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I decided to see how mileage varied with speed. For each data point, I ran my base 997.2 Carrera on cruise control for ~5 miles or more on a flat freeway section (I-8 east of San Diego) with the following results:
mph mpg (actuals)
55 31.7
60 30.5
65 29.7
68 28.2
70 27.1
notes:
The speeds shown are actual speeds measured by GPS. The speedometer indicated speed read 2 mph higher. The uncertainty in mileage is ~±0.2 mpg.
Like the speedometer reading high, the on-board computer indicated mileage read high. The true mileage was calculated by taking the on-board computer indicated mileage and correcting it to the actual mileage. The mileage shown by the on-board computer reads ~1.6 mpg higher than the actual mileage (my old 996.2 had an error of ~0.5 mpg high). The ~1.6 value was determined dividing the distance traveled (0ver the last 22,000 mules) by the actual amount of gas used. Linear Regression analysis was used to calculate the 1.6 value (Actual = Indicated - 1.6; R² = 0.947) and determine the uncertainty in mileage.
I have no idea what the wind was, but the bushes on the side of the freeway didn't seem to be moving much, so I ignored wind effects in this set of measurements.
My experience with my 2000 Boxster S MT was totally the opposite.
My 2009 C2S MT with 95K miles (in 2015) with trip computer MPH - note MTs get less mileage than PDK. Note my 2009 C2S has a slight increase in MPG with increased speed.
Last edited by Bruce In Philly; 11-12-2019 at 10:12 AM.
One of the several frustrating aspects of living in virginia. The state budget is made from speeding tickets. 15over is automatically wreck-less driving and a court date. Sucks.
One of the several frustrating aspects of living in virginia. The state budget is made from speeding tickets. 15over is automatically wreck-less driving and a court date. Sucks.
Cw
I'd have to move, otherwise, I'd be serving a life sentence by now
5 mile intervals is not nearly long enough to gather an accurate data point(s). A properly maintained, base 911, at 70 mph, should easily be able to run 30 mpgs.
By comparison, here is my 991.1S with Powerkit on a recent trip in October. Does the 997 get that much worse mpgs?:
Update.... I updated my post above to include my 2009 C2S data.
Peace
Bruce in Philly
This is interesting! I normally assume that mpg goes down quickly as mph increases, more drag blah blah etc. When driving my 997.1 C2S at a steady pace of 60-70 mph I usually get an indicated mpg of 24 and change. Then last weekend I was swimming along with the evening traffic on the NJ turnpike at more like 80 mph, and after a while the indicated mpg crept up to 25 mpg....!
I thought it was just a rubbish measurement, but it seems to be compatible with your data...well what a very special car this is
Maybe something to do with the fact that the recommended (not the legal) speed on German Autobahns is 81 mph....
Are you skinny or a fat boy? Just curious what degradation in mileage I can expect with Thanksgiving and Christmas coming up as I cross the 200lb threshold.
Something to think about...... I wonder .... when folks are doing 80 MPH, are they holding the gas steady? In highest gear? Holding the gas pedal really steady is important to getting the best mileage. I know too many people that pump that darn pedal, and when they are speeding, can get nervous.
Set cruise control to 79 in six gear and you will get great gas mileage. Mid to high 20s. Of course, my mostly mixed city and highway, with nice rips up the long on ramp (only curves we have in Florida are ramps!) and I am back to 16 mpg. Still don't care.
11-11-2019, 02:52 PM
Mileage vs. speed
