Most fuel efficient speed to drive?

Back in the 80's I remember my dad telling me that the most fuel
efficient speed to drive his car was at 45 mph, and that it was a spec
that was published for many car models.

With the gas price absurdity these days, I'm just curious if is there
an equivalent most efficient speed for today's cars? (Specifically my
05 EX-4 accord)

-MVL

 

Approx. 0 kph (0*0.6213= 0 mph) provided the engine is not running.
;-)
Greg

 

Coupla anecdotes to throw into the mill:

My 1991 Civic seems to get its best fuel mileage somewhere between 35 and 55
mph.

Someone recently posted here that going over 3000 RPM reduced mileage. That
figure may vary somewhat, but I have noticed that cruising at 70 mph in 5th
gear (of course) with my car puts me over 3000 rpm and reduces my fuel
mileage. I do not get my best mileage on long highway trips, with the cruise
control set at 68 mph.

Avoid rapid accelerations.

It seems staying in gear (instead of coasting with the engine at idle) and
braking with the engine improves my Honda's fuel mileage, too, because
(according to someone here) of the way the fuel control etc. system is set
up.

 

For every carburated car i've owned, the optimal speed is around
60mph. This is, i believe, why the US speed limits were set to 55 in
the oil crisis.

depends on the hill, and what you do with the coasted momentum really
- use it to boost your vehicles speed, and you culd save fuel.

Never really bothered to be honest, since fuels so cheap in the US.

 

Instead of speed watch the RPM. The sweet spot for most Hondas
are from 2100 to 2400 rpm. Since some people may be driving on
the wrong gear at 55-mph.

The new VTEC-E has a different higher sweet spot. Check your
information source.

 

That's because the fuel only cuts out (some carburetors too) when
in gear and above 1180-rpm and gas pedal released (carburetor
rpm is different). Idling is big business. Cruising is only a fraction of
the injection pulse width if you measure it. Your best bet is to keep
moving (not to slow, not too fast) in gear to increase your miles
per gallon.

 

That was one reason. 55mph was determined in tests to be a good compromise
between speed and mileage. The cops loved it too, once they discovered they
could make megabucks from all those new scofflaws driving on roads designed
for much higher speeds than 55. Hello, Fuzzbuster!

The other reason was the new pellet-type catalytic converters that were
mandated for 1975. It seems that they had a tendency to fire pellets out
the tailpipe if exhaust pulses got too violent. No pellets = no conversion.
Modern monobloc cats are immume to this.

It sure is (thank god).
http://www.aip.com.au/pricing/oecd.htm


I've also not found much difference in my mileage with car speed. Not
enough to make me do anything about it, anyway.

On average, I get about 27 mpg. A low of about 25 is seen at sustained
sppeds over 90 mph, and a high of about 30 at sustained speeds of about
60mph. I only conducted my speed tests once (60 is boring, 90 is too cop-
friendly), so I can't confirm my results.

 

Dunno. My 91 Civic is fuel-injected.

We disagree.

 

Wind resistance is another factor that has not been discussed so far.
The relationship between wind resistance and speed is complex.
However, at low speeds, the amount of wind resistance depends linearly
on speed. For example, is the speed of the car is increased form 20
mph to 40 mph, the wind resistance. will roughly double. However, at
higher speeds, the wind resistance will begin to increase more
sharply, so that doubling the speed may triple or quadruple the wind
resistance.

Part of the reason the spped was set to 55 was because with cars back
then, that was the approximate speed at which wind resistance began to
be an important factor. This became obvious if one listened to wind
noise. At 45 or 50 mph in my 1976 station wagon, wind noise was barely
noticeable. But at 75, conversation became difficult.

Modern cars are much more aerodynamic than they were 30 years ago
(except for those Chrysler products with huge, boxy, ugly grills), so
the wind resistance probably does not become an important factor until
a higher speed, but at some point it is going to take much of the
available engine power to just keep the car moving.



Elliot Richmond
Freelance Science Writer and Editor

 

Wind resistance (as measured by power required to overcome aerodynamic
friction) should vary roughly in a square or cubic relationship with speed.

I suspect what's really at work here are the conditions under which your
typical car engine is designed to operate most efficiently. That the typical
passenger car is not designed for optimal efficiency at 20 mph nor 75 mph
makes sense, since the average driver's average cruising speed is probably
closer to 35 to 60 mph.

I wouldn't use the decibel level to indicate anything more than wind
resistance goes up with speed according to a square or cubic relationship.

(I lean towards cubic, from a units analysis standpoint, but I may be
missing something from empirical studies on car drag.)

All engine power is strictly to keep the car moving. "A body in motion tends
to stay in motion, unless [the nasty F-phenomenon kicks in, which it will]."

We can't eliminate engine (internals), wind, or rolling friction.

Free.

 

Agghhhh ... not this conversation again!? ;p

Grrrrrrrrrr .....




a

 

Sounds rather low. I would have thought 55-65mph a sensible speed in 5th,
though quite how anyone can set cruise control (or even use it for that
matter!) at that low a speed on a motorway is quite beyond me!

Give it a break! You guys are pretty much *given* petrol. At least this is
a Honda group, so most of you aren't as selfish as your fellow countrymen in
their gas-guzzlin' 4x4 pieces of crap. Try paying £60 to fill your tank
each time (over $100).





a

 

Not if it had poor aerodynamics, which would have been pretty common back
then (especially if the car involved was not brand-new in the '80's but a
car 5 years old, maybe dating from the mid-'70's, even). At low speeds,
aerodynamic drag will be unimportant but it increases with the square of the
speed.

 

The lowest speed without lugging the engine in the highest gear(5th). On
level ground 45 to 50mph in 5th gear sounds about right(off the top of
my head) and probably puts out about 2200 RPM. Anything below 1800 RPM
starts to labor the engine. Above 65 mph much of your fuel is consumed
pushing air out of the way of the car. All experts & mechanics(I am
neither) out there, please correct me if I am mistaken.
Rich

 

This question was printed in the latest Road & Track magazine... there's
a very good, extensive answer there (check out the issue with the
"flat-out one-mile" tests on the cover).

 

Actually, you can also find the R&T article here:
http://www.roadandtrack.com/article.asp?section_id=19&article_id=2590&page_number=2

 

if i drive 70-90km/hr in my 04 civic (automatic tranny) i usually get
50-55 mpg consistantly

 

You drive in kmph and figure distance/fuel unit in mpg?

 

yeah, canadians sorta use a combination of the 2 systems. or at
least i do.

 

I communte through many stop lights and stop signs each day. I've been
looking for the most fuel efficient acceleration for my 05 Accord (4
cyl).

I've seen posts saying that cars in general drive most efficiently if
driven near peak torque RPM, which for the accord is around 4500. My
car's an auto, so I don't have too much control over RPM's. But the
automatic likes to hover around 2000 RPMs, so the only way to be close
to 4500 is almost flooring the starts.

But I've also heard that the harder starts are less fuel efficient.
Based on the Nav MPG, I think I've found keeping RPM's under 2000
(sloooow acceleration) is more efficient.

I don't mind speed of acceleration, since my commute is on 1-lane roads
where no one can pass, and regardless of how fast I drive, I just get
stuck behind another car at the next stop light.

Anyone able to comment one way or the other on hard vs. soft
acceleration?

-MVL