Higher mpg with 91 octane gas

[Nil]

Hi

I have been driving my 2000 corolla around with regular 87 octane gas. In the summers and till last month, the mpg inside the city has been 27 mpg and around 37-38 on the highway. Recently I filled the premium 91 octane gas. However, I noticed that my mpg on the highway dropped to 34. Is it because of the winter additives in the gas or because my car is having some problems ?
I had replaced 4 spark plugs because they were oil-fouled and an upstream O2 sensor couple of months ago to fix a sudden drop in mpg.

[fishexpo101]

Could be a number of things happening here. Assuming the engine is in good shape / good tune. Could be a function of winter blended gasoline and who blended that particular premium octane gasoline. Some use more alcohol to push up octane numbers - drawback is that it tends to hurt overall fuel economy, since ethanol has less energy content compared to gasoline.

Winter blends also bring in additives that tend to reduce energy content by 10-25% - which pretty much hits what you are getting now.

Real way to check is to refill with 87 octane - and check fuel economy. No real reason to use higher octane, unless you notice the engine is "pinging" more than usual. Happens as the miles start to roll on the engine, due to normal build up of combustion chamber deposits. Not a real fix, more a temporary solution, until you can enact a more permanent change.

Some cases, you'll see higher octane yielding higher MPG numbers - usually in the cases of engines that ping. Pinging leads to the ECM pulling timing, which in turn reduces power output. Will be different for each car - some just respond better than others or are more sensitive to octane of the fuel.

Also couldn't hurt to pull the plugs and take a peek at them. Also double check the O2 sensor, especially if the mileage starts to swing wildly and you've used a non-Denso O2 sensor. I haven't completely figured it out - but our cars (engines/ECMs) seem very sensitive to sensor impedances. If there is a slight mis-match, will give you all sorts of issues.