mileage at 60mph vs 70mph

I'm planning a trip cross country, about 3,000 miles, and am trying to estimate how much extra time and money it takes to cruise at 70mph vs

60mph. I've read that around 50-55mph is where optimal mileage occurs, but realistically, driving that slow on the open highway often comes with a lot of horns blowing and fingers wagging your way. So 60-70mph seems like a reasonable range to consider. I think these figures are about right; if not, somebody please correct me:

A. As best I can determine, it looks like my '97 4cyl Camry, with two people and minimal luggage, on the highway should get about 28mpg @

70mph, and about 32mpg @ 60mph.

B. If so, then:

cost @ 60mph:

3,000 miles / 32 mpg = 93.75 gallons @ $4.15 = $389.06

time:

3,000 miles @ 60mph = 50 hours

cost @ 70mph:

3,00 0miles / 28 mpg = 107.14 gallons @ $4.15 = $444.64

time:

3,000 miles @ 70mph = 42.86 hours

60mph: $389.06 50.00 hours

70mph: $444.64 42.86 hours

----------------------------------- $ 55.08 7.14 hours

so, it costs about 7 hours to save about $55. In other words, if you drive slower, you?ll earn $55.58 for those 7.14 extra hours you?ll spend on the road, or $7.78/hour -for those 7 hours only, not for each hour of the entire trip. Is that about right?

Better yet, does anybody have more accurate mileage estimates for driving speeds, like a curve plot from 50mph - 80mph, etc?

And I've heard waxing the car can boost mileage up to 5%, which is probably true to some degree, but is there any real evidence out there? Just trying to think of everything before pulling out of the driveway...

Reply to
Henry David
Loading thread data ...

Are you related to Built_Well? I think you need at least 6 months more analysis before you start your trip.

Reply to
Mark A

Max milage is maybe 40-45 mph or just when you go into high gear, so you will have to slow down your trip , have fun.

Reply to
ransley

I really commend you on your concerns in this regard. I know that I am much more concerned about speed-related MPG now days. However, unless a lot of other people are also thinking the same way, you'll get run over doing a "mere" 70 MPH out there. A couple years ago, I was travelling the I-25 corridor of Colorado. I had to do about 90 just to keep from getting steamrolled. Sometimes for your own safety you have to "go with the flow", even when you don't want to.

Reply to
Mark E. Bye

Some things are worth the extra time and money. I don't have any experimental data, but after a fresh wax, my car seems to glide through the air, noticeably smoother and quieter. Take your time and enjoy the land of the free and the home of the brave.

Reply to
pwichert

Say, where did you get the " 28mpg @ 70mph, and about 32mpg @ 60mph" numbers? I'm not saying they're wrong, but just wondering. Do you have/did you create your own mpg vs. speed chart?

I remember reading that optimal mileage is 45 mph, from some article in the newspaper when I was a kid, but maybe optimal engine operating points have changed...? (Plus, reporters are rarely mechanical engineers.)

Michael

Reply to
mrdarrett

The curve of mileage versus speed for various cars will vary significantly. Major factors are rolling resistance (related to tires, effect is linear), air resistance (resistance goes up proportional to the square of the speed), drivetrain efficiency (power loss is linear for a given gear), engine efficiency (can be difficult to predict - for a first order estimate you can assume it is constant over a small rpm range).

I think your steady-state fuel economy at 60 mph would actually be significantly higher than 32 mpg (closer to 40). I think you are about right for a steady state 70 mph. However in both cases your actual average mileage will be considerably lower than the steady state average because of hills, changes in speed (on / off ramps, slow downs, etc.).

I think if you try to drive 60 mph, on roads where everyone else is driving 70+, you will find that you are making so many speed changes that your mileage will suffer. Also, it has been my experience that in heavy traffic, your steady state fuel economy can actually go up, if you go with the flow. I don't know if this is because you are "drafting" (which reduces aerodynamic drag) or because you are minimizing speed changes (brake use in particular, which reduces other losses).

Here are some references:

formatting link
Regards,

Ed White

Ed

Reply to
C. E. White

The 'about 32mpg @ 60mph' was pulled from this claim of unknown reliability:

'On the same day I ran the Firefly test, I took a relative's 1997 Toyota Camry automatic down the same test road; it achieved its 30 mpg EPA highway rating at 115 km/h (71.5 mph). Quite a difference.'

formatting link
As I have the California smog stuff, I knocked off 2mpg -just a guess.

From the Natural Resources Canada site:

'...increasing your cruising speed from 100 km/h to 120 km/h will increase fuel consumption by about 20%.'

formatting link

So I converted the above to miles, and used the 20% mileage difference between 100 km/h to 120 km/h as a basis for backing into the 60mph estimate.

All rough estimates, to be sure -the thing I was after was just how much is it worth to the driver, in terms of time and money, between the two speeds.

Reply to
Henry David

Somehow my 1994 130K miles 4 cyl Camry gets 37MPG on highway driving at 70-72MPH recently. The same car got a little better MPG when driving at 57.5MPH a while back. Then, it got only 27MPG driving at

60MPH. It is probably relating to the weather, temperature, and condition of the car. I just recently replaced the fuel filter.

Does anyone know if a car gets higher or lower MPG on wet road (raining)?

Reply to
cpliu

You will get slightly better gas mileage when raining or high humidity. It is the same principle as water injection to get more power from an engine.

Reply to
sam

Water injection helps prevent pre-detonation (spark knock) by lowering peak combustion chamber temperatures. For engines with a high compression ratio, this allows full throttle operation without excessive spark knock. Few stock vehicles will get any advantage from this. And even if they did get an advantage, it really is only an advantage as far as creating maximum power is concerned (WOT operation). At part throttle, there is no significant advantage to water injection or high humidity as far as fuel consumption is concerned.

And more importantly as far as gas mileage is concerned - high humidity increases the density of the air, increasing drag and lowering fuel economy and wet roads increase rolling resistance, also lowering fuel economy.

So, the net is, should will almost always get slightly lower gas mileage when driving in the rain. I noticed this to be true a few years back when trying to measure fuel economy at various speeds. While I was doing this, we had a rain storm and the fuel economy measurably dropped. Even after the rain stopped the fuel economy was measurably lower until the roads were almost dry. However, I think the biggest effect on gas mileage during a rain storm will be the wind. A

20 mile and hour head wind will measurable lower your fuel economy. A 20 mph tail wind will improve it. Cross winds also affect fuel economy, but not as much as pure head or tail winds.

Ed

Reply to
C. E. White

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.