I'm sorry this is rather long, please do read it though, as I'd really appreciate your comments. My husband has a company car in his job with a large private company. He is about to move on to a better job as his current employer's terms and conditions are awful. When he leaves, we think a problem is going to arise and I would appreciate your opinions as I am trying to help him to do something to prevent it occurring.

He enjoys his company car but finds that he averages over 20mpg less than the manufacturer's published combined mpg figure, which is 76.3mpg (from the Vauxhall website for the particular model) despite being very good at maximising this through skilled driving and awareness. The terrain he covers for work is such that it is hard to maximise these figures. (Long distances on country roads and lanes, through towns and villages, many hills, stop-start and crawling necessary.)

His employer's method of calculating how much he should pay for private mileage is one that I have never encountered before, and I can't find anything on the internet although I am usually a fairly expert "Googler". I'm beginning to think they might be unique....

The car is fuelled with a company credit card or from the company diesel pump, and his private mileage repayment is calculated by working out how much of that fuel was spent on business miles. They do this by calculating how much fuel would have been used if the car achieved the manufacturer's mpg rate, and anything on top of that is deemed to be private mileage, for which the employee is charged. Of course, some of these "private" miles are actually business. He is not required to submit actual mileage amounts for business or private use, as they are not used in the calculation.

This system would work if all vehicles achieved this figure without fail. However, when a car is only able to average around 53mpg, the employee ends up subsidising the business mileage themselves - to a fairly large degree if this mileage is high.

Other colleagues have experienced this problem too, but the amount to which they are affected varies with the terrain they cover. Several have complained to the employer, who has begrudgingly reduced the target figure by 10%. Requests to lower it further have been met with the delightful "if they don't like it they can get another job" response. My husband's mpg is around 69% of the manaufacturer's figure, so this 10% reduction does not resolve the issue.

I am aware that manufacturer's figures are often hard to meet. The company sent a manager out on a short "test" trip in the local area and he managed a figure somewhere around 10% less than the published mpg, so that is their justification for reducing it by this amount.

My husband pays a set amount monthly out of his meagre salary towards his private mileage, and also pays for his own fuel if we need to use the car for a long trip such as a holiday, so that there isn't too much "private mileage" fuel on the company credit card.

So....here's the problem. He is about to give in his notice, and is aware that there is quite a deficit on his private mileage debt with the company. This has been increasing since he got the car as he was aware of this calculation problem and refused permission for the company to take any more off him until a resolution could be reached....but when he leaves, they can just deduct the whole amount out of his salary.

So...has anyone heard of a company calculating business mileage in this way? If so, is it a recognised, legal method? What do you think we should do? My first thought was that he should submit an official grievance at the same time as giving notice, although I doubt it will resolve the problem.

Thanks for reading this....I look forward to your answers.