Drivers in Rickmansworth, on the north-west fringe of London, are enjoying the lowest price for petrol anywhere in Britain, with unleaded on sale for less than 120p a litre for the first time in more than four years.

The dramatic fall in the price of oil – Brent crude is down from around $115 to $85 a barrel in the past three months alone – is finally showing up on forecourts across Britain.

In Rickmansworth, an Esso and a BP station are battling it out a few yards apart on the Uxbridge Road to offer the lowest price, at 118.9p – but motorists who take advantage of a slew of supermarket deals can now buy petrol for as little as 102p, with further price falls expected in coming weeks.

Filling up a Ford Focus cost around £71 when prices peaked in April 2012 at 141.9p, but will now set a driver back around £63 on average – or less than £60 if you happen to be on the Uxbridge Road. According to the AA, the price cuts are putting more than £2m a day back into motorists’ pockets, and should boost consumer spending in other parts of the economy.

But how do you find the best prices – or even get down towards £1 a litre?

• Check petrolprices.com This free-to-use site lists the lowest price petrol station in your area and is updated daily. For example, on Wednesday this week it was showing the average price for unleaded in the UK as 125.6p a litre, with 118.9p the lowest anywhere. Simply type in your postcode – we tested the Guardian’s office at N1 9GU – and the site shows the cheapest nearby station, which in our area was 123.9p a mile up the road, or 120.9p if we were prepared to head five miles away to an Esso in Bermondsey.

• 20p a litre off at Tesco First, a word of warning. The supermarkets have cleverly planted in motorists’ minds the idea that they are cheapest, but our snapshot survey found they are often 2p a litre more expensive than rivals such as Esso or Jet.

Tesco’s Clubcard Fuel Save, due to run until 28 February 2015, allows Clubcard holders to save up to 20p per litre of fuel. For every £50 you spend in store or at tesco.com/groceries, you get 2p off a litre of fuel. Then for every further £50 you spend, you get another 2p off a litre and so on, up to a total of 20p a litre.

It is worth noting that your fuel savings are separated into monthly pots. Each monthly pot is valid until the end of the following month, and the next pot to expire must be used first.

The cheapest Tesco petrol we could find this week was 122.9p a litre, so 20p off would take the price to just 102.9p. Note that Tesco petrol prices vary nationally – it’s currently 124.9p in Inverness, for example.

• 7p a litre off at Sainsbury’s Not all customers are being offered this deal – it depends on how you use your Nectar card – but many can obtain 7p off a litre of petrol if they spend £15 on laundry and cleaning products and toilet paper. Early next year Sainsbury’s is also launching a deal of 5p off a litre through Nectar, with the discount paid when you return to the petrol station to fill up again.

We found Sainsbury’s selling petrol at 121.9p in Greenwich and Watford this week, and if shoppers use the 7p voucher this would bring the price down to 114.9p. Note that Sainsbury’s does not operate a single national price – for example it charges 2p more a litre in Cornwall than in south-east London.

• Morrisons Fuel Saver This is an ongoing scheme, launched in September 2012, that gives you 1p off a litre for every £10 gift card you buy at the supermarket. For example, buy a £50 gift card at Morrisons to spend in Topshop, Next or H&M and you get a coupon for 5p off each litre of fuel.

There is no maximum spend on gift cards (redeemable at more than 50 different retailers) but the coupons last only 63 days from the date of issue.

• Don’t be fooled by supermarket vouchers, says Asda The vouchers from the likes of Sainsbury’s are a gimmick, says Asda. It analysed a 12p-off-a-litre deal from Sainsbury’s in 2011, linked to spending in the store, and said that the same goods plus petrol bought in Asda would be cheaper at the basic price. Asda claims to have consistently lower prices than rival supermarkets.

However, when we tested prices within a 20-mile radius of Manchester, both Sainsbury’s and Morrisons were selling petrol at numerous locations for 121.9p, but Asda’s cheapest price was  122.7p.

• Cashback cards – save £9 a month on petrol Santander’s 123 credit card gives users 3% cashback on spending of up to £300 a month at most petrol stations, including BP, Shell, Esso, Texaco and the main supermarkets. So if you go to Esso or BP in Rickmansworth and pay with a Santander credit card, it brings the effective price down to 115.3p a litre. Note that there is an annual £24 fee for the card, and always pay off the card bill in full every month or the interest costs wipe out the value of the discount.

• Shell Drivers’ Club – 0.5p off a litre Join the Shell Drivers’ Club for free, and get £2.50 off every 500 litres you buy. That works out at 0.5p off per litre, but it’s not worth it if your local station is pricey compared to rivals.

Can you cut costs just by driving more smoothly? Patrick Collinson learns how to save hundreds of pounds off fuel.