Why would you get lower mpg? I've noticed that as well..

 

If you use gas with an octane rating lower than recommended, the engine will retard the timing to prevent engine detonation. Basically, the car will make less horsepower and get worse gas mileage.

Think of it like this. If you have a 250 hp car that requires premium fuel (91-93 octane) and gets 25 mpg with it, if you use 87 or 89 octane fuel, you will get 240 hp and 22 mpg.

Many people think they are saving money by using a lower octane fuel, but the reality is, if gas is $3 a gallon for 87, you pay ~20

 

to the OP - do you get 93 Octane locally? If so you're lucky! California gets 91 octane, we used to get 92 years ago.

The local race fuel here I can get is 100 octane, not worthwhile in a street car. if you have a high compression engine and some advanced timing then the higher octane will be worth while.

 

I'm not sure that I agree.

The engine will only retard timing if it senses knock. I don't think you'd get knock at 5k rpms on the highway without significant load... even on 87 octane.

And the higher the amount of "octane," the less amount of freely combustable material, I thought. That's why it's less volatile and resists detonation.

So if you're not to the point of detonation and retardation... it would follow that you might get MORE mpg from lower octane gas... to a point.

 

[QUOTE=Saki GT,Feb 20 2009, 10:27 PM] If you use gas with an octane rating lower than recommended, the engine will retard the timing to prevent engine detonation. Basically, the car will make less horsepower and get worse gas mileage.

Think of it like this. If you have a 250 hp car that requires premium fuel (91-93 octane) and gets 25 mpg with it, if you use 87 or 89 octane fuel, you will get 240 hp and 22 mpg.

Many people think they are saving money by using a lower octane fuel, but the reality is, if gas is $3 a gallon for 87, you pay ~20

 

[QUOTE=CKit,Feb 20 2009, 05:03 PM]

I've wondered about running lower octane gas on a road trip where you're just sitting on the cruise control.

 

I've alternated between 89 and 93 over my commute for a number of tankfuls to determine the impact on mpg. Zero detectable difference, it ranged between 26 and 30, average of 28 on either. I still go with 93 octane.