A There are lots of factors that will improve a car’s mpg — warmer weather (cars need more fuel to start in cold weather), travelling less in heavy traffic and, of course, a lighter right foot. But if you record an improvement while driving in your usual manner over regular routes, it would suggest the fuel is responsible.

All fuels have to meet basic standards. Each oil company has its own recipe of additives, the most common being detergents aimed at keeping the fuel injection system clean and efficient. This could be the reason for your newfound fuel efficiency.

Your increase in mpg has obviously been obtained at an extra cost per litre, so you’ll need to work out whether it makes financial sense. The brand names each claim their additives are better for the long-term health of the engine, so this could be an extra consideration, although we haven’t seen independent figures to establish the extent of the benefit.

Q I have three points on my driving licence from a speeding offence in June 2002. My insurers were aware of this when I renewed on April 19 but I recently received another penalty for a second speeding offence that occurred on April 6. The offence took place before I renewed and I was unaware of it at the time. Do I need to contact my insurers or can I allow the new points to replace the ones that are about to expire? If I do inform them, will my monthly payments go up? — LB from Hampshire

A It’s important that you tell your insurance company. If you are involved in an accident, especially one that is speed related and involves the police, your insurance company could invalidate the claim on the grounds that you have withheld information.

Premiums can sometimes be unaffected by the first three penalty points on a licence but a survey by the AA discovered an average of £50 per year is added by insurance firms for the next three points.

Penalty points stay on your licence for three years — so with the £60 fixed penalty fine for speeding, the second offence could cost you more than £200 over three years. There is no harm in reminding your insurance firm that the three points from June 2002 are about to expire.

Q I run my car on LPG (liquid petroleum gas). Some of my friends tell me there are government proposals to greatly increase the duty on LPG to bring it closer in line with other fuels. Do you know of any such proposals? — GW from London

A Liquid petroleum gas enjoys significantly lower duty than conventional fuels such as diesel and petrol. It also benefited from a fuel duty freeze in 2004. However, in this year’s budget Gordon Brown announced that the Treasury intends to reduce the differential with other fuels by the equivalent of 1p per litre each year until 2007-8. The increase will take effect from September 1 this year.

The cost of LPG varies from area to area and company to company. However, the AA fuel price report for April shows that the average UK price for a litre of LPG was 39.1p, compared with 86p per litre for unleaded and 90.1p for a litre of diesel. So even with the increases LPG is still much cheaper.

Q I would like to buy a convertible but I can find them only with two rear seats and I have three young children. Although the children would fit in the back, there is no third seatbelt. Is there a solution to this problem or I am destined to dream on until they grow up and leave home? — MB from London

A You may have to harbour those fantasies for a little longer. There are no convertibles for sale in the UK with three individual rear seatbelts. Much of this is to do with the shallow rear seat necessary to make room for the folding roof mechanism.