Car running costs Fuel additives

petrol diesel additives

Almost all the additives we tested didn't live up to their claims

Do you feel like your tired old car could do with a bit of extra pep? A whole host of fuel additives are available, variously claiming to increase power and improves fuel economy. They can seem very tempting - but do they actually work?

Fuel additives: costs without benefits?

The array of products out there is vast, and prices per bottle vary from just under a fiver to about £20. 

However, the true cost of fuel additives is far from clear because the amount of additive needed to treat a litre of fuel varies hugely between brands. 

In fact, choosing some brands can push up the cost of filling your tank by almost 50%. So are the actual benefits worth the extra spend?

We put a selection of fuel additives for both petrol and diesel engines to the test at our laboratory to find out whether the claims can be justified – and, in most cases, the answer is no. 

While some additives did make slight changes to power, torque or exhaust emissions, none of them was significant enough to result in noticeable improvements. Others didn't boost the octane rating, as claimed. You can however save money on fuel with current deals.

The fuel additives we tested

Here's a list of all the additives we tested: 

STP Power Booster (250ml) £4.99

Wynn’s Supremium (250ml) £5.99

Redex 0 To 60 Octane Booster (500ml) £9.99

Nitrox Hot Shot Power Boost (500ml) £7.99

NOS Octane Booster – Street Formula (355ml) £9.99

Millers Diesel Power Sport 4 (500ml) £11.99

Wynn’s Supremium Diesel (250ml) £5.99

Redex 0 To 60 Cetane Booster (500ml) £9.99

PD-5 Fuel treatment (250ml) £18.50 

Fuel additives: Which? says

You may be tempted to try some of these additives in an attempt to perk up your car’s performance, improve its economy or reduce exhaust emissions.

But the fuel additives we’ve tested seem very unlikely to return noticeable improvements in power, economy or emissions. They're generally not worth the extra money.

If your car seems lacklustre, thirsty or smoky, the first thing to try is to give it a good motorway run (at least 50 miles) to clear the cobwebs and hopefully jolt the car’s electronic control unit (ECU) out of ‘urban mode’.

If this doesn’t work, you may have a problem with the car, in which case don’t rely on fuel additives – get advice from a reputable garage instead.

Watch the Which? fuel additives testing video


Please enable JavaScript to access this content.

Watch the video to see how we tested fuel additives using cars on a rolling road.

More on this...

Other sections in this guide

  1. Overview
  2. How to find cheap fuel
  3. Find the cheapest fuel with deals
  4. How to use less fuel
  5. Fuel additives