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#1056799 - 01/03/08 11:01 AM Re: My car get better gas mileage doing 70-75 mph [Re: Jon]
XS650 Offline



Registered: 12/21/03
Posts: 12385
Loc: Northern CA
 Originally Posted By: Jon
On one of my motorcycles, I get the best mileage at 65. I get 50+ mpg on a 1200cc touring bike that makes near 100 hp. I get 40-45 at 55-60. The numbers are as consistent as the day is long.

It is true that drag increases with the square of velocity, but that is being multiplied by a (typically) very small number (the drag coeffecient), so it's effect is small until the velocity gets to be larger. However, the larger issue for many vehicles is where the engine is in terms of RPM and it's peak torque at different RPMs. My '92 Buick Roadmaster got 24 mpg at 70 (which was 1700 rpm) and 22 at 60 (which was 1500 rpm or so). Why? I dunno. Gearing. Peak torque on that was 2200 rpm.

Another example, at the other extreme, is how much more horsepower it takes to go from 190mph to 200mph. It can take 30 HP or more for some motorcycles.

Gearing and engine efficiency *combine* with drag to produce the most efficient travel.


Peak engine efficiency does not occur at peak torque, that's an urban legend.

Peak efficiency at full throttle occurs at about 85 to 90% of peak torque rpm.

At less than full throttle, peak efficiency rpm drops way down from that.

For most cars, actual controlled testing instead of anecdotal stories shows they will develop best fuel economy someplace between 35 and 50 mph in their highest gear.

That even holds true for a 400 hp C6 Corvette with a manual transmission that has the engine turning 900 rpm at 40 mph.
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#1056839 - 01/03/08 12:13 PM Re: My car get better gas mileage doing 70-75 mph [Re: SuperEd73]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
 Originally Posted By: SuperEd73
I know the figures are close but I did follow the same route/gas refill routine both times.


Wind can make a difference, too. I've gotten GREAT gas mileage driving south in the winter..but the wind was blowing from the north.

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#1056868 - 01/03/08 01:09 PM Re: My car get better gas mileage doing 70-75 mph [Re: brianl703]
ThirdeYe Offline


Registered: 02/23/07
Posts: 7448
Loc: Walker, MI
My gas mileages always gets exponentially worse once I go over 75mph, around 60-70 my gas mileage is above average with about 31-35mpg, depending on how much traffic, weather conditions, amount of variation in speed, etc. I'm just using the cheapest tires on my car, but with a current alignment/balancing and correct air pressure in all of the tires. When I drive over 75 I struggle to break 30mpg. Also keep in mind that my car has a fairly low drag coefficient of .33.


Edited by ThirdeYe (01/03/08 01:10 PM)
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#1056879 - 01/03/08 01:24 PM Re: My car get better gas mileage doing 70-75 mph [Re: XS650]
Gary Allan Offline


Registered: 09/28/02
Posts: 39806
Loc: Pottstown, PA
 Quote:
For most cars, actual controlled testing instead of anecdotal stories shows they will develop best fuel economy someplace between 35 and 50 mph in their highest gear.

That even holds true for a 400 hp C6 Corvette with a manual transmission that has the engine turning 900 rpm at 40 mph.



Why did you have to bring all that up?

Slower pays. No two ways about it. Now some configurations are going to be effected less by wind resistance ..so the penalty is less noticeable/has lower impact. The thing is, most of you that report better mileage aren't comparing it to itself in the other mode. You go on one trip @ 80mph and get 2-3mph more and consider that proof that you're doing better ...but you never do the same trip at 55 to see the other side of the coin.

I could get 30 mph out of my BMW driving it in geezer mode (up from 24 typical-and it ran like stink the next day). I could get a 2.8 Citation to get 35 in geezer mode ...placing an egg between my foot and the gas pedal. I could get over 40 out of a Chevette ..same deal. These were all runs of over 300 miles per day over a few years; using 50 as my top speed as opposed to 65 (this was the Reagan federal highway speed enforcement years of 55mph limit). I didn't get to do this often, time was usually at a premium.

