Costco 91 vs 95 in New Zealand: Which Petrol Should You Buy?

New Zealand has one Costco fuel station, at Westgate in west Auckland, and it gives you two petrol choices: 91 and 95. This post covers what those numbers mean, whether Costco sells 98, which cars actually need 95, and how much more 95 costs at Costco, using prices from our own tracker.

Short answer: Costco Westgate sells Unleaded 91, Premium 95 and diesel. It does not sell 98. Buy the grade your car's maker asks for: 91 if the fuel flap or manual says 91, 95 if it says 95. On 15 October 2026, 95 cost $3.507 a litre against $3.317 for 91, a gap of 19c a litre, or about $9.50 on a 50-litre fill.

What petrol does Costco sell in New Zealand?

Three fuels: Unleaded 91, Premium 95 and diesel. Costco's own store data for Westgate, which we checked on 15 October 2026, lists exactly three fuel types: "Diesel", "Unleaded 91" and "Premium 95". Our warehouse data matches.

Westgate is Costco's only fuel station in New Zealand. It sits across the road from the warehouse at 2 Gunton Drive. When it opened in April 2022 it had 27 self-service pumps, and Costco said it was partnering with Mobil for petrol distribution, Driven reported at the time. For today's prices, see the Costco Westgate fuel price page.

Does Costco sell 98 in New Zealand?

No. Costco's Westgate listing has no 98, and our price feed has never shown one. There is no 98 pump to choose.

If you want 98, you have to buy it somewhere else. Mobil lists Synergy Supreme+ (98) alongside Synergy Supreme (95) and Mobil Extra Unleaded (91). BP lists BP Ultimate 98 above its Regular 91 and Premium 95. Not every brand has one. On 15 October 2026, Z's fuel page listed only Z91 Unleaded and ZX Premium Unleaded (95), and Gull's fuel page listed only Regular 91 and Premium 95. Gull used to sell Force 10, a 98-octane petrol with 10% ethanol, but its current fuel page does not mention it.

What do 91 and 95 mean?

The number is the Research Octane Number (RON). It measures how well petrol resists knock, which is fuel igniting too early under compression. Higher-compression and turbocharged engines need higher octane. A higher number is not "cleaner" or more powerful petrol on its own.

New Zealand's grades are set by the Engine Fuel Specifications Regulations 2011. Regular grade petrol is petrol supplied as having a RON of at least 91.0 but less than 95.0. Premium grade petrol has a RON of 95.0 or higher. The regulations also set minimum Motor Octane Numbers of 81.0 for regular and 85.0 for premium. There is no separate legal grade for 98. A 98 is premium petrol marketed at a higher number.

Is NZ 91 the same as US 91?

No. New Zealand, Australia, Japan and Europe all use RON. The US and Canada post the Anti-Knock Index, an average of RON and the harsher MON test, so their pump numbers run lower for the same fuel. A US Costco's "91 premium" is a high-octane fuel. A New Zealand 91 is regular. Our US Costco gas grades guide covers the American numbers. Don't compare them directly.

Australia uses the same scale as New Zealand, but its Costcos sell 91 or E10 plus 98, with no 95. See Costco fuel grades in Australia.

Does Costco petrol in New Zealand contain ethanol?

Costco doesn't say, and we found no ethanol-blended fuel listed for Westgate. Ethanol blends are rare in New Zealand. Z's and Gull's current fuel pages list no ethanol petrol, and the planned Sustainable Biofuels Obligation, which would have pushed fuel companies to supply biofuels, was dropped by the Government in February 2023 (MBIE).

The rules allow petrol with up to 10% ethanol, and blended petrol has to be labelled at the pump. When ethanol blends were first approved in 2003, the standard pump label was "contains up to 10 percent ethanol". If Costco ever sells an ethanol blend, the pump will carry that label.

Which cars need 95?

Your car's maker decides. Check inside the fuel flap or in the owner's manual. The AA's guide to petrol puts the rule simply: "if your car requires 91 octane then that's all you need, but you can use 95 or 98", and "If your car requires 95 octane you must use 95 or 98, but not 91".

Some general patterns, which never override your manual:

• European cars. The Motor Trade Association says "The lowest octane fuel available in Europe is 95, so any European brand of vehicle should operate on this" unless the maker says otherwise.
• Turbocharged and performance engines. These are the cars most likely to call for 95 or 98.
• Imported cars. The MTA also warns that "the imported version of the same vehicle may need to run on a higher-grade fuel" than the version sold new here. Many used Japanese imports run happily on 91, but check the specific model.
• Japanese imports that call for "high octane". Japan's regular petrol starts below ours, but its high-octane grade is 96 RON or above, according to Honda. A Japanese-market car built for high-octane petrol needs at least 95 here. Costco's 95 is the highest it sells, so ask the importer or a mechanic whether 98 is better for your car.

If you're unsure about an older import with no manual, a franchised dealer for the brand can usually tell you the fuel spec from the model code.

What happens if you use the wrong grade?

95 in a car built for 91: no harm, and usually no benefit you will notice. The MTA says "Buying higher octane petrol than you require is a waste of money." The AA agrees that a 91 car "can use 95 or 98".

91 in a car built for 95: don't. The AA warns that "Drivers risk causing serious damage to their engine if they use a lower octane than recommended." The early ignition shows up as knocking or pinging. Many modern engines have knock sensors that cut back ignition timing to protect themselves, which costs power and economy. One accidental tank in an emergency is not the same as running a 95 car on 91 every week, but don't make it a habit.

Petrol in a diesel, or diesel in a petrol car: don't start the engine. Call for help to have the tank drained.

How much more is 95 than 91 at Costco?

On our 15 October 2026 reading at Westgate:

Premium 95 cost 19c a litre more than 91, about 5.7% more. That is $9.50 extra on a 50-litre fill, or $190 more for every 1,000 litres you buy.

Over the 30 days from 16 September to 15 October 2026, we have both prices on 29 days (we missed 18 September). The 95 premium averaged 18.1c a litre, between 14c and 24c. So today's 19c gap is typical. For comparison, Driven reported a 10c gap when the station opened in April 2022 ($2.50 for 91, $2.60 for 95).

Prices change often. Check the Westgate price page before you drive. For how Costco compares with other Auckland stations, see is Costco petrol cheaper in Auckland.

Is 95 worth it in a car built for 91?

Not on cost. For 95 to pay for itself at a 19c gap, it would need to give you around 6% better fuel economy. The AA has said 95 may give slightly better economy in a 91 car, but not enough to make up the price difference. If your car is built for 91, buy 91 and keep the $9.50.

If your car needs 95, the gap is not a choice. Buy 95.

Is Costco 95 good quality?

It meets the same national specification as every other 95 sold in New Zealand. That is the only thing we can say for certain. Costco New Zealand publishes no fuel specification and no additive claims for its petrol that we could find. Its Westgate listing names the grades and prices, and nothing more.

Costco's US claims about Kirkland Signature gasoline and its Top Tier detergent rating are about the American product. Don't assume they apply here. See is Costco gas Top Tier for what Top Tier means in the US.

The AA's view on brand differences is that claims one brand's petrol performs better than another's are "hard to prove or disprove", and that fuels sold in New Zealand "are well suited for a variety of vehicles".

When can you fill up?

Costco fuel is for members only (membership and paying at the pump). The Westgate station opens at 6am every day and closes at 9:30pm Monday to Friday, 8:30pm on Saturday and 7:30pm on Sunday, per Costco's store listing on 15 October 2026. Holiday hours differ, so check Costco Westgate fuel hours before you go.