At around what speed do most modern cars hit their highest MPG?

[Schadenfroh]

Assuming flat surface with no stopping on the interstate with a minimum of 45 MPH. I have always heard that most cars achieve optimum MPG at around 55MPH, is that the case for most consumer level 2-door and 4-door small to medium cars with either 4 or 6 cylinders?

 

[dug777]

I'd guess it was in the highest gear at the lowest speed the engine isn't lugging.

 

[Billb2]

50-60 mph.

 

[Dr. Detroit]

Originally posted by: Billb2
50-60 mph.

Absolutely. That is why the speed limit used to be set at 55mph. It was not set so much for safety but for fuel economy.

 

[Zenmervolt]

Originally posted by: dug777
I'd guess it was in the highest gear at the lowest speed the engine isn't lugging.

The winner.

For most cars it's between 35 and 45 mph. While the 55 mph speed limit was set to reduce the amount of fuel we used, it was chosen as a balance between speed and fuel consumption, not as the optimal speed for mileage. I can get mid to high 30's in terms of mileage from my '98 S70 at ~40 mph on level ground. At 55-60 I'll get 28mpg. Most cars show similar benefits at 35-45mph.

ZV

 

[glenn1]

Originally posted by: Zenmervolt

Originally posted by: dug777
I'd guess it was in the highest gear at the lowest speed the engine isn't lugging.

The winner.

For most cars it's between 35 and 45 mph. While the 55 mph speed limit was set to reduce the amount of fuel we used, it was chosen as a balance between speed and fuel consumption, not as the optimal speed for mileage. I can get mid to high 30's in terms of mileage from my '98 S70 at ~40 mph on level ground. At 55-60 I'll get 28mpg. Most cars show similar benefits at 35-45mph.

ZV

Wouldn't the speed to achieve optimal mileage depend on the coefficient of drag for the car? IIRC, wind resistance increases exponentially with speed.

 

[ProfJohn]

Originally posted by: Billb2
50-60 mph.

My G35 gets around 30mpg at 50 on a flat road.

At 70-80 it drops to 20 or even lower.
I wish they had a higher gear ratio so it would get the best millage at 70ish.

 

[Zenmervolt]

Originally posted by: glenn1

Originally posted by: Zenmervolt

Originally posted by: dug777
I'd guess it was in the highest gear at the lowest speed the engine isn't lugging.

The winner.

For most cars it's between 35 and 45 mph. While the 55 mph speed limit was set to reduce the amount of fuel we used, it was chosen as a balance between speed and fuel consumption, not as the optimal speed for mileage. I can get mid to high 30's in terms of mileage from my '98 S70 at ~40 mph on level ground. At 55-60 I'll get 28mpg. Most cars show similar benefits at 35-45mph.

ZV

Wouldn't the speed to achieve optimal mileage depend on the coefficient of drag for the car? IIRC, wind resistance increases exponentially with speed.

Drag increases with the square of speed, but that's relative. Yes, a car will get better mileage overall with better aerodynamics, but it achieves that benefit at all speeds, though the benefit is more pronounced at higher speeds as you note, but it will still do better at lower speeds. Reducing drag is a means of reducing the penalty for going faster; it cannot negate the penalty, but it can reduce it, sometimes significantly. The best mileage is still achieved at the lowest possible speed in the top gear while avoiding lugging the engine.

Drag coefficient is actually not a measure of overall drag though. Drag coefficient is the multiplication factor of the vehicle's frontal area that is needed to arrive at the total overall drag. For example, a perfectly accurate 1/10 scale model of a car has exactly the same drag coefficient as the full-size car, but the 1/10 scale model experiences less overall drag because it has less frontal area. Drag coefficient is useful for measuring the aerodynamic efficiency of a shape, but it doesn't take into account the size of that shape. In most cases, frontal area plays a greater role in total drag than the drag coefficient, though of course a truly bad drag coefficient can become the key driver.

For example, a 1994 Volvo 850 Turbo Station Wagon has a drag coefficient of 0.32, while my 951 (aka 944 Turbo) has a drag coefficient of 0.33. However, because the 951 has roughly 2.5 square foot less frontal area, the overall drag of the 951 is lower than the drag on the Volvo wagon (drag area of 7.16 sqft for the Volvo versus 6.56 sqft for the 951).

ZV

 

[Zenmervolt]

Originally posted by: ProfJohn

Originally posted by: Billb2
50-60 mph.

My G35 gets around 30mpg at 50 on a flat road.

At 70-80 it drops to 20 or even lower.
I wish they had a higher gear ratio so it would get the best millage at 70ish.

Too much overall drag at 70. It will never get optimal mileage at 70 regardless of gearing. To go 70 mph you need 8 times the power used to go 35 mph. (2x the power to double groundspeed multiplied by 4x the power to overcome the quadrupling of drag equals 8x the power in total.) I'd be willing to put money on the car getting 35+ mpg if you were to set the cruise at 40 mph on flat ground and were in the top gear of the transmission.

ZV

 

[VooDooAddict]

I'd guess it was in the highest gear at the lowest speed the engine isn't lugging.

