At what speed would a small Honda Civic get its best gas mileage, assuming it had optimal gearing for any selected speed?
Howstuffworks says about 50mph for smaller cars and 30mph for big trucks and SUV’s… but I don’t know if I agree.
50 MPH seems about optimal, since you’re in the highest gear, not taxing the engine, and slow enough to avoid the major hit your get from wind resistance. For most cars it’s probably near 2000 RPM in the highest gear, in my unsupported opinion.
This very dependent on how the car is designed and built. In general you will get the best gas mileage at the lowest speed at which you can drive in the highest gear and the engine has enough torque to smoothly accelerate. This may be different for manual and automatic transmission cars due to gear ratios, shift points and even engine management computers.
My guess is that a manual transmission Civic would probably get the best mileage between 40 and 50. The reason large vehicles may get better gas mileage at lower speeds is that they may be geared much lower and they may have huge increases in wind resistance losses as speeds start to increase.
So, you didn’t see what I tried to put in italics about optimal gearing for any selected mph?
I find that my Civic gets its optimal gas mileage between 50 and 60 mph (about 35 mpg). Once I get over about 70 it decreases. It gets its worst mileage in “city” driving (of course) where there is a lot of stopping and starting. (about 30 mpg) That’s still pretty good IMO.
I have a 1.5 Liter 16 valve OHC and a manual transmission.
I have a honda Insight… I can average 78 to 84 MPG going at a close to constant speed of 65. Gas Milage tends to drop to 75 to 78 when I average 70 mph.
65 gives the car enough pep to transverse small hills without losing a great deal of speed. I suspect in Kansas I could easy average 85 Mpg.
Have any of you tried driving at a 30mph constant speed for at least a a half a tank to test if it’s better than your highway speed? Also, have any of you gone in and planted a perfectly-sized gear for the 30mph test? Point being, this is a theoretical question, one for engineers and physicists, not Civic owners. Are there any of those on this site?
Is the question as asked even meaningful?
I am neither a physicist nor an engineer, but I thought that if gearing were equally good for any speed, then there would be no optimum, and fuel efficiency would simply drop monotonically with speed.
Though maybe there are other inefficiencies and losses in the engine, so this may not be strictly speaking the case. Still, if sub-optimal gearing is by far the most important factor leading to poorer fuel efficiency at slow speeds, then the simplifying assumption removes any meaning from the answer… Sorta like trying to calculate the air speed of a laden swallow in a vaccuum, or something…
Can you provide a link to the Howstuffworks page in question?
It sounds to me like they’re saying little more than “small vehicles have less air resistance than larger ones.” That seems pretty obvious on the face of it; what are you objecting to, exactly?
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The slower the better for gas mileage. A quarter mile an hour type speeds.
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The best gear for most of the distance is “none”. Coast for a while. Turn on engine, add a little speed, turn off engine, coast.
Even for “real cars”, take it sloooow.
Would a dual overhead cam get more or less mileage?
That can’t be true. An engine cannot run at an arbitrarily small output and use a proportionally small amount of fuel. An engine uses gas just sitting on the driveway idling, getting infinite gallons per mile. So there must be a non-zero speed at which the fuel economy is at maximum.
i am no engine expert but engines achieve maximum torque at midrange RPMs. for example for my car its at 4500 rpm. so while aerodynamics say most efficient speed is a crawl, the engine seems to say most efficient speed is at 4500 rpm 
also there is the effect of lift, the car’s body acting like a plane’s wing. so at higher speed there is less friction in the tires because some of the weight of the car is actually flying i dont know if this effect is strong enough to matter.
so i would have to guess the optimum speed is in the highest gear somewhere about halway between zero RPM and the RPM of maximum torque
and if “gearing is optimal” then i guess it would be still be about halfway between zero and optimal rpm.
of course a car with smaller engine would run near its optimum rpm at even relatively low speed and thus get better mileage. for my car though ( 3.5L ) to go to 4500 rpm i would have to be doing about 110 mph
SOHC and DOHC might both be 4 valve per cylinder. in other words as far as i know DOHC can either be 4 or 5 valves per cylinder. while SOHC might be 2, 3, or 4 valves per cylinder.
if you’re comparing 4 valves with 4 valves then i doubt there will be any difference.
and if you’re comparing different number of valves then i guess you would have to coniser other things, that is all things NOT being equal. for example an engine with 3, 4 or 5 valves could be made smaller than a 2 valve per cylinder engine, and still achieve same horsepower. a smaller engine should get better mileage all things being equal imho.
Maximum torque does not mean maximum fuel efficiency. It means that, at WOT, the volumetric and combustion efficiency of the engine are proportionately the best relative to the engine frictional losses and thermal losses. It is a “power efficiency”, but not a “fuel economy efficiency”.
There are multiple sources of energy loss in an engine, with the largest being thermal loss and frictional loss. You (typically) reduce frictional losses by running the engine at its lowest possible piston speed. You reduce thermal losses by reducing the total heat which is lost via cooling - this one is a bit trickier to manage, since the engine has to be maintained at a temperature where it can operate safely and reliably.
Violet - there is nothing inherent in the design of SOHC versus DOHC that can give better mileage, speaking from a purely general standpoint. DOHC (assuming this means a 3 - 5 valve engine) is intended to operate more valves to thus improve the volumetric efficiency of the engine. However, in striving for maximum fuel economy, we don’t care all that much about volumetric efficiency, so long as the car can produce the power needed when needed. Thus, a SOHC design can be said from a general standpoint to be more efficient, due to the lower friction and inertia of the cam train.
I thought the main difference between DOHC and SOHC were to provide different valve timings? Kind of like the Hondas did/do with the variable valve timing on a single cam. You know, open the valves really far at high RPM and just barely let 'em crack at low RPM. Different series (like the EX vs. the CX) used different combinations to give better economy (the EX for performance, and the CX for economy). Been years since I’ve owned a Honda, so VTEC and EX and CX may be out of date.
Different valve timings are simpler with DOHC, you are correct, but it’s not the primary reason its used, since DOHC engines without varaible valve timing have been around since the 1950’s and possibly earlier.
And, of course, one can have multivalve engines with a single cam. IIRC, some Honda 16-valve engines used a SOHC for a while to run 16 valves.
According to How Stuff Works:
Variable Valve Timing could theoretically be done with any camshaft configuration (pushrod, SOHC, DOHC), as talked about here http://auto.howstuffworks.com/camshaft3.htm.
To try and bring this back on topic, the valve and camshaft configuration doesn’t really have any bearing on gas mileage.
as for thermal losses. methinks they would be minimized at maximum RPM. because rate of heat flow is dependent on temperature gradient, which remains constant more or less regardless of RPM. thus thermal loss is almost constant while total developed power is greater at higher rpms, so thermal loss as a percentage should be minimized at higher RPMs.
smaller engine means less surface for heat loss, means higher mileage
honda insight has 1.0 liter …
One more vote for lowest feasible RPM in highest gear generally results in best MPG.