If you’ve two or more cars in your household, you may be able to save £1,000s using a multi-car policy… then again doing that may leave you paying £1,000s over the odds. Frustrating isn’t it. There’s no law as to when it’s cheaper, but I have come up with a rule of thumb that can help.

Multi-car insurance really is marmite. In a Facebook poll of 2,100 people I did recently, 40% with more than one car said multi-car won for them, 60% said separate policies did. And the differences can be startling.

On the ‘love it’ side is boredwithbeingIgnored who wrote on my web forum: "Over three cars (incl our 18yr old son's), we saved £4,600 as our MultiCar policy reduced it to a much more manageable £2,600. Thanks MoneySavingExpert."

On the ‘hate it’ side Suzanne emailed: "Husband had three cars on multi-car, followed your guide and bought three [separate] policies, saving £1,400."

Get the opposite quotes to what you’ve already got first

This is the simple rule of thumb. If you’re at renewal and are in a household with more than one car, and you’ve got a multi-car policy, then start by looking for the cheapest separate policies for each car. If you’ve got separate policies then start by checking multi-car insurance.

Now this might not be as catchy a rule to remember as ‘don’t eat yellow snow’ – but there is some logic behind it. This is because what tends to happen is multi-car newbies get hot offers to suck them in, then that advantage usually erodes at renewal.

The top multi-car deals

Frustratingly, you can’t use comparison sites to do multi-car searches (some look like they do, but they just forward you to one insurer rather than doing a comparison). So instead, you have to do this manually as trial and error. Try as many of these as you can stomach...

Multi-car policies: Here all cars are on one policy. The big name in multi-car is www.admiral.com which would always be my start point, yet it’s not alone both www.aviva.co.uk and www.lv.com are players in this area as well.

Multi-car discounts: Here you get a discount on the cost to insure each 'additional' vehicle. There's www.morethan.com (15%), www.axa.co.uk (10%), www.esure.com (10%), www.priviledge.com (% varies) and www.sheilaswheels.com (10%).

There are also a range of firms that in one way or another potentially offer those with multiple cars discounts. Yet here the cars aren’t all in one policy. They are separate policies where you get a discount. Therefore, the easy way to do this is to use a comparison site (see below) for each car and just keep a note of these firms’ prices for each vehicle.

Plus as the discount is off each additional policy, insure the car with the cheapest premium first because you'll usually get the discount on the next 'additional' (and subsequent) cars – and if they’re most expensive you save more. This can be especially useful for example if one car is for a young driver, and far more expensive than others.

Multi-policy discounts. Here the discount is for getting an additional policy, eg, it could be a car policy and a home policy, but two cars also counts. For this, try www.directline.com (which isn’t on comparison sites) and www.churchill.com - the discount varies.

The cheapest standalone policies

There’s a whole art to getting cheap car insurance for individual cars including; haggling, cashback sites, how to home into the perfect policy requirements and more. You can read my full guide to it at www.moneysavingexpert.com/carinsurance or for under 25s at www.moneysavingexpert.com/youngdrivers.

Yet in brief, for each car, just use a comparison site for each vehicle. Yet as they don't search identical insurers, and can have different prices for the same firm, it's best to do a few checks for a wider spread. The current order to try (do as many as possible) is www.confused.com, www.moneysupermarket.com, www.gocompare.com and www.comparethemarket.com/car-insurance.

Do I have to be at renewal to do this?

If you’ve different cars in the home they may be up for renewal at separate times – which may make you think you can’t switch to a multi-car policy. Well don’t worry. The first thing to do is get a quote and see if it’s likely to be saving you substantial cash.

If you find it’s likely to be much cheaper, then provided you haven’t claimed (or reported an incident in that insurance year) on the policy that’s not at renewal since its last renewal, then for £50ish admin fee you can normally cancel your policy, and get the rest of the year refunded (you won't earn that year's no-claims bonus) and switch to the deal.

Martin Lewis is the Founder and Chair of MoneySavingExpert.com. To join the 12 million people who get his free Money Tips weekly email, go to www.moneysavingexpert.com/latesttip.