Better mileage at 70 vs 65 ?
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My 2001 Taurus had it's best fuel economy at 70. It would get 33-34MPG at 70. 65,60 ... 29MPG. Never understood it.
I agree with JODs graph.I can't accept the fact that a vehicle will get better MPG at higher speeds. I also agree that engine efficiency can be higher, at higher engine RPMs (BMEP) but, it will not overcome arodynamic drag.
I think there is something to the higher revs helping a little with engine efficiency. Especially those without variable valve timing or lift. Most cars have 4 valves per cylinder and set up to flow well at high rpms with WO throtle, so at low throttle and rpm, they are not as efficient as they could be.
I find my Focus quite sensitive to throttle position at lower rpms, a little more causes the instant mpg to plummet into the teens... But at 60mph+ those same throttle positions will result in stay in the high 20's mpg.
If driven carefully, 50mph can produce much higher mpg, but its much more dependent on the driver.
I find my Focus quite sensitive to throttle position at lower rpms, a little more causes the instant mpg to plummet into the teens... But at 60mph+ those same throttle positions will result in stay in the high 20's mpg.
If driven carefully, 50mph can produce much higher mpg, but its much more dependent on the driver.
Originally Posted By: IndyIan
I think there is something to the higher revs helping a little with engine efficiency.
That's entirely possible--operative word being "little". That pales in comparison though to the increase in power requirements though one you get above 45mph or so. I have seen cars tested that get slightly better fuel economy at 45 vs. 40mph, but above that, drag dominates the equation in all cases.
As far as hills, it's entirely possible that if you're committed to 60mph on all hills, that a higher speed on flat ground and then dropping the speed on the hill may use less fuel than maintaining 60mph(it's pretty likely); but if you're maintaining the exact same speed, then slower is going to use less fuel--always.
I think there is something to the higher revs helping a little with engine efficiency.
That's entirely possible--operative word being "little". That pales in comparison though to the increase in power requirements though one you get above 45mph or so. I have seen cars tested that get slightly better fuel economy at 45 vs. 40mph, but above that, drag dominates the equation in all cases.
As far as hills, it's entirely possible that if you're committed to 60mph on all hills, that a higher speed on flat ground and then dropping the speed on the hill may use less fuel than maintaining 60mph(it's pretty likely); but if you're maintaining the exact same speed, then slower is going to use less fuel--always.
All things being equal, I don't buy it.
Imagine, with a scan gauge fitted, driving a steady 75mph then backing off the throttle to a steady 70 and seeing the consumption GO UP!
If the car Gears down, or the engine falls out of it's torque band, I'd understand it.
Imagine, with a scan gauge fitted, driving a steady 75mph then backing off the throttle to a steady 70 and seeing the consumption GO UP!
If the car Gears down, or the engine falls out of it's torque band, I'd understand it.
I'm in the group that considers this very unlikely, and don't know of any documented case where this happened.
As has been mentioned, drag force varies with square of velocity, so I would expect any vehicle that could achieve this would need both a very low coefficient of drag and frontal area, along with certain engine and gearing modifications.
Regarding those modifications, to paraphrase an article I read recently
"For driving at a steady speed one cannot choose any operating point for the engine, rather there is a specific amount of power needed to maintain the chosen speed. Because cruising at an efficient speed uses much less than the maximum power of the engine, the optimum operating point for cruising at low power is typically at very low engine speed, around or below 1000 rpm. This explains the usefulness of very high "overdrive" gears for highway cruising."
Which would seem to verify what HTSS_TR said about "the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging."
As has been mentioned, drag force varies with square of velocity, so I would expect any vehicle that could achieve this would need both a very low coefficient of drag and frontal area, along with certain engine and gearing modifications.
Regarding those modifications, to paraphrase an article I read recently
"For driving at a steady speed one cannot choose any operating point for the engine, rather there is a specific amount of power needed to maintain the chosen speed. Because cruising at an efficient speed uses much less than the maximum power of the engine, the optimum operating point for cruising at low power is typically at very low engine speed, around or below 1000 rpm. This explains the usefulness of very high "overdrive" gears for highway cruising."
Which would seem to verify what HTSS_TR said about "the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging."
I have to think that, if this effect did exist (apart from the hill complication) that the move to six speed plus transmission mitigates it a bit. In the old days of large gear splits, perhaps; a few more rev's to prevent shifting.
I've never tested it, but I think my truck does better in 5th than in 6th. The reason being that 1500rpm puts it around 48mph or so. Wind drag is much lower, and it climbs hills perfectly fine w/o shifting. And it will hold 5th down to below 40mph for some real mpg boosting (it will coast in 5th at 30mph--5th is overdrive!). It's a bit of a slow ride though at that point...!
Direct drive ought to be most efficent, but rear gearing plays a role too, and I'm not sure what the real loss is (I believe the big rigs assume 1% loss in the gearing BUT those gearsets are I'm guessing a mite bit larger than anything in a passenger car). I've seen a few transmissions where there isn't a direct drive ratio for some reason (6L80, the 4spd in some years of the RAV4). Probably washes out in the end.
