I recently bought an S40 and I didn't realize that it needed premium gas until today, so I've been putting regular in for a while.

my question is:

will switching over to premium now have any negative effects?

 

It doesn't need it. Performance numbers are with premium, so you may have slightly less power or lower gas mileage with 87 octane. But it won't hurt anything. Switching won't hurt anything either.

In my car, I got the same MPG with 87 as 91. Others have reported higher MPG with 91

 

No, your car will thank you. As you have seen, it'll run fine on lower octane fuel but may run better with the higher stuff. You may also notice better gas mileage which will offset the higher cost.

 

ok thanks for the info guys.

also, i'm sorry if this has been asked before, but I'm pretty clueless about this kind of stuff;

is my s40 limited to it's current size of rims?

would getting bigger rims on it have any bad effects?

 

What year S40? I believe the 2005 came with 3 options total; 15, 16, and 17 inch wheels. I have the 16 inch wheels on a T5 AWD car. The speedometer will read low if you go to a larger diameter wheel/tire package. Not sure if the ECM can be reprogrammed for anything over the stock options.