Fuel economy, outside of actual operational conditions, is primarily effected by miles per warm up cycle. 75-80 yielded the BEST mileage that I ever saw out of my minivan; nearly 25mph. It was merely due to the 400 mile/one warm up situation that would normally have included 8-15 warm ups the rest of the time it was in use. Even my wife's 25 mile one way commute produced lower numbers (around 20mph). I'm sure that if we had the patience to do the whole 400 miles @ 55mph (or even 65mph) the yield would have been even better ...but I80 in central PA is ONE BORING ride if you don't have company in terms of traffic. 90mph may make the drive interesting enough to keep your attention.
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#1056884 - 01/03/08 01:31 PM Re: My car get better gas mileage doing 70-75 mph [Re: M1Accord]
Bob Woods Offline


Registered: 09/11/03
Posts: 1342
Loc: North of Dallas Texas
Buy a Scangauge it gives you an instant readout of your mileage, makes it easy to see what happens to mileage at different speeds. http://www.scangauge.com
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2002 GMC 5.3 extended cab
1997 Taurus

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#1056935 - 01/03/08 02:49 PM Re: My car get better gas mileage doing 70-75 mph [Re: brianl703]
ProfPS Offline


Registered: 05/25/05
Posts: 834
Loc: CT, YL of USSA aka Oceania
 Originally Posted By: brianl703

Wind can make a difference, too. I've gotten GREAT gas mileage driving south in the winter..but the wind was blowing from the north.


I don't think it was the wind, it was because you were driving down hill. (that's what my father always said: driving down hill when going south and up hill when going north [on the globe])

I neglected to say that my car has an average mpg reading, not an instant mpg reading. On my 13hr/750 mile trip using cruise control on a several hour portion of the trip where the road was fairly flat, the average mpg decreased when driving between 55-65 and increased when driving at 70. I don't know at what speed over 70 it will start to decrease; I guess I have to plan a trip out west to drive on one of those 75-80 mph posted interstates.
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#1057075 - 01/03/08 07:07 PM Re: My car get better gas mileage doing 70-75 mph [Re: ProfPS]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Engine and drivetrain friction are pretty much linear with respect to speed.
Wind resistance is proportional to the square of the speed.
I don't see how greater drag and friction increases gas mileage.

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#1057106 - 01/03/08 07:47 PM Re: My car get better gas mileage doing 70-75 mph [Re: mechtech2]
oilyriser Offline


Registered: 04/30/03
Posts: 7077
Loc: Ontario, Canada
I drive slow, but not road speed. I try to maintain a constant airspeed in my car. Going against the wind, I slow down; with the wind, I speed up. Over my lifetime, the extra time spent slowing against the wind will be made up by going faster with the wind.

If a round trip has one half against the wind, and one with the wind, you will use less gas if you slow down when going against the wind and speed up when with the wind, than if you do the whole trip at a steady speed.

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#1057133 - 01/03/08 08:32 PM Re: My car get better gas mileage doing 70-75 mph [Re: wgtoys]
PT1 Offline


Registered: 02/06/07
Posts: 5746
Loc: near the mistake
 Originally Posted By: wgtoys
"So my question if the "drive speed limit of 60 mph because every 5 mile over that is a big waste of gas" a big government conspiracy lie to scare us into slowing down to grandpa pace?"

In a word, no. At highway speeds overcoming air (wind) resistance becomes a very large factor. The power to overcome air resistance increases roughly with the cube of the speed.

If you have studied rudimentary physics you will understand what this means. If not, you may just have to settle for conspiracy theories.



So, true! With my GMC Yukon XL 6.0L at 65MPH i get 20.8 MPG at 70MPG I get 18MPG at 75MPG it drops to 16MPG.
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#1057286 - 01/04/08 05:05 AM Re: My car get better gas mileage doing 70-75 mph [Re: PT1]
ProfPS Offline


Registered: 05/25/05
Posts: 834
Loc: CT, YL of USSA aka Oceania
Added power to oversome wind resistance may be true but there are other factors that effect speed vs. mpgs. Areodynamics, wind direction, wind speed, road surface condition, weather, temperature, humidity, altitude, weight, rolling resistance, engine displacement, engine efficiency, and drivetrain gearing (to name a few) will be factors beyond the basic physics of acceleration/deacceleration and moving a mass through a non-vacuum space.

Not all cars will achieve higher mpg's at speeds above 55mph; it depends on the vehicle. I don't want to pick on your Yukon, but I think the results would be different if you compared a 2wd Yukon with passenger tires in Kansas on a cool day (and making it a low-rider too ) to a 4wd Yukon with all-terrain light truck tires in Denver on a hot day.
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Thank you Commodity Futures Modernization Act of 2000.

