Optimum fuel saving speed

buick
lesabre

#1

Has anyone done any scientific research on what the optimum speed for maximum fuel efficiency is? I hear things like “every x mph above 60 mph uses y% more fuel” but I suspect it is just someone’s opinion. Is 60 mph really the magic number, or is it more like 35, “Victory Speed” during World War II.


#2

I’m going to guess that would depend on the vehicles weight and engine computer settings. Trucks and Cars idle and cruise differently. I think the government did a study and came up with 55 mph for their final analysis. But who knows.


#3

Well, logically, I think that the speed at which you would use the least gas is the lowest speed at which your transmission shifts into its highest gear and locks its torque converter.

Of course, logic may not be a valid way of determining this, but since your engine would be turning over at the lowest RPMs possible when the transmission is in its highest gear and its torque converter is locked, I think that this idea has some merit.

On most modern cars, this would translate to a steady speed of ~40-45 mph on a level road.


#4

There are too many variables here–vehicle shape, size, load, weight, tires, weather conditions. But for most late model cars with automatic transmissions somewhere in the mid-40mph range.

I was recently talking to someone who claimed he got 40mph in his 4 cyl Camry on a trip from Seattle to Spokane. Once on the highway he kept the windows shut, A/C off, and set the cruise control to 52mph. I can only imagine how many people he ticked off on that trip.


#5

I don’t think 55 was the optimum speed so much as it was the point where diminishing returns start.
Nixon originally proposed an emergency 50 mph speed limit but it was raised to 55 as a compromise.


#6

Different engines have different “sweet spots”.
It takes lots of experimentation in your vehicle to learn what that is.
In certain areas of speed vs time you have to compare the fuel used for the same identical trip.
ie; Slower speed but longer driving time -vs- higher speed and shorter drive time.
The conundrum ?
Which higher speed ? which slower speed ? yeilds the most efficeint overall trip mpg ?

We have agrued this subject to death before and it is an extermely individual vehicle & conditions situation with NO simple answer.
Sure one could drive only 35mph on the speed limit 75 I40 corridor but that would be highly impractical, dangerous, and at only 35mph, illegal. Not to mention the normally hour & 53 minute drive ( Gallup to Albuquerque ) would take 4 hours.

So then you start looking at practical application.
On the speed limit 75 interstate, does your vehicle do better @ 65 for 2hrs 15min, 75 for 1hr 53min, or somewhere in between ?

Generally speaking, the slower PRACTICAL speed ( for situation ) will garner the better comparative mpg.


#7

I had this truck, a 1983 Ford F-150 with reg. cab, 300 cid eng. auto. trans 4WD and cruise control with 38 PSI in the tires. At 58 MPH it seemed to get 17.7 MPG. Plug in all that data and multiply by pi and the radius cubed and you will get the optimum speed for your car.


#8

I’m In My Car Right Now And I’m At The Exact Optimum MPG Speed.
It’s in park and the engine is off.

Seriously though, it would be good to know an optimum MPG speed range, maybe added to the EPA’s MPG rating for a car model-year/make/model/engine/transmisssion.
CSA


#9

I’m just guessing, but for most of todays passenger cars, a speed of 35-40 mph will produce the best miles-per-gallon figure. The top of the curve will be pretty broad, so anywhere in the 30-45 MPH speed range will produce very close to peak mileage. Once you get over 45 MPH, aerodynamic drag starts getting serious and fuel mileage falls off rapidly…

Perhaps the car magazine road testers should find and publish the speed at which maximum mileage was obtained and publish it for each car they test…


#10

I think it’s vehicle depended. Generally speeds less than 55 would get the nod, but cars like the very slick Corvette but with high cruising speed gear ratio with lots of torque, may get the nod at a higher speed than a Yaris. This is one of those debates that will go on forever as opinions will vary with the car each owns.


#11

I think it depends on the number of forward gears. Newer cars have up to 8 forward gears; many now have 5 or 6. My 2005 Accord has a 5-speed auto and it shifts to 5th around 50 to 55. I chug along at about 1800 RPM in 5th.


#12

It does seem very much vehicle dependent. My S-10 varies from 13 mpg on interstates pushing 75 plus to 26+ on the Parkway at 50. Several E-150s that I owned seemed to do well when driven at about 65 and there seemed to be no improvement when dropping to 55 but at 75+ those vans became gas hogs at <11 mpg. And a Ford F-250 with a 400M engine would burn a gallon every 11 miles regardless how (within reason) it was driven. Even towing a car on a flat bed.


#13

I heard the same thin stated slightly different. The slowest speed you could be in the top gear without lugging the engine.


#14

What would be interesting would be a “time vs. money” graph, to show (a) how much more it costs to do a trip at 75 vs 55, (b) time saved, and © the “hourly rate” you pay to go fast (or for you optimists, the “after-tax wage” you’d accrue from slowing down.)

One of the “no-brainer free money” tips is not to accelerate up to red lights! (BIG pet peeve)


#15

With the tools commonly available how closely do you think you could measure fuel consumption? (robbibng the Myth Busters lab is not allowed).Myself I think plus/minus 2 mpg is about the closest you will get.


#16

With todays timed pulse fuel injection, it is possible for the on-board computer to compute exactly how much gas was burned for every mile traveled…


#17

I believe the most optimum speed a car needs to travel at is 0mph. With your car shut off, it’s not using ANY fuel, unless there’s a leak somewhere.


#18

Yeah, you’re burning no fuel…but you’re not going anywhere…so your calculated MPG is…

DANGER! Will Robinson! Dividing by zero!!! Worlds are imploding!!


#19

Hee, hee.


#20

So far noone has shown the optimum speed with any scientific basis to support it. Above 50 mph the wind resistance starts to increase exponentially. Mean a 2% increase in speed creates a 5% increase in wind resistance, or drag. There will be lots of graphs and formulas to figure the exact answer. My best guess is 40 mph as that puts most cars into the final overdrive gear and has relatively low wind resistance.

Driving at 40 mph might not be very practical, but if saving fuel is the most important goal - 40 mph it is.