Yes, and boy, does getting into 6th make a big difference, too!

That's about 26mpg for you US folks.

Best mileage on a fillup I can remember in a long while. I usually get 10L/100km per fillup.

Could be the alignment helped (less tire scrubbing), or more likely, my new commute doesn't allow long stretches at 80mph.... It's now closer to 55-60mph.

Peter

.... is 55 mph in the highest gear.
So your speed reduction most likely is responsible for your better mileage.
Happy Boxstering
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

But, as noted, at constant speeds in the highest gear, almost by definition. I'll spare all the science. But it has to do with pumping losses, effective compression, friction losses, and air resistance being proportional to (roughly..) the square of the speed.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

I tracked this way long ago on my 2000S, in 2002, and of course, YMMV:

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Edited 1 time(s). Last edit at 10/01/2012 09:38AM by Bruce In Philly (195K+). (view changes)

In particular, hwo did you control for consistency of speed (lack of braking and accel) which is the #1 factor in poor mileage.

This, coupled with better efficiency at moderate/lower speeds, is why hybrids, which regenerate braking energy into accel energy, do better in city cycles than highway ones.

So absent that, what i see is "the more highway driving the better" -w hcih is very sensible.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

I calculated MPG using the actual miles and gallons used as denoted on the pump when I filled - I did not use the computer's calculation of MPG. For AVG MPH, I used the on-board computer for this. I never fully understood how the on-board computer calculates average MPH, but really it was all I had to use. Now obviously, the higher average MPHs were primarily highway and lower MPHs were city and mix. I do know that the high MPHs were from a drive I took from Philadelphia to Atlanta where the only slow-speed driving I did was on the entrance/exit ramps and pulling into gas stations.

While not a true scientific set of measurements, I do believe the data accurately state that faster is better for mileage. If you can find a flaw with all of this, fire away as you may just be very correct; but my data is what it is.

Only other data point to give you is that my city driving style is quick but not aggressive (whatever that means).

Bruce



Edited 2 time(s). Last edit at 10/01/2012 10:17AM by Bruce In Philly (195K+). (view changes)

the biggest impact on mileage is accelerating-decel-re-accel.

Think F=MA. Its not F=MV. So controlling for constant, stead, cruising is absolutely critical.

Since most lower speed driving includes lights, curves, traffic, etc - it will almost always show lower mileage. But this is despite the fact that if one were traveling at a steady speed, it would yield even better economy.

That's what needs to be controlled for. I didn't read that yu did any of that....did i miss it?

(in fact, it would be quite difficult)

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

I've yet to use the Auto Stop/Start function except once or twice to get an impression of how it works. And it does work well, but it's disconcerting to feel the engine die: a lifelong conditioning to having a motor idle when at a red light, etc. I've mentioned elsewhere that oil pressure registers as 1 PSI when AS/S is activated. (No jokes, please: that's why I used the slash mark!)

Very good points of course.... but, who drives in a scientifically controlled manner? My driving is as I drive and those were the results. While I would not bet a dime that any one of those line items in my chart was spot accurate, I do believe that the data is directionally correct in that faster average speeds, for the average Porsche driver, will yield the same results. Dangerous nutty drivers and track drivers may have a completely different profile than what I posted. But I assume, maybe falsely, that with almost 200K miles and 12 years of ownership that, on average, I am an average Porsche driver. YMMV

Oh, I reset the computers with each fill up when I was tracking the stuff above.

Bruce

in this case i believe it is "steady cruising and fuel economy: as opposed to "speed and fuel economy".

I frequently get my best fuel economy on "b" roads in the country, running 45-55, in 5th ( i have no 6th).

My other best is really long freeway drives with nearly no stops - e.g.: the trips to VIR or Mid Ohio.

Those often occur at higher speeds however. More i prefer not to say

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Quote
Bruce In Philly (195K+)
I calculated MPG using the actual miles and gallons used as denoted on the pump when I filled - I did not use the computer's calculation of MPG. For AVG MPH, I used the on-board computer for this. I never fully understood how the on-board computer calculates average MPH, but really it was all I had to use. Now obviously, the higher average MPHs were primarily highway and lower MPHs were city and mix. I do know that the high MPHs were from a drive I took from Philadelphia to Atlanta where the only slow-speed driving I did was on the entrance/exit ramps and pulling into gas stations.

While not a true scientific set of measurements, I do believe the data accurately state that faster is better for mileage. If you can find a flaw with all of this, fire away as you may just be very correct; but my data is what it is.

Only other data point to give you is that my city driving style is quick but not aggressive (whatever that means).

Bruce

carefully. Well, I do not enter the data into a spreadsheet but I note at the time the mpg and the average speed and it was a bit unexpected to over time note that a higher average speed came with better gas mileage.

One of the things I like to do on a long drive is to see how close I can keep the average speed to the posted speed so I strive to do as little driving as possible that doesn't not directly relate to how much further down the road I get.

While I avoid overly aggressive driving I do get the car up to speed pretty quickly and pass slower traffic when it is safe to do so

Also, I will at times push the envelope and run over the limit using the speed of other vehicles to know what is likely to be ok. My V1 and CB radio offer good protection against coming upon the occasional speed trap. My last ticket on the open highway came from running 10mph over the limit with my V1 turned off. I simply forget to turn it on when I left the hotel that AM. Ouch!

