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2018 Volkswagen GTI - fuel type

If my car suggests mid-grade fuel (87), can I use half 85 and half 91? does it get mixed up?

Just use 87 , why would you go to all that trouble to save pennies . Also that is a really good way to irritate the person in line behind you.

“Suggests” or recommends or requires mid grade? If it requires mid grade, the USE mid grade.

First off, why would you do that? Why go to the trouble at all? Yes, you can mix, but use more 91 than 85 to be on the safe side and yes it does get mixed up.

Stations around me are selling mid-grade at halfway between regular and premium prices so there is no savings at all and loads of risk to your expensive car.

Curious, where do you live?
Where I am in Florida rating available are 87,91, and 93.
Non-ethanol 89,90

Wouldn’t recommend to monkey with mixing the grades if mid-grade is required. Could result in engine damaging pinging and void your warranty.

85/89/91 is often the range in higher elevations areas, like Denver.

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The Golf R requires premium fuel, the Golf GTI uses regular unleaded fuel. Using a higher octane may offer better performance for those who push it to the limit. Using a higher octane won’t cause the engine to ping and void the warranty.

A car that requires 87 octane (US) at sea level can safely use 85 at %000 ft elevation because the thinner air at 5000 ft lowers the effective compression. The exception might be on some high end turbocharged cars.

I know that a few years ago, Lincoln made a model that compensated for altitude by having the computer increase boost pressure.

If you mix 85 and 91 octane gas the result is 88 octane. I did it years ago when I had a car that required mid grade and the stations had a big price jump from regular to mid grade and a much smaller jump from mid grade to premium.

I don’t know if Sunoco still does it, but the buyer turned a knob to choose the octane level desired and then pumped gas. I believe they mixed gasoline at the pump rather than keep five or more octane levels in underground storage.

However, using a lower octane than required might.