Wouldn't the speed to achieve optimal mileage depend on the coefficient of drag for the car? IIRC, wind resistance increases exponentially with speed.

Agreed.

It's a combination of those factors.

I also however like to add in another factor when talking about fuel 'economy' and true savings. Time.

I find just under 80 to be optimal travel on major expressways. Gets me there noticeably quicker then 55-60 on over 90 trips and doesn't completely decimate my gas tank.

Fuel in my current car ('01 Passat, 2.8L V6, AWD), fuel consumption starts to climb heavily after 75-80 (depending on weather conditions).

 

[Billb2]

My Buick Lesabre, 3.8L V6, 60 mph, 37 mpg, 1800 rpm, ... level ground, no head wind, cruise control on. YRMV.

 

[spidey07]

My best mileage is around 80mph, 32 mpg. 2004 tl in 6th gear around 2300 rpm. 60 lugs 6th gear too much and is too high for 5th for good mileage.

 

[overst33r]

Originally posted by: spidey07
My best mileage is around 80mph, 32 mpg. 2004 tl in 6th gear around 2300 rpm. 60 lugs 6th gear too much and is too high for 5th for good mileage.

70 seems your sweetspot.

 

[spidey07]

Originally posted by: mariok2006

Originally posted by: spidey07
My best mileage is around 80mph, 32 mpg. 2004 tl in 6th gear around 2300 rpm. 60 lugs 6th gear too much and is too high for 5th for good mileage.

70 seems your sweetspot.

Nah, 80 is actually the sweet spot. I get 32 mph driving 80-90 mph and 28 when I drive 60-70.

 

[overst33r]

Originally posted by: spidey07

Originally posted by: mariok2006

Originally posted by: spidey07
My best mileage is around 80mph, 32 mpg. 2004 tl in 6th gear around 2300 rpm. 60 lugs 6th gear too much and is too high for 5th for good mileage.

70 seems your sweetspot.

Nah, 80 is actually the sweet spot. I get 32 mph driving 80-90 mph and 28 when I drive 60-70.

I wonder why that is, your engine certainly isn't lugging at 70 in 6th.

 

[AdamK47]

80 to 90

 

[MovingTarget]

Considering current gas prices and overall consumption, would it be prudent to go back to a national speed limit like we used to have? Say, 60-65 mph? If we all use less fuel as a whole, the demand and therefore price *should* go down. Those who can afford to drive vehicles long distance on the interstate getting single digit mpg affect more than what they themselves spend on fuel over the long run.

 

[AdamK47]

Originally posted by: MovingTarget
Considering current gas prices and overall consumption, would it be prudent to go back to a national speed limit like we used to have? Say, 60-65 mph? If we all use less fuel as a whole, the demand and therefore price *should* go down. Those who can afford to drive vehicles long distance on the interstate getting single digit mpg affect more than what they themselves spend on fuel over the long run.

No. You base your conclusion on all vechicles performing the same at 60 to 65 mph. They don't.

 

[Scouzer]

My car it's between 43-49mph. I have achieved 40mpg with this, whereas on the highway at 68mph I usually get about 32.

 

[overst33r]

Originally posted by: AdamK47

Originally posted by: MovingTarget
Considering current gas prices and overall consumption, would it be prudent to go back to a national speed limit like we used to have? Say, 60-65 mph? If we all use less fuel as a whole, the demand and therefore price *should* go down. Those who can afford to drive vehicles long distance on the interstate getting single digit mpg affect more than what they themselves spend on fuel over the long run.

No. You base your conclusion on all vechicles performing the same at 60 to 65 mph. They don't.

Consumption will still go down as a whole. 99% of the vehicles out there will get better mpg at 55-60 vs 70

 

[AdamK47]

Originally posted by: mariok2006

Originally posted by: AdamK47

Originally posted by: MovingTarget
Considering current gas prices and overall consumption, would it be prudent to go back to a national speed limit like we used to have? Say, 60-65 mph? If we all use less fuel as a whole, the demand and therefore price *should* go down. Those who can afford to drive vehicles long distance on the interstate getting single digit mpg affect more than what they themselves spend on fuel over the long run.

No. You base your conclusion on all vechicles performing the same at 60 to 65 mph. They don't.

Consumption will still go down as a whole. 99% of the vehicles out there will get better mpg at 55-60 vs 70

How about they just apply that law to the right lane and make the left lane an any MPH lane. Then prosecute those who drive slowly in the left lane to the fullest extent (ie death). I would vote YES to that proposition.

 

[Zenmervolt]

Originally posted by: mariok2006

Originally posted by: spidey07

Originally posted by: mariok2006

Originally posted by: spidey07
My best mileage is around 80mph, 32 mpg. 2004 tl in 6th gear around 2300 rpm. 60 lugs 6th gear too much and is too high for 5th for good mileage.

70 seems your sweetspot.

Nah, 80 is actually the sweet spot. I get 32 mph driving 80-90 mph and 28 when I drive 60-70.

I wonder why that is, your engine certainly isn't lugging at 70 in 6th.

Probably because when the conditions allow him to be at 80 mph he can maintain a constant speed more easily but when he's forced to drive at 60-70 he has to vary speed much more often due to other traffic on the road.

ZV