That's certainly the key element there. The question is, would they be tuned more conservatively if rated for Regular?

I was pretty happy when I saw only 1 HP loss on that test, but then the 7 ft/lbs was a bit discouraging, being the more meaningful number.

Right now is the best time to experiment with the lower octane, at any rate. When gas is cheap like it is now, you pay more of a, er, premium for Premium. A lot of people do it backward, spending the extra on Premium when prices are lower and skimping when they're high, but the fairly fixed difference is at its lowest percent when prices are up.

 

That's a good question and really I dont think we will ever find out the answer on it 100%. according to my butt dyno, which is the most important dyno, its not making a large impact (lets say its borderline noticeable). your butt may vary.
I used to use mid-grade quite often before going turbo. now I only use premium but I am also either on a turbo ecu or standalone so its completely different.

If you are using the car mostly for commuting, it makes sense to save a little coin sometimes, cause to be honest even if you have lets say an extra 10 ft/lbs of torque you still wont be winning any races in a stockish sc300/400.
But to the guy with a lead foot who is just trying to get max out of what they have, premium is the way to go. unfortunately, making more power and spending more $$ go hand in hand.

as far as the original question, side effects are that your car will be a smidgen slower, but you might not notice it. there is no reason you would hurt the engine.. unless the gas is bad and has water in it.

 

So, I filled up tonight for the first time after a partial-tank Regular (7.9 ga added when topping off) experiment. I usually average 15-16 mpg with mostly city driving if I minimize the short trips (use the other car for that) that are barely long enough for the temperature gauge to rise. 18-19 mpg with a mix of city and highway.

This fill-up was 18.5 mpg, consisting mostly of longer city driving (5-10 mile jaunts with a bit of stop and go). I filled up again with Regular, so I'll know a lot more next time around, but there's certainly no strong evidence yet that fuel economy will suffer. If there's any loss in performance, it's too small to write off as just my imagination because I'm paying closer attention to it.

 

Thats an interesting way to think about that. I never considered it that way but you are right, about the % difference compared to the absolute difference.

 

I've explained it to a few people over the years and ... crickets. Sometimes I'll get a grudging, "you're right, but I don't want to pay more when it's already expensive."

 

Follow-up...

After three fill-ups of 87 octane Regular, I'm getting a consistent 18 mpg with mostly city driving, and 19-20 mpg with highway driving mixed in. In the past, I've been around 15-16 mpg with the same mix of city driving, and maybe 18-19 mpg for the partial highway ratio.

I think that's enough full-tank sample data to rule out rounding errors on subsequent fill-ups. So, I see two possibilities: 1) the spring/summer gas blend coincides with this experiment and has the car running more efficiently (I did get some pretty good avg's last May), or 2) it actually runs better on Regular than on Premium.

 

I fill up with premium every time, but I do tend to drive it hard. If I had an automatic and just puttered around town most of the time, I'd probably try regular.

 

t2d2's data seems to support that the stock ecu can handle regular just fine... at least mpg wise. If it was really "tuned up" for premium "only" that would not have gone so well.

 

^ Yep, and I've felt no ill effect performance-wise. If I'm down on power at all, it's too little to tell.

 

Interesting. Toyota really did emphasize "bulletproof" when they tuned these engines.

 

i'm gonna try regular today, i have like 3 gallons left in the tank right now so it should be a good test. the highest MPG i got with combined highway/traffic/small town roads was 22.2 MPG. I'm wondering how it will affect the mileage since i don't drive the car fast ever so idc about performance much.

 

22.6 and 22.8 mpg are the two highest averages I've recorded, both with mostly highway driving and a few hard accelerations along the way. I haven't done an all-highway test on Regular yet.

 

well i'm a quarter of the way down on the tank of regular, feels exactly the same as it does on premium so far. mileage seems to be the same so far, doesn't seem to be going down slower or faster. i'll report in like a week or two when i fill up.

 

well i filled up again, got 21.6 MPG, absolutely no change and i was driving in the city a lot and hauling tiles around for a day. probably will get even better mileage on this tank since i haven't driven anywhere unordinary.

 

Filled up again, highest mileage I've gotten yet. Drove 406 miles on about 17.8 gallons IIRC. So getting about 22.7mpg. I have been hearing this ticking noise which seems to be coming from the injectors, but that started before i tried regular gas.