Anyhow, it is nice to see someone else arrived at the same conclusion, that it wasn't just me.

Sincerely,

MarcW.

the same way I do in my automotive test eqiupment firmware.

Speed is sampled periodically from trip start to trip end and accumulated along with of course the number of samples taken then at the end of the trip the average speed is caculated.

How other data of this sort like max speed, distance traveled, idle time, time in speed bands, accel/braking/lateral g events, fuel consumption for gas, diesel, flex fuel, alternative fuels,hybrids, electrics is arrived at I can't divulge.

Sincerely,

MarcW.

I'm surprised at the difference you found in the OBC and calculation of your average MPG. At each refueling, I reset my mpg, average speed and trip odometer. I record it all in a spread sheet. Pretty simple way to keep tabs on what the car is doing. I've never found the difference between OBC and actual mpg to differ by any more than 0.1 mpg. (Yes I do it in the metric setting but same result.)

As for AVG MPH. I believe the computer just calculates the time and distance you travel from starting the engine to switching it off.
Simple example.
You start the car and immediately drive off at 60 mph for 45 minutes then have to stop and let the car idle for 15 minutes, then turn off the engine.
So in that hour/ 60 minutes, you've driven 45 mi, so your average speed was 45 mph.
The OBC keeps track of this from one start/switch off/ to the next. So in mostly city/town driving (assuming a 30 mph limit), you might average something like 15 - 20 mph, with all the stops you have to make.

As far as determining optimal speed for best mileage, I think you'd have fill up and drive a route at one speed, then fill up, drive the same route at the next test speed and so on to get an acurate measure of which is the optimal speed for best mileage. Otherwise, there are a lot of variable that can have an effect on mileage - acceleration, traffic, # of stops, driving up hill, down hill, etc.

I'd never achieve this from Sedona, AZ to Flagstaff, AZ, but it was possible from Flagstaff to Sedona. (Even if I went both ways at the same speed)

If you notice in my data, the biggest variance in computer calculated MPG is at the slowest average speeds (well sort of anyway). I suspect, but am not completely sure, that the computer's algorithm is recency based, in that what you were doing at the end of your trip has a bigger impact than what a true average was. Just a guess.

Bruce

Quote
Bruce In Philly (195K+)
If you notice in my data, the biggest variance in computer calculated MPG is at the slowest average speeds (well sort of anyway). I suspect, but am not completely sure, that the computer's algorithm is recency based, in that what you were doing at the end of your trip has a bigger impact than what a true average was. Just a guess.

Bruce

I don't think that what you do near the end or beginning of a trip has any more or less impact. Again, going back to the simple example.
Drive for one hour. (That is the engine is running for one hour, during which average MPH is calculated.

Let the engine idle for 15 minutes, drive at 60 mph (one mi./minute) for 40 minutes, let the car idle for 5 minutes. So in that hour, you'd average 40 mph.

When it comes to city driving, it's REALLY hard to get the same mileage twice in a row because the number of lights you hit red and traffic congestion, all will have an impact on mileage.

The city of Hamilton (pop about 700,000) has its lights timed along one of its main streets. If I hit the first light green and travel at a stead 35 mph, I can drive through the whole city (roughly 7 miles) without making one stop. I haven't check mileage in this one, but I know I'll get way better mileage than if I hit the first light red and have to keep starting and stopping.

Some people just don't seem to know about the light setting and they'll jack rabbit start from each light ahead and sit and wait at the next red light over and over. Their average speed is going to be the same as mine, but I'll get much better mileage since I'm driving a constant speed.

As mentioned in the iPad thread, the trip odometer likely reset at 6213 (6214 rounded up) miles. The last display I took note of was somewhere over 6200 miles, and I guess the internal calculation is based on kilometers, as in 10,000. I didn't catch when the Consumption display resets, but it seems to be somewhere around 2500 miles. There's probably a metric equivalent there, too. My best guess for my estimated 8246 mile trip is 27.75 to 28 even. The lowest displayed I saw was about 25.5 and the highest was a 29-29.3 over maybe 30 miles or so. I know I saw a 496 mile range on one fill-up, and unless I'm dreaming, 505 once. Several times the range was over 480 miles.

Trivia: The 912's engine consumption graphs as the inverse of torque. Maximum torque is at 3500 rpm, and that's the point of minimum fuel consumption, again, isolated to the engine itself. Pre EPA standards has vehicle consumption at 27.6. I think the most I saw with my '66 was around 24-25.

By the way, for any of you oil consumption worrywarts, the factory spec for the 912 was 1.32 -1.76 quarts maximum per 620 miles. After I "rebuilt" the motor by matching up the completely interchangeable cylinders and pistons for minimum clearance, I was using about one quart per 1000 miles. I think I used Castrol GTX 20-50. Dino oil, of course.

Oil consumption with the Boxster 3.4 was maybe half a quart between start and finish of the 8000 mile + trip. I wasn't worried about the oil being stressed as I must've been in 6th gear 95% of the time or distance.