Ethanol in Fuel — A Story of Power, Economy, and Potential Engine Ruin

I’m currently enjoying some blissful weather on the recce of the Spain tour. I was stood looking like a teapot whilst filling up the hire car where I noticed something that made my inner petrolhead do a double take. At some fuel stations in Spain, they sell Super Unleaded 98 RON with E10 and Standard 95 RON with E5. Now, for those of you who just read that and thought, ‘Well, that’s all gibberish to me’, don’t worry — you’re not alone. Fuel labels these days read like an algebra exam, and if you don’t have a degree in chemistry, you could be forgiven for thinking that your car’s about to be filled with either premium rocket fuel or what appears to be diluted vodka.

But fear not. I’m here to clear up the confusion, explain what ethanol actually does, and, most importantly, answer the burning question — Is ethanol making my car faster, slower, or just ruining my chances of ever making it home?

What is Ethanol, and Why is it in My Petrol?

Ethanol is basically alcohol. If you’ve ever had a sip of whisky and thought, ‘That’ll do nicely, congratulations’, you’ve just experienced what your car does every time you fill up with E10. It’s the same kind of alcohol, but rather than drinking it (which, I must stress, you definitely should not do), cars burn it as fuel.

Governments love ethanol because it’s renewable. Unlike petrol, which takes millions of years to form, ethanol can be made from plants, including corn and sugarcane. This means that, in theory, ethanol is a greener, more sustainable alternative to good old-fashioned fossil fuels. In reality, though, it’s a bit like putting an electric motor in a Ferrari — you might be saving the planet, but you’re also slightly ruining the fun.

Does Ethanol Increase or Decrease Power and Fuel Economy?

This is where things get a bit tricky. Ethanol has a higher octane rating than petrol, which means it can increase performance in the right conditions. This is why you’ll see race cars using E85 (85% ethanol, 15% petrol). With the right engine, ethanol can boost power by allowing more aggressive ignition timing and higher compression ratios.

However — and this is a big however — ethanol also contains less energy per litre than petrol. So, while the octane rating might let a high-performance engine extract more power, the average road car just ends up burning more fuel to go the same distance.

To sum it up:

• High-performance cars tuned for ethanol? More power!

• Normal road cars? Less fuel economy, no extra power, and a slightly emptier wallet.

E5 vs E10 vs E85 — What’s the Difference?

• E5 (5% Ethanol, 95% Petrol) — The classic. This is what most premium fuels used to be before E10 came along. It still has ethanol in it, but in small enough quantities that it doesn’t cause much trouble.

• E10 (10% Ethanol, 90% Petrol) — Now the standard in many countries. Slightly worse fuel economy than E5, and in older cars, it can cause all sorts of chaos (we’ll get to that in a minute).

• E85 (85% Ethanol, 15% Petrol) — Race fuel. Only works in cars specifically designed or tuned for it. Smells a bit like a distillery and makes turbocharged cars very happy.

One important note: The ethanol rating is not the same as the RON (Research Octane Number) rating. People often think that E10 means 10% more octane, which is absolute nonsense. A fuel’s octane rating is separate from its ethanol content. You can have a low-RON E10 (like 95 RON E10) or a high-RON E10 (like 98 RON E10). Ethanol does help increase octane ratings slightly, but it’s not a direct link.

Why Ethanol is Bad News for Older Cars

Now, if you drive a modern car built in the last 15 years, you can relax — E10 won’t kill your engine. You might see a small drop in fuel economy, but other than that, your car won’t care.

If, however, you own anything made before around 2005, E10 is basically a ticking time bomb. Why? Because ethanol is hydrophilic, which is a fancy way of saying it absorbs water. This leads to:

• Corrosion in fuel lines and tanks (older cars don’t have ethanol-resistant components).

• Damage to rubber seals and gaskets (ethanol eats through old-school fuel hoses like a rat through a packet of crisps).

• Potential carburettor chaos if you own something truly old-school.

If you drive a classic Ferrari, Porsche, or even an early 2000s hot hatch, E10 is not your friend. Stick to Super Unleaded (E5), or better yet, use an ethanol-free alternative if you can find one.

Is Ethanol a Problem for Newer Cars?

For anything made post-2005, not really. Modern cars have fuel systems designed to cope with ethanol, so while E10 might reduce your miles per gallon, it won’t melt your engine or turn your fuel lines into soup. That said, some performance cars — especially those that thrive on high-octane fuels — might prefer 98 RON E5 instead of E10, simply because ethanol reduces energy content slightly.

If you own a newer turbocharged, high-compression engine, you’re probably better off sticking to Super Unleaded (98 RON E5) if you want maximum performance.

What’s the Best Fuel to Use?

• Got an old car? Use Super Unleaded E5 (98 RON) or an ethanol-free alternative.

• Got a new car? E10 is fine, but you’ll get better fuel economy with E5.

• Got a performance car? Stick with 98 RON or higher, ideally E5.

• Got a race car? Tune it for E85 and enjoy the fireworks.

Final Thoughts — What Should You Put in Your Tank?

Ethanol in fuel is one of those things that sounds like a great idea on paper but, in reality, is a bit of a mixed bag. It’s better for the environment (in theory), but it reduces fuel economy and can wreak havoc on older cars.

If you own something made before 2005, treat E10 like an ex who still wants to ‘stay friends’ — avoid it at all costs. If you own something modern, you’ll survive on E10, but if you want to give your engine the best, stick to premium Super Unleaded (E5).

As for E85? Well, unless you have a heavily tuned performance car and a deep love of filling up every 200 miles, it’s probably best left to the racing circuits.

And that, dear reader, is why Spanish petrol stations selling 98 RON E10 and 95 RON E5 had me scratching my head. Because it means that, technically, their cheaper fuel is better for older cars than their premium option. And if that’s not a perfect summary of the modern fuel industry, I don’t know what is.

Written by: Paul Pearce