T [1] here 's a curious - some would say quaint - Latin phrase some insurance professionals use to describe the partnership between insurer and insured. That phrase is uberrimae fidei also it means "utmost good faith". Put simply, it means that the partnership is based on both parties being totally honest with each other rather than telling any untruths - by commission or omission. In short, it's the reason you don't lie on your car insurance applications.Whether quaint or otherwise, this idea of utmost good faith is so central to the principles of the insurance business that it lies in the centre of every insurance contract. In the absence of such honesty, the contract itself can be null and void. What this means, needless to say, is that unless you tell the truth on your own car insurance proposal forms, the insurance can be declared invalid. Despite the fact that you've been paying the premiums every month, if the insurer can show that you've acted in anything less than utmost good faith, in terms of making a claim, the policy can be declared invalid and you also find that you're uninsured.This is something that could leave you not merely seriously out of pocket, when you have to fund all repairs and damage yourself, but may possibly also put you in serious breach of the law by driving while uninsured. Therefore, the message is pretty clear - don't lie on your own car insurance application forms.One of the areas where people seem tempted to be significantly less than truthful relates to previous motoring convictions. They quite rightly recognise that past infractions will undoubtedly be considered by the insurer when calculating the risks, potentially leading to an increased premium as a result. In try to avoid paying the higher premium, some people might be tempted only to lie, or never to divulge the entire truth about their motoring convictions. In the event of an accident, especially one to which the police are called and one involving a subsequent summons to court, it's likely that the driver's record should come out. With this in mind, there's a risk of the insurance provider then declaring that the insurance coverage is actually invalid.Some applicants for car insurance cover are sometimes economic with the truth when assessing the state of their health and their fitness to operate a vehicle. If the car insurance application form asks for any sort of medical declaration concerning your fitness to drive, it will be extremely short sighted once again to lie about any of it. Still worse, of course, would be to have told exactly the same lies or even to have exercised exactly the same deceit with the Driver and Vehicle Licensing Authority (DVLA) in order to secure or keep your driving licence. This is usually a criminal offence, your driving licence would be invalid and, alongside it, will be any insurance you had arranged.When it come to car insurance applications you need to be completely honest about modified vehicles. Modifications - especially those that alter the performance of the car - are key to the insurer's assessment of the chance of insuring the vehicle. If they feel they've been misled, or lied to, the chances are that they can consider whether or not to honour the insurance contract in its entirety.Jon McGovern is from Tescocompare.com [2], the insurance comparison site where you could compare auto insurance [3] policy features and prices.