Ideal speed for best MPG on 997.1?
05-27-2014, 05:53 PM
#1
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Thread Starter
Ideal speed for best MPG on 997.1?
I played around with my speed on a long stretch of highway to figure out what speed (in 6th gear) yields the best MPG... it was a very difficult thing to do as Porsche doesn't show current MPG, and only shows an average after a certain time that you need to reset to get an idea.
Here's what I've found by using cruise control at some set speeds (in a C4S):
50MPH: 7.1-8.5 l/100Km ~ 28 - 33 MPG
60MPH: 7.5-8.5 l/100Km ~ 28-31 MPG
70 MPH: 7.8-9 l/100Km ~ 26-30 MPG
80 MPH: 9.5-10 l/100Km ~ 23.5-25.5 MPG
I was unable to get it better than 33MPG, and it varied quite a bit, and it was impossible to keep it at 30 MPG+ for long stretches. But, with cruise control (always the most efficient for me), by keeping the speed between 60-70 MPH, you can easily keep it above 25 MPG and if the road is flat closer to 30. I managed to get about 22 MPG at the end of the day with mostly highway and a little bit city driving and one stretch of grid lock (20mins)... which is pretty good if you ask me for a sports car with 0-60 of 4.5 seconds!
Is there an actual proper data on this? to state at exactly what speed the engine is most efficient?
PS. This is not scientific, and I don't think 80 MPH should be that worse than 70 MPH in our cars, but maybe during that time there was more work for the car to do... not sure.
Here's what I've found by using cruise control at some set speeds (in a C4S):
50MPH: 7.1-8.5 l/100Km ~ 28 - 33 MPG
60MPH: 7.5-8.5 l/100Km ~ 28-31 MPG
70 MPH: 7.8-9 l/100Km ~ 26-30 MPG
80 MPH: 9.5-10 l/100Km ~ 23.5-25.5 MPG
I was unable to get it better than 33MPG, and it varied quite a bit, and it was impossible to keep it at 30 MPG+ for long stretches. But, with cruise control (always the most efficient for me), by keeping the speed between 60-70 MPH, you can easily keep it above 25 MPG and if the road is flat closer to 30. I managed to get about 22 MPG at the end of the day with mostly highway and a little bit city driving and one stretch of grid lock (20mins)... which is pretty good if you ask me for a sports car with 0-60 of 4.5 seconds!
Is there an actual proper data on this? to state at exactly what speed the engine is most efficient?
PS. This is not scientific, and I don't think 80 MPH should be that worse than 70 MPH in our cars, but maybe during that time there was more work for the car to do... not sure.
Last edited by alexb76; 05-27-2014 at 06:17 PM.
05-27-2014, 06:09 PM
#2
Generally anytime throttle goes down or rpm goes up, you are using more gas. Cruise control saves gas by taking out the unnecessary fluctuations of throttle/rpm. You see MPG decrease as you go faster because you are maintaining higher rpm.
To find most efficient point, just find the fastest speed you can maintain in the highest gear/lowest rpm using the least amount of throttle. I think in most cars its around 45mph.
To find most efficient point, just find the fastest speed you can maintain in the highest gear/lowest rpm using the least amount of throttle. I think in most cars its around 45mph.
05-27-2014, 06:17 PM
#3
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Thread Starter
Generally anytime throttle goes down or rpm goes up, you are using more gas. Cruise control saves gas by taking out the unnecessary fluctuations of throttle/rpm. You see MPG decrease as you go faster because you are maintaining higher rpm.
To find most efficient point, just find the fastest speed you can maintain in the highest gear/lowest rpm using the least amount of throttle. I think in most cars its around 45mph.
To find most efficient point, just find the fastest speed you can maintain in the highest gear/lowest rpm using the least amount of throttle. I think in most cars its around 45mph.
05-27-2014, 07:26 PM
#4
Just seems that the faster you go, the more throttle is required to overcome the increased wind resistance induced by the increased speed. If your drag was less at 80mph vs 55mph (not possible I know) then you would see the opposite effect. I would think that the sweet spot, if found would constantly be a changing one based on the weather conditions.
05-27-2014, 07:31 PM
#5
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Thread Starter
http://auto.howstuffworks.com/fuel-e...uestion477.htm
So even when the car is sitting still it uses quite a lot of fuel. Cars get the very worst mileage at 0 mph; they use gasoline but don't cover any miles. When you put the car in drive and start moving at say 1 mph, the car uses only a tiny bit more fuel, because the road load is very small at 1 mph. At this speed the car uses about the same amount of fuel, but it went 1 mile in an hour. This represents a dramatic increase in mileage. Now if the car goes 2 mph, again it uses only a tiny bit more fuel, but goes twice as far. The mileage almost doubled!
In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.
This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.
You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.
So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:
- Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
- Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
- Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.
In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
In effect the efficiency of the engine is improving. It uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. So in terms of fuel used per mile, the faster the car goes, the better use we make of that fixed amount of fuel required.
