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100 MPG Is an Engineering Triumph — But Electric Cars Can Do It for Less

 

I recently watched a brilliant video from Autocar, where one of their writers sets out to answer a deceptively simple question:

Can an old diesel car really average 100 miles per gallon in the real world?

The car is an Audi A2 1.4 TDI, and the result is genuinely impressive.

If you haven’t seen the video yet, it’s well worth a watch:

👉 https://youtu.be/q1YmIpCEym8

What I liked most about it is how honest it is. This isn’t a laboratory test or a steady motorway crawl — it’s real UK driving, with towns, dual carriageways, motorways, rain, traffic, and compromises.

Watching it made me wonder something slightly different:

If a car can do 100 mpg, how much does that actually cost — and could an electric car cover the same distance for less?

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How Hard Is It to Achieve 100 MPG?

One thing the Autocar video makes very clear is that 100 mpg is not easy — even in one of the most efficient diesel cars ever built.

The Audi A2 starts with big advantages:

• Lightweight aluminium body (around 900 kg fully fuelled)
• Very low drag coefficient (Cd 0.28)
• Small frontal area
• Efficient turbo-diesel engine

Even then, achieving 100 mpg required:

• Careful route planning
• Gentle acceleration and braking
• Long stretches at 50–60 mph
• Making use of slipstreaming (at sensible distances)
• Accepting that losing momentum could drop the average into the 90s
• Verifying trip computer accuracy against real fuel usage

This isn’t “normal” driving — it’s the absolute peak of combustion-engine efficiency.

And that’s exactly why it deserves real praise.

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What Does 100 MPG Actually Cost in the UK?

Let’s convert that achievement into pounds and pence.

At the time of writing, the average UK diesel price is around £1.43 per litre.

A UK gallon is 4.55 litres, so:

£1.43 × 4.55 ≈ £6.50 per gallon

If a car genuinely averages 100 mpg, then:

• 100 miles costs about £6.50

That’s excellent.

But there’s an important limitation:

Diesel has no off-peak rate.

You pay roughly the same whether it’s day or night.

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What Does 100 Miles Cost in an Electric Car?

Now let’s run the same maths for an electric vehicle.

I spend a lot of time driving Nissan Leafs, and earlier I used 3 miles per kWh for my calculations.

That figure was deliberately conservative.

Real-World Nissan Leaf Efficiency (UK)

In everyday UK driving, most Nissan Leafs actually achieve better than 3 mi/kWh:

• Typical mixed UK driving:~3.2 – 3.6 miles per kWh for modern (2020s) models
• Long-term real-world use:Around 3.4 mi/kWh reported in extended UK tests
• Favourable conditions (warmer weather, calmer roads):Some drivers regularly see 3.8 – 4.0 mi/kWh

What Affects Efficiency?

• Driving speed (slower = better)
• Weather (cold winters reduce efficiency)
• Model year and battery size
• Use of heating and air conditioning

Rule of thumb:

Most Nissan Leaf owners in the UK see around 3–4 miles per kWh in real-world everyday driving.

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EV Cost Per 100 Miles (Using Realistic Numbers)

Let’s look at what that means for cost, assuming Economy 7 overnight charging:

Efficiency	Energy Needed	Cost @ 7p/kWh	Cost @ 18p/kWh
3.0 mi/kWh (very cautious)	33 kWh	£2.33	£5.99
3.5 mi/kWh (typical UK use)	29 kWh	£2.03	£5.22
4.0 mi/kWh (good conditions)	25 kWh	£1.75	£4.50

Even using the least flattering realistic figure, the EV is still cheaper per 100 miles than a genuine 100 mpg diesel car.

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100 MPG vs Electric: Cost Comparison

Vehicle Type	Cost per 100 miles
100 mpg diesel car	~£6.50
EV (cheap off-peak electricity)	£1.75 – £2.33
EV (higher off-peak electricity)	£4.50 – £5.99

And unlike diesel, that cheaper energy is available every single night.

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The Key Difference Diesel Can’t Overcome

The Autocar video brilliantly demonstrates the limits of combustion efficiency.

To achieve 100 mpg:

• The right car is essential
• Conditions must be favourable
• Driving must be disciplined
• Small setbacks matter

 

With an EV:

• You plug in overnight
• You wake up with cheap energy
• You drive normally

And tomorrow night, you do it again.

Electricity can be cheap every night.

Diesel never is.

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This Isn’t Anti-Diesel — It’s Just the Maths

Cars like the Audi A2 deserve admiration. They represent the very best of combustion engineering.

But the numbers tell a clear story:

• 100 mpg is a remarkable engineering achievement
• EV running costs at that level are routine
• Cheap off-peak electricity quietly changes the equation

No hype. No ideology. Just real-world UK maths.

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Final Thought

100 mpg is absolutely something to celebrate.

But when an electric car can cover the same 100 miles for £2–£6, night after night, without special preparation or perfect conditions — the economics have already shifted.

Electric doesn’t shout about it.

It just quietly costs less.

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