What Car? Q&A - Can I stretch my fuel economy?

Q: Have you heard of MPG caps, a product supposed to make your fuel last longer? They're huge in the USA, and a friend is trying to sell me the caps, but I have read a few reports about possible damage to the fuel system.

A: We remain unconvinced by these products, and measuring mpg involves so many variables (weather, driving style, and so on) that it's almost impossible to test them. This web page has some interesting points: http://www.fuelsaving.info/ffi.htm.

There's no independent evidence to suggest that these have been thoroughly tested, or are effective.

Whether it would damage your engine is another matter, however you should check that putting additives into your fuel doesn't affect your warranty or lease agreement.