Fuel discount vouchers from major supermarkets may not be giving you the bargain you think.

A Herald on Sunday survey has revealed that even after factoring-in the price reduction you could save more on your groceries by shopping around.

We bought the same 10 items from stores owned by each of New Zealand's major supermarket chains and got the same basket of shopping from other outlets, such as greengrocers, butchers and dairies.

The standard supermarket fuel discount for customers who spend $40 or more is 4c a litre off their fuel - giving a total saving of $2.20 when filling up New Zealand's best-selling car, a Toyota Corolla.

That means shoppers who went to Pak'nSave would be only 3c better off than if they had shopped around.

Woolworths customers who used the docket would have been almost $4 out of pocket.

Grocery Retail Association chief executive Trina Snow said the calculations showed consumers had to work out whether it was worth their time to bother with fuel vouchers.

She didn't think supermarkets were trying to trick customers because they tended to offer higher discounts on fuel when petrol prices increased.

Customers who don't live near a butcher or greengrocer may spend more petrol driving to buy cheaper food. "I personally can't be bothered with the vouchers because I don't think it's a huge saving, but it depends on the individual," she said.

"You need to work out how much you are spending and how much you're saving - for those who are shopping at supermarkets for a bargain this would be interesting to them."

Food and Grocery Council chief executive Katherine Rich said a $2 saving on petrol was unlikely to be enough to sway someone to shop at a certain store. But she said fuel vouchers were likely to be a perk for smart shoppers.

"I think shoppers see it as icing on the cake," she said. "Most consumers are smart and know how much it costs to fill their tank and they'll shop accordingly, but the numbers could be an eye-opener for some."

Murray Jordan, retail general manager for Foodstuffs, which owns the Pak'nSave, New World and Four Square, said the vouchers were not used as an incentive to get people to shop there and that most people who shop at Pak'nSave bulk-buy to feed a family for a week, and generally spend $100 to $350. "When you add the discounts on top of this it's a compelling offer ... added to that we have fuel outlets on our carparks so people don't have to make a specific visit to a petrol station."

Bill Moore, of Progressive Enterprises, which owns Woolworths, Countdown and Foodtown, said fuel vouchers were another way of saving money, as were the company's Home Brand range and Onecard loyalty scheme.

"Everyone lives different distances from their local stores or petrol station and food prices vary.

"The maths would be different when talking about 6, 12 or 20c off [a litre] as can be the case in some of our promotions.

"We offer fuel vouchers because research shows that our customers really appreciate and use them (and they) are being redeemed in greater quantities than before the recession."