This trend continues to a point. Eventually, that road load curve catches up with us. Once the speed gets up into the 40 mph range each 1 mph increase in speed represents a significant increase in power required. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts dropping. Let's plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph compares with a 1 mph increase from 50 to 51 mph. To make things easy we'll assume a, b and c are all equal to 1.
You can see that the increase in power required to go from 50 to 51 mph is much greater than to go from 2 to 3 mph.
So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:
- Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
- Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
- Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.
In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
05-27-2014, 07:47 PM
#6
Rennlist Member
It's been my experience that high RPM doesn't necessarily mean poor gas mileage. What drives gas mileage is the load on the engine. For instance, a while back I was driving a big 26' Uhaul box truck with an auto-transport hitched to the back. The newer trucks have an efficiency gauge which is nothing more than a fuel-flow meter. When I would get to an incline, I would pop it out of overdrive (RPM's would go up), but the efficiency meter would stay in the green. If I waited for the transmission to do it by itself, I would be driving in the red for a bit until it downshifted, then it would go into the green.
If this doesn't make sense, think about riding a bicycle (with shifting gears) up a hill. What is going to make you work harder . . . keeping the bike in a high gear or dropping it down into a low gear? Of course it's easier to ride the bike up hill in the lower gear, even though your legs are moving faster. I'm sure an engineer will chime in with f=ma or e=mc2 discussions, but this is just my experience.
Sean
If this doesn't make sense, think about riding a bicycle (with shifting gears) up a hill. What is going to make you work harder . . . keeping the bike in a high gear or dropping it down into a low gear? Of course it's easier to ride the bike up hill in the lower gear, even though your legs are moving faster. I'm sure an engineer will chime in with f=ma or e=mc2 discussions, but this is just my experience.
Sean
05-27-2014, 09:32 PM
#7
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My wife's commute is 90% highway and she usually gets 21-22-23 mpg in her '11 Carrera S Cab. Come the weekend, or if she's traveling, and it's my turn to drive, she'll ask whether I siphoned the gas tank because I get more like 12-14.
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05-28-2014, 11:22 AM
#10
Three Wheelin'
Think about it like this: You need about 65% more horsepower to go 65 MPH than you do to go 55 MPH.
So, top gear @ a speed not lugging the engine and at the lowest speed you can go without need to shift for more power for inclines.
Cruise control: A way for MOST people (Joe Public) to improve fuel economy, but if you're paying attention to throttle position, traffic and inclines, you can out perform cruise control, as it has a mission (get and keep a specific speed) and fuel economy will suffer because of this mission at certain times... time which you can foresee and avoid... such as inclines and your own desire to resume speed at your softer pace.
.
So, top gear @ a speed not lugging the engine and at the lowest speed you can go without need to shift for more power for inclines.
Cruise control: A way for MOST people (Joe Public) to improve fuel economy, but if you're paying attention to throttle position, traffic and inclines, you can out perform cruise control, as it has a mission (get and keep a specific speed) and fuel economy will suffer because of this mission at certain times... time which you can foresee and avoid... such as inclines and your own desire to resume speed at your softer pace.
.
05-28-2014, 01:24 PM
#11
Three Wheelin'
Equating speed with efficiency is a slippery slope. The thing is that roads driven on will vary as will loads (weights) of the car as well as ambient air pressure (altitude), concentration of O2 per unit volume, air resistance (drag) and rolling resistance (road surface/tire interaction), tire pressure, etc. - and you'd have to start with the assumption that all engines are equally tuned. And there are probably more variables as well. In the end, while you could certainly figure out speed and efficiency it would be for a specific set of criteria (much like EPA mileage estimates). In the end, a ballpark figure (like you calculated) applicable across all driving conditions, locations, etc., is the best that you can hope for - unless there are people out there with entirely too much time on their hands who want to collect data that will have no real useful application.
FWIW, the mileage for your 60 and 70 mph groups is in the same range as I got several years ago when driving up to Thunder Hill for a DE. It was in May, temps in the 60's and in the northern part of the CA Central Valley which is relatively flat. Air pressure in the tires was 36 R / 33 F.
FWIW, the mileage for your 60 and 70 mph groups is in the same range as I got several years ago when driving up to Thunder Hill for a DE. It was in May, temps in the 60's and in the northern part of the CA Central Valley which is relatively flat. Air pressure in the tires was 36 R / 33 F.
05-28-2014, 03:41 PM
#13
I think I'm doing something wrong :-) My average right now is 12.3 mpg and is usually always somewhere between 12-15 mpg. I can maybe get 20-22 on a long highway trip if I try real hard to slow down.
05-28-2014, 04:55 PM
#14
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05-28-2014, 05:58 PM
#15
Three Wheelin'
Since we're 'enjoying' this... I mathed it.
Wanna go 65 instead of 55? That's 65% more horsepower! In a 997, probably means using about 70 HP @ 55 to using 120 HP @ 65 MPH!
As proof that I am not interested in saving fuel, please take note of my other thread in which I ran out of fuel.
Wanna go 65 instead of 55? That's 65% more horsepower! In a 997, probably means using about 70 HP @ 55 to using 120 HP @ 65 MPH!
As proof that I am not interested in saving fuel, please take note of my other thread in which I ran out of fuel.