Influence of window opening vs. A/C use on fuel economy

Fair'nuff... my Accord gives me usually 500-550km on a tank (80% city driving),
which works out to around 28mpg (alright for an '87 with almost 420,000km), so
it really isn't worth the difference for me.

 

System pressure cycles the compressor, by nature of its design.

Spdloader

 

System pressure cycles the compressor, by nature of its design.

Spdloader

 

It is good information, but the true differences are difficult to assess, as
many segments are averaged together, so one cannot calculate the standard
deviations for each group. It does seem that on 9/7 there was less head
wind.....

 

It is good information, but the true differences are difficult to assess, as
many segments are averaged together, so one cannot calculate the standard
deviations for each group. It does seem that on 9/7 there was less head
wind.....

 

I don't believe the original gas mileage matters, only the differences.

Maybe one can assume that windows open would be less of an effect on a
vehicle with more mass and available torque (unless the vehicle allowed a
higher volume of air)?

 

I don't believe the original gas mileage matters, only the differences.

Maybe one can assume that windows open would be less of an effect on a
vehicle with more mass and available torque (unless the vehicle allowed a
higher volume of air)?

 

Vehicles with lower fuel economy are less sensitive to differences, because
the losses are already pretty high. In the Toyota Prius fora people are
shocked to find that running the heater can decrease the in-town fuel
economy 10 mpg... the finer the edge the more quickly it dulls.

Mike

 

Vehicles with lower fuel economy are less sensitive to differences, because
the losses are already pretty high. In the Toyota Prius fora people are
shocked to find that running the heater can decrease the in-town fuel
economy 10 mpg... the finer the edge the more quickly it dulls.

Mike

 

Better check the date. Tests were run on 8/31, 9/4 and (mostly) 9/5.

The behavior of the Scan Gauge is a little flaky in regards to
measuring fuel consumption. I recalibrate it at every fuel stop and
it can vary by 10% or more. Since the calibration changed between
test sessions, you can't compare the absolute milage figures between
sessions. Also, gradually changing terrain, elevation, wind and
different test speeds complicate the situation. That is why I looked
at relative fuel economy between the sessions. Below is the
individual session data in case you or someone else wants to do more
sophisticated analysis. Just keep the above in mind.

Test 1 8/31 76 mph

A/C Window m/g miles
on closed 18.2 10.0
off closed 19.5 10.0
off wide opn 20.2 10.0
off closed 21.2 10.0
on closed 17.4 10.0
off closed 18.9 10.0
off wide opn 18.7 10.0
on closed 18.8 10.5
off closed 19.7 10.0
off F3 R3* 19.3 10.0
on closed 19.1 10.0
off R6 18.5 10.0


* F3 R3 = front window down 3", Rear window down 3"


Test 2 9/4 75 mph

A/C Window m/g miles
off closed 21.0 10.0
off R4 20.0 10.0
off closed 22.5 8.7
on closed 20.9 10.0
off closed 22.4 11.4
off F2 R4 22.4 10.0
off F2 R4 21.4 11.4
off closed 21.7 10.0
off closed 21.0 10.4

Test 3 9/5 75 mph

A/C Window m/g miles
off closed 19.6 10.0
off R4 20.5 10.0
on closed 18.5 7.8
off closed 20.0 10.0
on closed 19.2 10.2
off R4 20.8 10.1
off closed 21.7 10.0
on closed 20.1 10.0

Test 4 9/5 73 mph flatland

A/C Window m/g miles
off closed 21.6 10.0
off F3 R4 21.1 10.2
on closed 21.2 10.0
off closed 22.5 10.0
off F2 R4 22.4 10.9
on closed 20.6 11.6
off closed 21.4 10.0
off F3 R4 21.7 10.0
on closed 20.6 10.0
off closed 22.8 13.7
off F3 R4 21.4 10.1
on closed 20.3 10.5
off closed 21.6 12.0
off F3 R4 21.8 11.0
on closed 21.4 10.0
off closed 23.4 13.0

 

Better check the date. Tests were run on 8/31, 9/4 and (mostly) 9/5.

