The 2001+ Miatas, which the Emperor is one of, requires premium (91 octane) fuel because they bumped the compression, among other things, to increase the horsepower. Because of this I have been paying 20¢ more a gallon than must folks.
The car will run on lower octane gas because the ECU (brain) will compensate and adjust the timing so the engine won’t knock. When this happens the car will lose a few horses and the gas mileage will also suffer some. I have never cheated and run lower grade stuff, because even when the car is on E it only takes 10 gallons to fill up, so considering the downsides, saving two bucks more a tankful is just not worth it.
When I first got the car I was startled by a sort of abrupt surge in engine power as it passes up through 3000 RPM when the car is not fully warm. I wondered if there was something wrong because none of my previous Miatas did that. I checked that fountain of Miata knowledge, the forums on Miata.net and learned that that behavior was common in the 2001+ cars. They added a mechanical variable valve timing thingamabob and that in conjunction with ECU created that little surge which smoothed out after the engine was up to temperature.
Funny thing happened last Friday though, because Hurricane Ike created havoc with the gas supply in our neck of the woods, when I finally found a place that actually had gas for sale they weren’t selling premium. I filled the car up with mid grade (89 octane) and so far my butt dyno has noticed zero difference in pep. And as a bonus that, still surprising after all these years, 3000 RPM surge is gone. I didn’t check to see if the mileage was down, so tonight I filled it up with mid grade again. At the next fill up I’ll see if I end up getting less than the usual 27 MPG.
Miata Top Transitions since 01/01/08: 360
Several years ago, I had a Pontiac 6000 that behaved badly on low-octane fuel. I tried high octane and mid-octane and both seemed to do well.
Then I got scientific (some might call it anal). I calculated the miles per dollar. Basically take MPG and divide by the cost. I found that I got better miles/dollar with mid-grade than with low or high.
I spoke too soon, the surge was back today, though somewhat subdued. Still no knock or sub par acceleration. I will probably just go back to premium on the next tankful, but I am curious as to see if there is any noticeable drop in mileage.
In order to really be sure, you’ll have to test it a few tank-fulls to get a good measurement. Too many variables, like time spent idling, how full you fill the tank, etc.
I did tests in my pickup with tailgate up vs tailgate down. I read reports both ways, some swore better up, some better down. All with scientific evidence to back up the story. My results, no appreciable difference.
It would be interesting to know…