2nd gen prius, will using premium fuel affect my mpg and in what way?

no, it will only affect your wallet

just use what is recommended in your owners manual

 

The octane number is the fuels resistance to detonation.
Higher octane fuel burns more slowly to have higher resistance to detonation.
Burning more slowly can actually cause a reduction in MPG in engines that are not designed to use higher octane fuels.
The Gen 2 recommended fuel is 87 octane. I would stick with it.

 

Carbon buildup can cause pinging and detonation. If you are hearing that, then higher octane fuel can help. Otherwise 87 octane, from the most popular station near you is best.

 

Gasoline quality, not octane, is what counts. Even many cars like Lexus that say they require premium will run fine on lesser octane with undetectable performance decline, so long as you are consistent in buying same octane else the computer chases its tail trying to optimize.

Some discount brands sell so-called startup gasoline they get cheap from distributors or blends that don’t have proper additive blends. At current ridiculous prices, the spread between craps gas and top tier is minimal.

See some layman’s info here.
https://www.popsci.com/sponsored-post/top-tier-gasoline-detergents

 

Oil consumption can also cause pinging.

If the engine pings with regular fuel 87 R+M/2 (91 RON) then using premium instead will "bandaid" the problem and get you better fuel mileage. An engine in good condition will not benefit from more octane than the recommended octane.

My suggestion is to see if you do get the same fuel mileage with regular as with premium. If you notice you get better fuel mileage with premium then try to determine what's wrong with your engine and see if you can fix it as the problem will probably only get worse. Next thing you know you have to use racing or aviation fuel to get good fuel mileage if you don't fix it now. So is it burning oil? Or is it coked up with carbon? Is it running lean? Or is your engine fine and gets its best fuel mileage with 87 R+M/2 octane?

 

Coils or plugs?

 

A lean condition would cause the Check Engine Light to turn on. So does constant pinging caused from any reason. Very bad pinging will cause the CEL to flash meaning "Get off the road and turn off the engine before you blow a piston!"

A hot running engine (stuck thermostat for an example) can also cause more pinging leading to high octane bandaiding the problem. Carbon build up might also cause more pinging too.

An engine that was previously machined and thus has a higher compression ratio (for an example, a milled head or milled block or both) can also cause more pinging.

 

What's the best gas

 

Most large brands, Shell, Mobil, BP are good quality with plenty of additives. Additives cost money to blend in. Discount brands can be ok, but hit or miss depending on who their wholesaler is. At current prices, stick to quality brands for the few pennies more they may cost.
When we had refinery upsets years ago during startups, the refinery would offload the off spec ‘startup gasoline’ to wholesalers to sell. I’m sure that has not changed.

 

Best at what?

In my opinion, the most frugal gas purchase is the most popular regular unleaded has 87 AKI octane near your home, Gas 'settles' if not used, so old gas tends to be bad gas. You want gas which has not had time to settle.

If you must buy by buzzwords, ' top tier gas' is a good buzzword at a popular station.

 

My Toyota owner's manual specifically recomends Top Tier gasoline.