is aero still important at 40mph
 

And what kind of mpg might I expect dropping all the way down to 40mph in say a pickup that currently gives me 14mpg at 70mph due to a taller than the roof of the pickup topper and a 100% flat back?

I hope to eventually aeromod things and posted once about building impromptu mods that wont pay for a single trip. So I decided that when I move things i'll just drive slow on country roads. But then realizing I usually have more time than money and don't have ALL that far to go rurally, why not just always drive really slow? Would there be much point aeromodding if I just start driving 40mph on the rural highways most of the time when hauling things in my pickup when i'm in no hurry?

I get 32 mpg in my boxy 2006 Scion xB @ 70 mph.
I get 41 mpg @ 60 mph.
I get 49 mpg @ 50 mph.
These are all multi-hundred mile drives setting the cruise control.
30 and 40 mph trips get 54 mpg or greater, per my Ultra-Gauge.

My wife's 2012 Elantra gets 29 mpg @ 80 mph, and 48 mpg @ 60 mph. Slowing down is the best thing you can do for fuel economy on long highway drives.

But if i'm slowed down, would going to the trouble and expense and potential ridicule of funny looking aero still make as much of a difference? :)

There are some routes/situations where I have no alternative but to drive fast like my daily commute, so I plan to aeromod my compact when I get a chance. When i'm hauling stuff I can go as slow as I like though because that's during downtime. Will a whale tail even help much at 40mph?

It's a commonly repeated myth that aerodynamics only "come into play" at highway speeds. (You often see people mentioning 60 mph in this myth.)

If you plug your vehicle stats into our aero calculator tool (in the tools section of the garage... link at the top of the page under the header image) you'll see that even at just ~40 mph, about half your fuel is being burned to overcome aero drag.

So while slowing down will definitely help most, ignoring easy aero mods is leaving money on the table.

My driving experience is that the "seat of the pants" feel regarding aerodynamic alterations/changes can be felt at 35 mph, and maybe as low as 30 mph.

If you are on a bicycle you can feel changes in aerodynamics at much lower speeds just by leaning forward or sitting upright.

Square your speed to get the percentage difference in aero drag.

30 MPH squared is 900
60 MPH squared is 3600

Doubling your speed quadruples the drag.

90% of the force required to pedal a bike at 30 MPH is aero drag.

regards
Mech

Well maybe I'm a good tipper and I want to reward the oil companies.
What's wrong with that? :cool:

I believe at the Reynolds numbers we're interested in the envelope is 25-250mph. But as stated, there is squaring involved.

Look at it this way: When you improve the aerodynamics, the faster you go the more you benefit. :thumbup:

Highway speed air resistance is only one part of the equation. If you had a 20 mph headwind when you are going 30 or 40 mph your air resistance is a whole lot more than what you see on your speedometer. Likewise if the wind is at your rear, air resistance is a little less than what you see.

Unfortunately if the wind is from any other direction areo mods become less effective and you will see variations in the theme based on the roads orientation to the wind direction.

I get a pretty consistent mpg in the 40s between 50 and 70 mph. The xB has the same engine and similar drivetrain (the Yaris has taller gearing) and is designed off the same platform as the Yaris, but less aerodynamic body. I'll try and record the exact numbers at speed, but I remember not dropping below 40 mpg until about 80 mph.