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#1057457 - 01/04/08 11:24 AM Re: My car get better gas mileage doing 70-75 mph [Re: ProfPS]
BuickGN Offline


Registered: 10/18/07
Posts: 3756
Loc: CA
I do lots of four hour trips and have noticed the TL gets best mileage at 65mph. I've gotten 35mpg on a 280 mile trip with no stops. The GN on the other hand gets best mileage at the lowest speed it can hit OD with the convertor locked which is 55mph. I have a feeling if I got a chip that locked the convertor sooner it would only get better mileage but a 3,800 stall convertor doesn't get good mileage unlocked even on level ground.
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84 Buick GN. 10.60@127 old times. 602hp 620lbs.
06 Acura TL. Lots of suspension and brake mods.

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#1057526 - 01/04/08 01:03 PM Re: My car get better gas mileage doing 70-75 mph [Re: ProfPS]
PT1 Offline


Registered: 02/06/07
Posts: 5746
Loc: near the mistake
 Originally Posted By: ProfPS
Added power to oversome wind resistance may be true but there are other factors that effect speed vs. mpgs. Areodynamics, wind direction, wind speed, road surface condition, weather, temperature, humidity, altitude, weight, rolling resistance, engine displacement, engine efficiency, and drivetrain gearing (to name a few) will be factors beyond the basic physics of acceleration/deacceleration and moving a mass through a non-vacuum space.

Not all cars will achieve higher mpg's at speeds above 55mph; it depends on the vehicle. I don't want to pick on your Yukon, but I think the results would be different if you compared a 2wd Yukon with passenger tires in Kansas on a cool day (and making it a low-rider too ) to a 4wd Yukon with all-terrain light truck tires in Denver on a hot day.


True! I have had 3 of these vehicles all configured differently and the mileage was different on each based on all of those factors. The best was a 2002 Suburban 5.3L 4wd which got 21.2 mpg at 65 mph in 2wd setting. All had 3.43 gearing and 4L60 transmissions. The mileage was consistent in year round temps.
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#1057532 - 01/04/08 01:11 PM Re: My car get better gas mileage doing 70-75 mph [Re: PT1]
Gary Allan Offline


Registered: 09/28/02
Posts: 39806
Loc: Pottstown, PA
One of you physics enabled types do a comparison based on drag coefficient, please? Give an index to how much difference the spread is for two different vehicles. I imagine that we'll find that the "increases by the square" impact is a whole lot more for some than others. What may make 5% penalty for some slick configuration ..may be over 20% for a rolling brick.


Edited by Gary Allan (01/04/08 01:12 PM)
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#1057537 - 01/04/08 01:22 PM Re: My car get better gas mileage doing 70-75 mph [Re: Gary Allan]
BuickGN Offline


Registered: 10/18/07
Posts: 3756
Loc: CA
 Originally Posted By: Gary Allan
One of you physics enabled types do a comparison based on drag coefficient, please? Give an index to how much difference the spread is for two different vehicles. I imagine that we'll find that the "increases by the square" impact is a whole lot more for some than others. What may make 5% penalty for some slick configuration ..may be over 20% for a rolling brick.


I forget the actual CD of my TL but I remember being very surprised at it being close or equal to the C5 Vette. The Buick is a brick. The Buick also has tons of low end torque while the Acura needs to rev a little. Acura gets worse mileage below 60mph while the Buick gets better the slower I go until the TCC unlocks. I suspect if I lock it manually 40mph wouldn't be out of the question for peak fuel economy.
_________________________
84 Buick GN. 10.60@127 old times. 602hp 620lbs.
06 Acura TL. Lots of suspension and brake mods.

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#1057564 - 01/04/08 02:08 PM Re: My car get better gas mileage doing 70-75 mph [Re: BuickGN]
Gary Allan Offline


Registered: 09/28/02
Posts: 39806
Loc: Pottstown, PA
Oddly, my wife's 4.0 Wrangler appears mostly indifferent to speed. Naturally we've never done 80mph for extended periods of time, but it doesn't seem to care much one way or another. I did break 21 mpg one time on a longer trip, but I attribute it to one warm up over a tank instead of 12+.

Not in contradiction with my belief that slower is better (nor what XS650 said) ..but I believe that the power development characteristics of a given engine can surely influence the efficiency gain that one can realize with varied speed; especially when you're forced into certain confines of practical ceilings and floors in what you can manipulate in a given set of drive train semantics.
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