amartin

Very interesting data here-- I made a trip from Austin to San Antonio and back (the EXACT same route). 84 Miles each way.

Going TO SA, I was on cruse control @ 84 mph, I got 15.7mpg.

Going BACK to Austin, I was on cruse control @88 mph, I got 22.5mpg (the best I've EVER gotten).

The trip to SA was @ 4am and ~65-70 degrees out side.

The trip to Austin was @ 3pm and ~95 degrees out side, with more traffic and throttle changes, though 95% of it was still done on cruse control.

The ONLY other difference is I got some gas from a random gas station, (I'm using 87 octane both ways), and topped off so I could record the MPG going down there.

So, my theory is: (1) Some addiitaves in the gas are confusing the ECU and causing it to run richer than needed, or

(2) At 84mph, my RPMs were in a fuel map-range which was farrr richer than when I was at a HIGHER RPM (88mpg)

Any other ideas!?

FoxTypeR

I'd say the most likely reason is the ECU changing the fuel mapping based on your driving habits. I always get higher mileage on the return trip than the run going out.

vegeta

"(I'm using 87 octane both ways)"

Why are you using 87? it kills the car.

Toly

When the temperature is hotter, the air is thinner. The computer may accordingly lean the fuel, giving you better mileage.

Nubo

Could be a tailwind vs. headwind situation. At 80+ mph, even a 5-10 mph wind could make a big difference. And at 95 degrees the air is substantially less dense than at 65. Also, any elevation difference? The gasoline could also be different. Here's an article that discusses how the gasoline formulation can vary from one place to another

http://www.sfgate.com/cgi-bin/articl...sn=001&sc=1000

Glyphon

or not. 87 is a perfectly fine grade to use in the car. the computer just changes the timing at which the plugs fire, which only causes slight performance lost.

GhostRidr

The old octane raiting as related to performance and mpg argument is an old one. I've had them before on other message boards. In the end, when people were challenged to show any studies or proof that higher octane had any real signficant effect, not one person could. On the contrary, I was able to show many more links to opposite, even one where engineers and such were putting low octane in the their Mercedes, and every other car out there that "recommends" higher octane fuel in the manuals (which seems to be just about everyone these days). The most I could ever find was one expert saying that at the very very most, MAYBE you could get a 5% increase in maximum performance/benefit (this was HP if I recall). Transfer that number through tranny loss, to direct RWHP, and even with that theory, at maximum, it was paltry. It is therefor, my opinion, that the only thing higher octane fuel does for the driver, is simply give more money to the oil companies.

I will say though, old muscle cars, with high compression, and no computers to adjust timing, then yes, those cars definitely do need the higher octane.

Now, having said all that, that was specifically related to conventional engines. I don't know how it would be in rotaries, but would bet it's the same thing. And of course, people will swear to God that premium fuel does have benefits, that they've seen it, and don't care what the experts say.

Tamas

iTrader: (1)

That's a myth. The car runs perfectly with regular gas. I don't even notice a performance degradation, but didn't do tests regarding this either.

Aseras

my car gets MUCH better economy on 87. I alternated tanks for the first 3,000 miles I owned my 8 and the 87 always got better mileage on my 104 mile rountrip to work.

dwill9578

why, right on the fuel door...........Super or High Octane whatever. It's a high compression motor that most likely you paid a good amount for??? Why not make it run to it's best potential? Over 10 cents a gallon??

pcimino

How did you measure your mileage? Generally mileage isn't too accurate for one tank, becasue the filler nozzle cutoff works on back pressure and could be adjusted differently for different stations. So you may not have filled up the tank to the same place each time. The easiest way is to top off the gas in the fill pipe so you can see the level of the gas, then you'll know you filled it up the same both times.

Temperature changes could have affected the tire pressure, but I doubt so dramatically, most of the tire heating would be from driving.

I vote for the tail wind theory, plus inconsistent fill up.

According to Tom & Ray (NPR "Car Talk") they had a caller who wondered if they were hurting their car with lower octane. They explained that higher octane delays combustion for higher compression engines. In older cars, the rule of thumb was use the lowest octane that doesn't cause the engine to knock. In modern cars, the computers can adjust the timing to avoid knock. No significant difference in MPG in their opinion.

HOWEVER, they cautioned that cars that state from the manufacturer to use high octane do so for engine longevity, as much as anything else. I don't remember the exact explaination (someone help out) but I think it was something along the lines of a higher compression engine would be more susceptible to carbon build up.

Aratinga

I think you were in fourth gear instead of sixth on the trip to SA, and in sixth gear on the return leg.

