I filled my tank with mid-grade fuel yesturday (90 octane) and afterwards felt my stomach sink after realizing that I only saved a dollar. First thought, "What have I done?!?" Second was, "If she starts clanking I'm still going to have to drive until the tank gets low." The final to wrap it all up, "Well at least I'll know how she'll do if I can't get premium." - Hey. Give me a break. She's my babe. ;) After a day of driving I'm feeling okay and I've got no worries.
chadjohnson gave me the go ahead to post the info he gave me to get better gas mileage...
Here's what I know about increasing fuel economy:
Maintenance:
The most important thing is to make sure your tires are inflated to the proper pressure. The owners manual (or a sticker on the door sill) will tell you the recommended pressure. The pressure printed on the tire is the maximum pressure, not the right pressure.
The GSR requires premium fuel. If you don't drive it hard (i.e. VTEC) you could probably get away with mid-grade (90-91 octane). Using a lower grade fuel could result in low mileage among other things. Premium gas also tends to have detergent additives, which leads to the next point:
Keep your fuel system clean. Check the maintenance schedule to see when the fuel filter needs to be replaced. A bottle of STP costs like 5 bucks, so it might be worth it. You shouldn't have to use this regularly (especially if you use premium gas) but since the car is new to you, it's worth doing IMO.
Keep your air filter clean. These need to be replaced about every 3 oil changes or whatever the manual recommends. As long as it's not super cruddy, it's fine.
An obvious point, but regular oil changes are a good thing.
Driving Style:
Drive in a way that minimizes the use of your brakes. Try to find opportunities to coast.
Go slower on the freeway. Between ~55-65mph you're getting the best mileage you're gonna get.
Try to maintain a constant speed on the freeway. Cruise control is your friend.
Avoid hard acceleration. VTEC is bad for gas mileage. It's fun though :-D Basically drive like a grandma if you want better mileage.
Here's an article from the FTC about octane The article says all grades of gasoline are required to have detergent. It doesn't mention anything about quality or quanity of that detergent, so I guess that's up in the air.
I'd say you could try switching to mid-grade, but if you hear any knocking or pinging, switch back to Premium.
And you don't have to drive that grandma-ey. Just keep it under control. You know how some people are always either accelerating or braking? Don't do that.