I've never tested it, but I think my truck does better in 5th than in 6th. The reason being that 1500rpm puts it around 48mph or so. Wind drag is much lower, and it climbs hills perfectly fine w/o shifting. And it will hold 5th down to below 40mph for some real mpg boosting (it will coast in 5th at 30mph--5th is overdrive!). It's a bit of a slow ride though at that point...!
Direct drive ought to be most efficent, but rear gearing plays a role too, and I'm not sure what the real loss is (I believe the big rigs assume 1% loss in the gearing BUT those gearsets are I'm guessing a mite bit larger than anything in a passenger car). I've seen a few transmissions where there isn't a direct drive ratio for some reason (6L80, the 4spd in some years of the RAV4). Probably washes out in the end.
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Interesting observations. I noticed this too with GM big sixes.Toyota research suggests in general best engine efficiency is at 40-45% of redline. The sweet spot depends on the interplay of aero and friction drag, torque curve, volumetric efficiency; not just speed alone.
The C3 gets worse mileage with cruise set at 50mph then when set at 70mph,but from 60 to 70 there isn't any measurable change in consumption.
My charger gets its best mileage at 55mph,and gets slightly worse the faster I drive.
I live in the prairies,no hills to speak of,so tracking fuel consumption is pretty easy since the only variable that changes are fuel and wind.
My charger gets its best mileage at 55mph,and gets slightly worse the faster I drive.
I live in the prairies,no hills to speak of,so tracking fuel consumption is pretty easy since the only variable that changes are fuel and wind.
Originally Posted By: JOD
Yeah, no question the sweet spot may be different--but not that different. Here's a chart of some really disparate vehicles and the curves are pretty similar (except for one). Regardless, they ALL slope down after 45mph
I agree, really. Improvement over a 5 mph difference might be a bit of a stretch, and the graph you provide is helpful. I'd wonder if there were any outliers in testing. This would be one of the cases where the outliers and errors are more interesting than the data.
And yes, fuel economy measurement is exceedingly difficult. I've said it here many times that I wouldn't accept an error bar of less than 10% for calculations and testing without specialized equipment and proper accounting of variables.
Nonetheless, I'll have to look back and see what I can find - at least one automaker had asked for an increase in one of the speeds of the EPA highway test. Obviously, it wouldn't be a significant increase.
Originally Posted By: HTSS_TR
I think the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging.
I would agree.
Yeah, no question the sweet spot may be different--but not that different. Here's a chart of some really disparate vehicles and the curves are pretty similar (except for one). Regardless, they ALL slope down after 45mph
I agree, really. Improvement over a 5 mph difference might be a bit of a stretch, and the graph you provide is helpful. I'd wonder if there were any outliers in testing. This would be one of the cases where the outliers and errors are more interesting than the data.
And yes, fuel economy measurement is exceedingly difficult. I've said it here many times that I wouldn't accept an error bar of less than 10% for calculations and testing without specialized equipment and proper accounting of variables.
Nonetheless, I'll have to look back and see what I can find - at least one automaker had asked for an increase in one of the speeds of the EPA highway test. Obviously, it wouldn't be a significant increase.
Originally Posted By: HTSS_TR
I think the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging.
I would agree.
Nope. At 55 mph in my Cruze I can get 48-49 mpg. At 70 mph it's 40 mpg. 65 is about 43-44 mpg. Similar situation with our Fit. It'll get 40 mpg at 65, but about 33-34 mpg at 70.
It was the same with my old Buick with its old-tech 3800 V6. It got about 35-37 mpg at 60 mph, and 26-27 mpg at 70.
It was the same with my old Buick with its old-tech 3800 V6. It got about 35-37 mpg at 60 mph, and 26-27 mpg at 70.
I have two vehicles with almost the same 2.2 l four and 5 speed manual transmission, except the gear ratios are drastically different.
My car, Chevy cobalt, has a very tall 5th gear (the gear ratio gives 3000 rpm at 83 mph) and while it gets really phenomenal mileage on a flat surface at 45, that speed is not realistic on the interstate, and all of the interstates around here are hilly. (Here in Kansas the only roads that are truly flat are the few that run along a river).
It gets better gas mileage in 'real world' cruising over 70. It cruises very nicely at 75-78, around 2700-2800 rpm, and generally delivers its rated 37 mpg, more or less, depending on other conditions, like temperature, wind speed and direction, etc.
In hilly terrain if I am going slower, like 50-60, and encounter a hill I have to really plant my foot to maintain speed, and the economy, according to both scangauge and my DIC, goes down substantially.This hurts the average mpg. If I maintain a higher speed the momentum helps carry me over the hill without as much foot feed. The engine revolutions per mile stay the same regardless of speed, unless I have to downshift. Going downhill it gets fantastic gas mileage pretty much regardless of speed.
The bottom line is that on long, cross country highway trips I do better at 70-80 than 50-60 in the Cobalt. The engine is geared so high that it doesn't deliver enough power at the lower rpm's that happen at lower road speeds. If I downshift that hurts the mpg even more.
