Friday, May 6, 2011:  Updated Saturday August 4, 2012

Questions/Myths about Vehicles answered by John

Lindsey from Texas:  What is the difference between driving with low air pressure in the tires and normal pressure?  What is the ideal tire pressure? 
I ran several tests using a 2002 Chevy Venture mini-van and a Scan Guage tool. I lowered the tire pressure in each tire to 25psi and took several samples while driving on a flat road at 30mph. I then pumped up each tire to 32psi and took several more measurements. Finally, I pumped all the tires up to 42psi and drove around for several more measurements. Here are my results: 

While driving at 30mph on a flat road. 
Tire Pressure    GPH (avg)    MPG (avg)
25psi               1.55gph          20.0mpg
32psi               1.23gph          23.85mpg
42psi               0.92gph          33.08mpg


Having low tire pressure will reduce your fuel efficiency over 40%.  Keeping your tires at close to the maximum pressure written on the sidewall will give you the most fuel efficiency. 

Niko from California:  Does driving with the windows open hurt fuel efficiency? 
To my surprise, and amazingly, no!   I had to run this test over and over just to make sure.  A few of my results even show an improvement in fuel efficiency when rolling down the windows.  My "baffle you" theory is modern vehicles have a more aerodynamic shape.  This creates a bubble of air around the vehicle as it drives.  Rolling down the window does not slow the car down because the bubble preserves the aerodynamics.  
Results may vary depending on the shape of the vehicle.  

Speed       Windows up    Windows down
75mph       29.5mpg           29.8mpg
70mph       25.5mpg           24.0mpg
65mph       26.1mpg           28.1mpg
60mph       29.0mpg           32.5mpg
55mph       33.7mpg           31.5mpg

Karen from Washington:  Is it bad to start the car with the A/C on?  Also will it hurt the car to keep the A/C on when driving up a big hill? 
Starting the car with the A/C turned on doesn't hurt it at all.  If you leave the A/C button on all the time, when you start your engine, the vehicle computer delays the A/C from turning on.  My van takes about ¼ second after the engine is turned on before it automatically engages the A/C compressor.  Cars are pretty smart.  On steep hills, when the engine is really working hard, most cars automatically turn off the A/C to save performance.  Next time you are driving up a steep hill this summer, leave the A/C on.  You may notice the air coming out of your vents won't be as cool as it should be.  This is an indication that your vehicle is switching off the A/C so all available power can be used for getting up the hill.  Or maybe it's about to blow up (j/k).  By the way, it is normal for the air conditioner compressor turns itself on and all the time.  But during the times when it is on, it only uses 3 horsepower.  On average it will draw even less power.  

A/C On/off     idle/parked    30mph        55mph
no A/C            4.9hp              18.7hp        27.0hp     
max A/C         7.1hp              22.8hp        30.0hp
delta               2.2hp                4.1hp          3.0hp

What is the ideal, most fuel efficient cruising RPM?  What is the best gear to be in?
For manual transmissions, lower is better as long as you don’t bog the engine down.  This will depend on the type of vehicle you drive and its transmission gear ratio.  For an automatic, a lighter push on the gas pedal will send it into a higher gear
Look at how the fuel efficiency is effected by what gear you are in.  These measurements were taken while driving on a flat road at a constant 55mph and manually shifting into that particular gear. 

Gear               rpm             mpg
Overdrive       1680rpm      35mpg
3rd Gear         2380rpm      30mpg
2nd Gear        3700rpm      18mpg

Angie from Utah:  Does it really save fuel to drive the speed limit?  Driving faster will get you there quicker, but is the extra burn worth it?  5 mph over?  10 mph over? 
Not only will driving the speed limit save you tons on gas, it will save you speeding tickets and in turn, give you lower insurance premiums.  Plus, its safer. 
MPG of 2002 Chevy Venture
mph             mpg
55mph          35mpg
65mph          30mpg
75mph          22mpg

For a 15 mile commute, driving 75mph takes 12 minutes.  Driving 65mph takes 13:50 minutes.  Driving 55mph takes 16:21. 
Assuming you drive to work and back 5 days a week for a year, you will drive 7800 miles a year just driving back and forth to work.  Using the figures above and assuming $4/gallon gas:
Driving 75 will require 354 gallons/year and cost $1418. 
Driving 65 will require 260 gallons/year and cost $1040. 
Driving 55 will require 222 gallons/year and cost $891. 