The behavior of the Scan Gauge is a little flaky in regards to
measuring fuel consumption. I recalibrate it at every fuel stop and
it can vary by 10% or more. Since the calibration changed between
test sessions, you can't compare the absolute milage figures between
sessions. Also, gradually changing terrain, elevation, wind and
different test speeds complicate the situation. That is why I looked
at relative fuel economy between the sessions. Below is the
individual session data in case you or someone else wants to do more
sophisticated analysis. Just keep the above in mind.

Test 1 8/31 76 mph

A/C Window m/g miles
on closed 18.2 10.0
off closed 19.5 10.0
off wide opn 20.2 10.0
off closed 21.2 10.0
on closed 17.4 10.0
off closed 18.9 10.0
off wide opn 18.7 10.0
on closed 18.8 10.5
off closed 19.7 10.0
off F3 R3* 19.3 10.0
on closed 19.1 10.0
off R6 18.5 10.0


* F3 R3 = front window down 3", Rear window down 3"


Test 2 9/4 75 mph

A/C Window m/g miles
off closed 21.0 10.0
off R4 20.0 10.0
off closed 22.5 8.7
on closed 20.9 10.0
off closed 22.4 11.4
off F2 R4 22.4 10.0
off F2 R4 21.4 11.4
off closed 21.7 10.0
off closed 21.0 10.4

Test 3 9/5 75 mph

A/C Window m/g miles
off closed 19.6 10.0
off R4 20.5 10.0
on closed 18.5 7.8
off closed 20.0 10.0
on closed 19.2 10.2
off R4 20.8 10.1
off closed 21.7 10.0
on closed 20.1 10.0

Test 4 9/5 73 mph flatland

A/C Window m/g miles
off closed 21.6 10.0
off F3 R4 21.1 10.2
on closed 21.2 10.0
off closed 22.5 10.0
off F2 R4 22.4 10.9
on closed 20.6 11.6
off closed 21.4 10.0
off F3 R4 21.7 10.0
on closed 20.6 10.0
off closed 22.8 13.7
off F3 R4 21.4 10.1
on closed 20.3 10.5
off closed 21.6 12.0
off F3 R4 21.8 11.0
on closed 21.4 10.0
off closed 23.4 13.0

 

Running the heater? Isn't this just waste heat anyway?

 

Running the heater? Isn't this just waste heat anyway?

 

The raw number would be lower, but the percentages should be similar.

 

The raw number would be lower, but the percentages should be similar.

 

I guess it has to do with the amount of juice needed to run the
fan.......????

 

I guess it has to do with the amount of juice needed to run the
fan.......????

 

The Prius doesn't normally run the engine when the car is moving slowly or
stopped, so the waste heat often falls short. The engine has to run more
just to make heat.

Mike

 

The Prius doesn't normally run the engine when the car is moving slowly or
stopped, so the waste heat often falls short. The engine has to run more
just to make heat.

Mike

 

I think the discrepancy comes from working with the reciprocal of what we
really want to measure: fuel per increment. For example, suppose it requires
5 gallons of gas to move the car 100 miles with the windows rolled up and
A/C off. That is 20 mpg. If the window drag at some speed consumes 0.2
gallons in 100 miles at that speed the economy drops to 19.2 mpg for a loss
of nearly 5%. If the drag is the same in a vehicle that requires 2 gallons
to move the car 100 miles (50 mpg) the window drag drops it to 45.5 mpg, a
nearly 10% hit. I'm not a big fan of the "liters per 100km" measurement but
it works a lot better than mpg here.

So, here's an example of how it goes - theoretically - with the window and
A/C. Assume (for illustration) that at 50 mph the window drag consumes 0.1
gallon per hundred miles. Similarly, assume the A/C consumes 0.1 gallon per
hour. For our hundred mile trip that would mean the windows would use 0.1
gallon and the A/C would use 0.2 gallons at 50 mph. If we increase the speed
to 100 mph the window drag, increasing with the square of the speed, becomes
0.4 gallons for the one hour the trip takes while the A/C loss drops to 0.1
gallon for the hour instead of 0.2 gallons for two hours.

For different vehicles the numbers would change; the A/C losses are higher
for a larger vehicle with more glass and the window drag will certainly vary
with body style. In the end, all that will change is the speed at which the
A/C is more economical than windows down.