Don't laugh. It's happened to me before. :o

rx8wannahave

lol...that sounds possible, but didn't the RPM's give it away?

speeddemon32

i think that the differance could be found with all of the below combined.

was the trip out there:
uphill, head wind, and denser air/richer fuel.

and the trip back:
downhill, tail wind, and thinner/leaner fuel.


by uphill, and down hill, i dont mean climbing a mountain, i mean even an ever so slight hill. sometimes a slight grade can be hard to spot, but over a few hundred miles it will make a HUGE differance in the amount of effort the engine has to make.

MazdaManiac

iTrader: (3)

Lower octane gas makes MORE power, provided you do not knock. Knocking is dependant on temperature, load, fuel quality, etc.
If you can use 87 without ping, do it. I cannot, but in the winter I can get away with 89 which gives me much better mileage.

jaguargod

I don't know where you are that premium fuel is only $.10 per gallon more expensive than regular, but I want to find out.

rX-n-sa

The trip from Austin to San Antonio is a decrease in elevation, so the return trip is an increase in elevation. Since the winds in this area come out of the south and southeast during this time of year, then I would say that the tailwind theory may actually hold some water. Why were you coming into SA that early in the morning? Just curious.

jangle

[QUOTE=pcimino]How did you measure your mileage?

The best way to measure your gas mileage is:

Reset your tripometer at your next fill-up.
Then, when you're ready to fill-up again, take that number and divide it by how many gallons it took to fill-up.

Example of my last fill-up:

250.3 miles / 12.537 gallons = 19.96 MPG

Aseras

makes sense for a piston car but rotaries are diffrent. the combustion chamber constantly changes size and a lower octane fuel has a better flamefront and burning characteristic in a rotary. Higher octane burns slower and doesn't fill the chamber quickly enough to get the maximum benefit. Rotaries were heavily marketed in the 70's for using low octane fuels. Onlt real concern with the rx-8 is the 10:1 compression which with 87 is borderline detonation, however you should never encounter a problem since there is dual spark ignition and the car has a very sensitive knock sensor that will retard the timing if you happen to get subpar 87 fuel.

In my expericence in the last 8,000 miles on my 8, 87 runs far superior in mileage and power than 93 does. Keep in mind I'm in florida, it's toasty and humid here. If you are somewhere where it's much drier and cooler, you may have problems.

rotarygod

With an 84-88 mph cruise speed, no wonder people get terrible mileage! Try 70 mph. It'll shoot up to about 27 mpg or so and you'll only get between cities a few minutes later.

rotarygod

All engine combustion chambers are constantly changing in size. That's how you get compression.

Who says 10.0:1 is borderline detonation on 87 octane? That's crazy. What if I tune it less agressively. Borderline gone! Compression ratio isn't the main deciding factor, tuning is. Compression ratio matters if everything stays tuned the same way for differently setup engines. Why would anyone do that?

Dual spark plugs complicate the matter if anything. A leading/trailing split being too close together will detonate before a farther split will, even with the same total advance. The knock sensor on the RX-8 is TOO sensitve. It will retard the timing based on sounds it hears sometimes that aren't knock.

Raz0rama

I vote for the 4th vs 6th idea too - went on a round trip to maryland last weekend - with 50% city driving and 50% one long road trip I got 240 miles on 13.4 gallons. On the way back, I noticed halfway through the drive I was still in 4th gear, I'd been on cruise the whole time and hadn't thought about the revs (average speed for me in cruise is 85mph - I like getting there 20 minutes earlier rather than saving $3.00 in gas). Felt pretty stupid, especially when after finishing up the tank with another 50% city driving, I got 209 miles on 13.8 gallons. Driving conditions and temperatures were about the same accross the board for both tanks. I usually average about 160-180 miles on 13 gallons in all city, and 260-280 miles on the same tank in all-highway driving.

I've never encountered getting better milage on return trips - usually get about the same, plus or minus 1 mile per gallon.

jbrasure

Cast my vote for headwind/tailwind. I used to have a Honda Insight, and that car had a computer which made it easy to see what was influencing gas mileage. Wind made a BIG difference.

SilverEIGHT

Twice coming back from Savannah I did that in 4th gear for 50 + miles on the Interstate I felt like such a dum bass. Aratinga, I'm glad to know I'm not the only one that has done that. :o

Nubo

For anyone wondering why wind (airspeed) would be a deciding factor --

A big share of a car's power at highway speed is spent overcoming wind resistance. The force of air drag is proportional to the *cube* of the airspeed. That's a very steep curve. It means that doubling the airspeed results in EIGHT times the drag. You might not even notice a 10mph breeze while walking outside. But at 85mph into a 10mph headwind, your airspeed is 95. Reverse direction and your airspeed drops to 75. Drag at 95 is DOUBLE the drag at 75.