At the cost of less than 4 ½ minutes each way, you will save $527 in gas each year by driving 55mph instead of 75mph.  That's $44/month. 
All those extra minutes spent and dollars saved, add up to a wage of $13.97/hour, tax free. 

For a short daily commute, a few minutes really doesn't matter much.  But what if you are going on a 600 mile road-trip?  In this case, the time really adds up.  To drive 55mph for 600 miles would take 10.9 hours.  Driving 75mph would only take 8 hours.  At a savings of $40 in gas, your trip will cost you nearly 3 extra hours.  To me, a vehicle full of sticky, tired, crying kids easily justifies driving as fast as possible.  

What is the most fuel efficient speed to travel at? 
That depends slightly on the type of vehicle you drive.  For highway driving, going the speed limit or even slightly under will always achieve the maximum fuel savings. 

Here is a plot of the fuel efficiency vs. speed for a 2002 Chevy Venture. 
mph         mpg
40            32
45            35
50            36
55            35
60            31
65            30
70            26
75            22

While driving 50 is the most fuel efficient speed, it's also happens to be the most painfully slow one too.  Driving 55 mph during a short commute doesn't use much more fuel and will still get you there in a reasonable time frame.

Ben from Utah: When parked, when is the break-even point of fuel consumption between idling and turning off the engine then restarting it? 
Using a scan gauge tool in a 2002 Chevy Venture van, I measured how much fuel it takes to idle for about 5 minutes. Then I measured the amount of fuel it takes to stop/start the engine 10 times. 

Idling in park burns 0.3 gallons of gas every hour or about 1 penny of gas every 30 seconds. 
Stopping/starting the engine 10 times consumes about 5 cents of gas.  That averages out to 1/2 cent of gas per stop/start. The break even point is 15 seconds. 

How much fuel is consumed leaving the car in drive vs. putting it in neutral while waiting?
A warmed up engine with the transmission in drive (but not moving anywhere) will consume 0.31 gallons per hour. In neutral will consume 0.29 gph. 
A cold start engine burns 0.8 gph at idle while in park but it steadily drops as the engine warms up.

How much gas does it take to run the A/C or heater on full blast?
When idling, running max A/C uses 59% more gas than just idling alone. Running the heater on max uses 6% more gas. Running the heater and defrost uses 56% more gas, (the defrost also uses the A/C compressor). The rear defrost (which is just a resistive heater in the back windshield) used 3-6% more fuel.

Engine mode                            gph
Idling with max heater            0.34gph
Idling with max A/C                0.51gph
Idling with no heat or A/C      0.32gph

Cruising on the freeway, a Chevy Venture only uses 27 horsepower. With max A/C, it uses 30 horsepower. This is only about 10-12% increase. Well worth it on a hot summer day in my opinion. 

How much horse power does a vehicle use to drive 55mph?  65mph?  75mph?

Speed     horsepower     mpg
55            26.1                   34.3
65            35.1                   30.3
75            47.0                   25.0

Gavin, New Zealand:  Does the octane in fuel affect your gas mileage?  Which one works out to be cheaper overall?  
A higher octane fuel will not increase your fuel efficiency one bit.  Don't waste your money.  If you car doesn't care, neither should you. 

Octane   Speed      Throttle position  Ignition timing  GPH
85          30 mph     5                           44                      1.06
92          30 mph     5                           40                      1.00
85          in park      0                           18                      0.30
92          in park      0                           18                      0.31

How does extra weight affect the fuel efficiency of a vehicle? 
I measured the fuel efficiency of a Chevy Venture and then I removed all the seats, weighed them and measured the fuel efficiency again. 
The curb weight of a 2002 Chevy Venture is 3699 lbs.  The weight of 4 removable seats and other misc stuff I had in the van is 288 lbs.  Removing all that stuff, I reduced the van's weight about 8% and at the same time, increased its fuel efficiency by 10%. 

Weight                                   gph          mpg
Van with seats = 3699lbs      0.92        33.08
Van no seats   =  3410lbs      0.83        36.54

Does cruise control help fuel efficiency?  
It will help your vehicle achieve better fuel efficiency if you tend to drive too fast. Otherwise, you can do better just by paying attention to the terrain of the road and allowing the vehicle to slightly speed up/ slow down accordingly. Cruise control tries to maintain a constant speed no matter what the terrain. This is a very in-efficient way to drive.  Slowing down while climbing hills (even an overpass) and speeding up on the downhills will save more fuel. 

A more fuel efficient cruise control would be a throttle position sensor control, or an intake manifold vacuum pressure control or a fuel gallons per hour control.