My Saturn VUE, on the other hand, gets better mileage at 60-65. The gear ratio gives 3000 rpm at 68 mph. It has no trouble on hills at that speed. It's not as aerodynamic, and it's about 500 lbs heavier, so it doesn't get as good a mileage at 65 mph as the Cobalt gets at 75. I would guess it could get even better at 55 mph, but someone would probably end up giving me a 'push' anyway, so I don't risk that.
My car, Chevy cobalt, has a very tall 5th gear (the gear ratio gives 3000 rpm at 83 mph) and while it gets really phenomenal mileage on a flat surface at 45, that speed is not realistic on the interstate, and all of the interstates around here are hilly. (Here in Kansas the only roads that are truly flat are the few that run along a river).
It gets better gas mileage in 'real world' cruising over 70. It cruises very nicely at 75-78, around 2700-2800 rpm, and generally delivers its rated 37 mpg, more or less, depending on other conditions, like temperature, wind speed and direction, etc.
In hilly terrain if I am going slower, like 50-60, and encounter a hill I have to really plant my foot to maintain speed, and the economy, according to both scangauge and my DIC, goes down substantially.This hurts the average mpg. If I maintain a higher speed the momentum helps carry me over the hill without as much foot feed. The engine revolutions per mile stay the same regardless of speed, unless I have to downshift. Going downhill it gets fantastic gas mileage pretty much regardless of speed.
The bottom line is that on long, cross country highway trips I do better at 70-80 than 50-60 in the Cobalt. The engine is geared so high that it doesn't deliver enough power at the lower rpm's that happen at lower road speeds. If I downshift that hurts the mpg even more.
My Saturn VUE, on the other hand, gets better mileage at 60-65. The gear ratio gives 3000 rpm at 68 mph. It has no trouble on hills at that speed. It's not as aerodynamic, and it's about 500 lbs heavier, so it doesn't get as good a mileage at 65 mph as the Cobalt gets at 75. I would guess it could get even better at 55 mph, but someone would probably end up giving me a 'push' anyway, so I don't risk that.
Originally Posted By: HTSS_TR
I think the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging.
3 variables here...
flat surface
lowest RPM in topmost gear
no lugging
The point made here by many was that changing any one of them will likely confound any predictable outcome.
Beginning hill climbs at too low RPM will require more gas than having started at a faster RPM...and coming down hill, if the car has FDCO technology, that would increase MPG if pulse/glide strategy is used correctly...
Clearly some effort must be consciously made to get some engines into top gear....for those with finicky ATs....
...and then there's lugging...it takes a light foot to find/maintain that lowest RPM/sweet-spot without lugging...
I think the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging.
3 variables here...
flat surface
lowest RPM in topmost gear
no lugging
The point made here by many was that changing any one of them will likely confound any predictable outcome.
Beginning hill climbs at too low RPM will require more gas than having started at a faster RPM...and coming down hill, if the car has FDCO technology, that would increase MPG if pulse/glide strategy is used correctly...
Clearly some effort must be consciously made to get some engines into top gear....for those with finicky ATs....
...and then there's lugging...it takes a light foot to find/maintain that lowest RPM/sweet-spot without lugging...
Originally Posted By: Garak
I agree, really. Improvement over a 5 mph difference might be a bit of a stretch, and the graph you provide is helpful. I'd wonder if there were any outliers in testing. This would be one of the cases where the outliers and errors are more interesting than the data.
And yes, fuel economy measurement is exceedingly difficult. I've said it here many times that I wouldn't accept an error bar of less than 10% for calculations and testing without specialized equipment and proper accounting of variables.
Nonetheless, I'll have to look back and see what I can find - at least one automaker had asked for an increase in one of the speeds of the EPA highway test. Obviously, it wouldn't be a significant increase.
I agree--the outliers definitely make more interesting data points! If an auto manufacturer is asking for testing at higher speeds, it's not because their car gets *better* absolute mileage, but that its relative order changes. Case in point: the Mercedes C180K in that test. It goes from near the bottom, to the best fuel economy at 120mph! While its mileage continues to drop, the curve is WAY more flat than the other cars. I'm wondering what exactly the did to mitigate the fuel economy losses at high speeds?
I agree, really. Improvement over a 5 mph difference might be a bit of a stretch, and the graph you provide is helpful. I'd wonder if there were any outliers in testing. This would be one of the cases where the outliers and errors are more interesting than the data.
And yes, fuel economy measurement is exceedingly difficult. I've said it here many times that I wouldn't accept an error bar of less than 10% for calculations and testing without specialized equipment and proper accounting of variables.
Nonetheless, I'll have to look back and see what I can find - at least one automaker had asked for an increase in one of the speeds of the EPA highway test. Obviously, it wouldn't be a significant increase.
I agree--the outliers definitely make more interesting data points! If an auto manufacturer is asking for testing at higher speeds, it's not because their car gets *better* absolute mileage, but that its relative order changes. Case in point: the Mercedes C180K in that test. It goes from near the bottom, to the best fuel economy at 120mph! While its mileage continues to drop, the curve is WAY more flat than the other cars. I'm wondering what exactly the did to mitigate the fuel economy losses at high speeds?
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