: ~14.7:1 (air:fuel by mass)
- Ethanol: ~9.0:1
Because ethanol needs more fuel per unit of air, engines must inject more total fuel when ethanol is present. Modern engines compensate automatically, but the increased fuel flow still reduces mileage.
3. Octane benefits don’t usually offset the loss
Ethanol has a high octane rating:
- Ethanol: ~108 RON
- Petrol: ~91–98 RON (typical)
Higher octane allows:
- More ignition advance
- Higher compression ratios
However:
- Most standard engines are not designed to take advantage of this
- Unless the engine is specifically tuned for ethanol, the octane benefit does not recover the lost energy density
In flex-fuel or high-compression engines, ethanol can partially offset mileage loss—but rarely eliminate it.
4. Cold start and warm-up penalties
Ethanol:
- Evaporates less readily than petrol
- Absorbs heat during vaporisation
Effects:
- Slightly richer mixtures during cold starts
- Longer warm-up time in cold weather
This further reduces mileage, especially on short trips and in winter conditions.
5. Secondary effects (not mileage, but relevant)
While not direct mileage factors, these influence overall efficiency and costs:
Advantages
- Cleaner combustion (lower CO and some hydrocarbons)
- Renewable component (policy-driven benefit)
- Higher knock resistance
Disadvantages
- Hygroscopic (absorbs water)
- Lower storage stability
- Can degrade older rubber and plastic fuel system components
- Reduced range per tank
6. Why some people don’t notice the difference
Some drivers report no noticeable mileage loss, because:
- Driving style variation masks small changes
- Trip computers are often optimistic
- Seasonal petrol blends already vary in energy content
- Improved combustion efficiency in certain engines
But controlled testing consistently shows a measurable reduction.
Summary:
Yes, ethanol in petrol reduces mileage
The reduction is roughly proportional to ethanol content
For common blends like E10, expect 2–4% fewer miles per tank
The loss is fundamental and unavoidable due to lower energy density
Other courses:
