Optimum speed for mileage per gallon

[vivesdn]

I imagine that this magnitude is dependent on a lot of factors, but does anybody know a good source to get the relationship betwen speed and mpg for at leas families of cars?

 

[TVP45]

Go to CarTalk.com and look at December, 1995.

 

[russ_watters]

The optimum speed (with no air conditioner running) is generally the lowest speed that you can easily maintain in your highest gear.

 

[rbj]

The optimum speed (with no air conditioner running) is generally the lowest speed that you can easily maintain in your highest gear.

what exactly do you mean by "maintain", Russ? i don't think "lugging an engine" is the most efficient use of it. even on perfectly flat and smooth pavement.

 

[ank_gl]

when the gearbox doesn't make weird sounds, i guess russ meant this

 

[rcgldr]

The optimum speed is generally the lowest speed that you can easily maintain in your highest gear.

I remember this same belief from the 1970's, but it wasn't true. For cars back then the optimum speed was around 45mph. This is because gasoline engines are very inefficient at producing small amounts of power, and running at higher speeds, with more of a power load, results in better milage, because the increase in rate of fuel consumed is less than the increase in the rate of speed.

Modern cars have fairly tall gearing in the form of overdrive, and better aerodynamics than cars of the 1970's, so they should be more efficient at higher speeds. Considering that EPA measures highway milage at 55mph (this has just changed), the car designers may have designed their cars to get the best milage at 55mph.

So other than a 1300cc VW bug from the 1970's, or a hybrid, the optimum speed for most cars will be between 45mph and 55mph.

 

[Integral]

..snip...

So other than a 1300cc VW bug from the 1970's, or a hybrid, the optimum speed for most cars will be between 45mph and 55mph.

Which is about the slowest you can comfortably go in the highest gear.

 

[russ_watters]

what exactly do you mean by "maintain", Russ? i don't think "lugging an engine" is the most efficient use of it. even on perfectly flat and smooth pavement.

That's where the "easily" part comes in.

 

[russ_watters]

Modern cars have fairly tall gearing in the form of overdrive, and better aerodynamics than cars of the 1970's, so they should be more efficient at higher speeds. Considering that EPA measures highway milage at 55mph (this has just changed), the car designers may have designed their cars to get the best milage at 55mph.

So other than a 1300cc VW bug from the 1970's, or a hybrid, the optimum speed for most cars will be between 45mph and 55mph.

I drive stick, but don't most automatics only have 4 gears? Also, gear ratios vary a lot by engine and type of car. Ie, an SUV will be geared a lot lower (or at least be able to handle a lower rpm). So I think that's too narrow a range - maybe 35-55.

 

[stewartcs]

I drive stick, but don't most automatics only have 4 gears?

My automatic has 6 gears.

 

[TVP45]

So, there's an easy way to do this. Some cars have a built-in mileage indicator. Rent one and drive it in all different ways and watch the indication. When I did this, I found I got the best mileage at about 1500 RPM in the highest gear. This was on level ground (such as we have in western PA) and corresponded to about 40-55 mph. Presumably this would vary a bit in different cars, but that answer seems to "ball-park" it.

 

[RonL]

For my first post i'll give a firsthand experience.
Last year i purchased a used F350 truck, with a 351 V-8, the transmission is a 5 speed ( the final gear is overdrive ).

Two trips out of town, with speeds of about 60 MPH, produced a fuel average of 11.25 MPG, my big supprise came when i started a part time job, about 13 miles from home, and traveling thru town in stop and go traffic ( top speed between 35-40 MPH ) average speed while moving is about 30 MPH, produced a fuel average increase to about 12.5 MPG.
My thoughts are less wind resistance, and lower average speeds, actually produced an increase in MPG, where i least expected it.

In line with what Russ said.

If someone will point me to the proper place, i have a few thoughts and questions, about how to add a power system ( electric ) to this truck, that will provide 13 miles each way. The current power system will remain intact for heavy or long haul use.

Thanks
RonL

 

[vivesdn]

This is because gasoline engines are very inefficient at producing small amounts of power, and running at higher speeds, with more of a power load, results in better milage, because the increase in rate of fuel consumed is less than the increase in the rate of speed.

Then, best mileage will be around the rpm's which give the highest torque with the highest gear. Does it make sense?

 

[ank_gl]

no...

 

[vivesdn]

No? Just no?

I imagine that regime for highest torque is likely to be the most efficient (power vs fuel), and using the highest gear will produce the lowest number of explosions for a given mileage.
Am I wrong?

 

[rcgldr]

I imagine that regime for highest torque is likely to be the most efficient (power vs fuel), and using the highest gear will produce the lowest number of explosions for a given mileage.

The amount of power generated for the amount of fuel consumed is best, but because of aerodynamic drag, it's consuming more fuel per mile traveled.

 

[Cameronblmr]

from what i have read, wind resistance is not linear. meaning that wind resistance at 60 mph is not 2x the resistance at 30mph, it is lower. although the engine is most efficent at it highest torque rpm, the increase in air resistance at this speed and in top gear more than compensates to make lower mph driving more efficent.

 

[DaveC426913]

This chart was done a few years ago on my 1993 Chrysler Intrepid:

http://www.davesbrain.ca/miscpix/mileage.gif

What I find odd about it is:
- the lack of curvature. I would have thought it would level out to the left.
- the minimal effect of the AC. It has a smaller effect than changing highway speed by 20kph.

 

[DaveC426913]

wind resistance at 60 mph is not 2x the resistance at 30mph

The formula is such that 2x velocity will result in 4x wind resistance.

 

[Cameronblmr]

good to know daveC, i didnt know what the actuall formula was. thanks

 

[mgb_phys]

The drag force is velcotiy squared but power used to overcome drag is velocity cubed. So the engine power is proportional to v^3 * density of air * front area * drag coefficent.
Which is why planes like to fly high, SUVs don't get good gas milage and removing roof racks is a good idea.

As a rule of thumb I think that minimal speed in top gear is normally the most efficent for most cars. The real world best technique is to try not to brake and accelarate, so leave a larger gap to the car in front so you can keep a constant speed.

 

[kharding]

Speed per RPM

Since RPMs have a pretty close correlation to fuel consumed, don't we want the maximum speed per RPM? So if 2000 RPMs get me 60mph that is .030mph per RPM. At 70mph, my tach reads 2500 so that's .028mph per RPM, showing 60mph is more efficient than 70mph. So with this, you can record speed and tach at various speeds and see which one gives the lowest mph to rpm ratio to determine the best cruising speed for optimum mileage. I've also noticed that with an automatic transmission I get spoiled and lazy and don't realize that cruising at 35 mph has the engine revving up close to where it's ready to shift but not doing so. By increasing to 36, it shifts and becomes more efficient. With a stick I wouldn't think of driving at that revved up speed, with a small car auto trans I barely even notice it. Just some thoughts I've had. Do they make sense to anyone else or am I missing something?

 

[mgb_phys]

Since RPMs have a pretty close correlation to fuel consumed, don't we want the maximum speed per RPM? So if 2000 RPMs get me 60mph that is .030mph per RPM. At 70mph, my tach reads 2500 so that's .028mph per RPM

Almost correct - except you are interested in DISTANCE/gallon not TIME/gallon.

So at 60 you use a certain amount of fuel per min and at 70 you use 2500/2000 = 125% as much fuel but only go 70/60 = 116% as far per min so you are loosing.

But if you went at 2000RPM at 60 and 2300RPM at 70 you would be doing better at 70.
Since you would only be using 2300/2000 = 115% as much fuel to go 116% as far.

 

[K.J.Healey]

RPM are not directly correlated to fuel consumption. You can be going at 50mph at 2200 in 5th gear both uphill and downhill. The RPM is the same, but the amount you have to open the throttle is different.

Also, assuming no change in gear, you should get the same mph/rpm for a whole gear, that's the whole point of a gear ratio. Its locked. (non cvt). I wonder if you could just get a measure of Throttle Angle vs. RPM (or mph, same thing). I think there's a 1:1 correspondence between throttle angle and fuel injector flow.

 

[K.J.Healey]

Almost correct - except you are interested in DISTANCE/gallon not TIME/gallon.

So at 60 you use a certain amount of fuel per min and at 70 you use 2500/2000 = 125% as much fuel but only go 70/60 = 116% as far per min so you are loosing.

But if you went at 2000RPM at 60 and 2300RPM at 70 you would be doing better at 70.
Since you would only be using 2300/2000 = 115% as much fuel to go 116% as far.

If you went 2000rpm at 60 then you MUST be doing 2333 at 70. Which must give the same ratio. The gears are locked. For every rotation of the engine it does a certain rotation of the wheel.

 

[K.J.Healey]

This chart was done a few years ago on my 1993 Chrysler Intrepid:

http://www.davesbrain.ca/miscpix/mileage.gif

What I find odd about it is:
- the lack of curvature. I would have thought it would level out to the left.
- the minimal effect of the AC. It has a smaller effect than changing highway speed by 20kph.

If it was a medium to small 6 cyl then I believe it, but in my 99 neon (2.0 dohc 4 cyl) if I turned the air on it nearly halved my mpg at highway speeds.
Ive noticed for some reason 6 cyls have less of a problem with the air on.

 

[kharding]

Thanks for the info. With limited automobile mechanical knowledge, my common sense failed me. So the best solution for both of my questions is to drive as slow as practical in the most efficient gear. (I'll carry a neckbrace in the glove compartment for when I get rear-ended.) But at the lower gears it doesn't make sense to drive revved up if I can give it a little more gas and get to a more efficient gear, true?

 

[mgb_phys]

Basically on a freeway you drive as slowly as possible in the highest gear.
More important is to avoid unnecessary braking and acceleration.
And if you can avoid driving a 4ton SUV to take one small kid to school that would help.

 

[TVP45]

Basically on a freeway you drive as slowly as possible in the highest gear.

Click and Clack have dealt with this at length on Car Talk. (You may have never heard of them- think of Benny Hill and Monty Python with engineering degrees and doing a car show on the radio. ) They recommend your solution, i.e., driving as slowly as possible in the highest gear, without the engine "lugging", which typically means perhaps 1500 rpm or so.

 

[Danger]

I'm not sure about the math aspect of it. My Roadrunner (650hp full race 440) got about 18 mpg at 59 mph. At 60, the secondaries on the carb started to open, and it dropped to about 14 mpg. In city stop & go traffic, or if I put my foot in it on the highway, it was about 4 or 5 mpg.
So my opinion is... it entirely depends upon the vehicle.

 

[mgb_phys]

or if I put my foot in it on the highway, it was about 4 or 5 mpg.

Have you thought of converting it to just burn dollar bills - might be cheaper!

 

[MechSoup]

I have always thought that fuel mileage was based on RPM. A lower, average, RPM would result in a higher fuel mileage.

 

[Danger]

Have you thought of converting it to just burn dollar bills - might be cheaper!

I'm sure that it would be... but we don't have dollar bills. Smelting Loonies and trying to feed them though the carb before they harden is more of an engineering challenge than I care to tackle.

I have always thought that fuel mileage was based on RPM. A lower, average, RPM would result in a higher fuel mileage.

True to some extent. If your revs are below your power band, however, you 'lug' the engine. That makes for lousy mileage.

 

[K.J.Healey]

But remember! It depends on the load as well. Just because I'm at 3000 rpm doesn't mean anything gas-wise. The fuel spent is constantly adjusted even at the same rpm. So if you fo uphill at the same speed, the rpm stays the same, but more work is needed to turn over the engine, so the throttle/fuel increases to keep it at the same rpm. Opposite with downhill.

Same applies to adding a trailer to the back. Same RPM, different fuel expenditure.

If you toss around words like "average" though, then you can draw some correlation of course. It becomes an efficiency thing with the power band, and I'd assume its a simple function of:

Wind Drag(Function of Speed) + Drivetrain Loss(Should be a constant) + Efficiency of Engine(Function of RPM, therefor due to locked gear, say 5th, a function of Speed) + Loss due to grade of road

I figure any weight of the vehicle at a constant speed only comes into play for the drivetrain friction loss.

The drag/drivetrain can be easily approximated. But there is no "function" for the power curve of an engine, you'd just have to map it using a dyno.

If you assume drag goes like : -kv^3
And drivetrain friction is a multiplier on the output of the engine : D*Out
And the Output of the engine is linear, then your best efficiency is at lowest velocity.

but if your engine power output is some sort of peak @ some rpm, then its possible that at that peak you get a better MPG than along the -kv^3 line.

 

[Adrock1795]

From what i can tell, on a gasoline engine when you hit the throttle it opens the butterfly valve on the throttlebody which increases the air intake which is then mixed with fuel to create a constant air-to-fuel ratio maintained by the ECU. The mixture of fuel and air is then is then released into the cylinder chamber when the camshaft hits the valve lifter.
Camshaft speed is dependent to RPM speed, so the faster your camshaft is rotating the more fuel/air goes into the cylinder per minute.

So to use the less possible fuel you would need to be at the lowest possible RPM without stalling the engine.

But to get the most efficient power from your fuel, you would need to drive at the lowest RPM where you hit peak torque which depends on the power band of your camshafts