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6.25 Pounds Drag @ 60 mph = 1 Road-Horsepower

By definition,
1 HP = 33,000 foot-pounds work/minute
One mile = 5,280 feet
1 hour = 60 minutes
60 mph = 1 mile/minute
So divide 33,000 foot-pounds by 5,280 feet to get
6.25 mile-pounds/minute = 1 horsepower

In other words, if a side view mirror is resisted by exactly 6.25 pounds of wind resistance at 60 mph, it is consuming a steady 1 horsepower. Big truck mirrors consume considerably more than 1 horsepower at 60 mph.

Would you like to test how much horsepower specific attached devices like mirrors and exterior spotlights consume at various road speeds? Here's a test procedure that can give sufficiently accurate comparison results to guide decisions.

Attach a common drawer slide to a vehicle's free-air-flowing attachment surface. Lumberyards sell low-priced, sufficiently-strong drawer slides which have low friction. Their end facing the air flow will add little wind drag. If you'd like, you can even zero your spring scale at 60 mph with no test device attached. Your test slide may be attached and detached with removable Velcro adhesive strips. Attach objects you'd like to wind-drag test to the slide. Attach a thin cable or strong fishing line to the slide, pulling it forward about half way between its front and rear stops. Attach a spring scale positioned so you can read it from inside your vehicle while driving. We don't want the scale to chip paint, so tape on some bubble wrap to create a soft bumper. Secure your spring scale's front to a forward attachment point on your vehicle with more cable or strong fishing line.

Wind drag will push your test objects rearward. Your spring scale & cables will resist that rearward push with tension. You will read that tension from inside your vehicle by reading your spring scale. By testing at 60 mph, we can divide observed tension pounds by 6.25 pounds to reveal road horsepower consumed at 60 mph.

Wind resistance pounds increase by speed squared. Horsepower required balance against speed-squared wind resistance increases by speed cubed. Start with 60 mph horsepower values, then adjust for speed accordingly.

If you run some of these tests, please post your results telling exactly what device you tested, wind resistance you observed at 60 mph and your calculated horsepower estimate.

If you're not sure where actual 60 mph appears on your speedometer, adjust your cruise control until Interstate mile markers come up at exactly 60 second intervals. Survey teams set those mile markers pretty accurately.
John
 

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A nice little nugget.

MPG is like most other types of performance: There is no single silver bullet but lots of little golden BBs. Ask the drag racers and strong sled pullers. They will tell you there was no one thing (other than money, and even that won't do by itself) that made them go fast, but rather a collection of little things all working together.

What LoveLearn has done is lay the groundwork for a case for replacing side view mirrors with CCTV or something. I saw a SAE paper a few years back where GM engineers stated that 17% of the aerodynamic drag on a W-body (Grand Prix, etc) comes from the side mirrors. I used to own a Grand Prix and its mirrors were well-faired but they still accounted for one-sixth of the aero drag.

We need low-drag rear view systems.
 

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Zero-drag mirrors is virgin territory.

I'm thinking CCTV with only little blisters housing lipstick cameras.

Maybe quick-on mirrors to satify cops in states where that is an issue.
 

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Heavy Assault asked:
"Basically larger tires and a lift will decrease fuel economy?"

Dave says:
Yup. Big time. Let me count the ways.

1. Increased rolling resistance
2. Increased drag from the tires (width & lugs)
3. Increased rotational moment of inertia. Big wheels become big flywheels you have to accelerate from every stop.
4. Increased frontal area. The double boundary layer under the truck is the draggiest part of the vehicle. Every inch under the truck is worth two inches height. If you lift a typical 4x4 SRW truck 8" you have increased frontal area by 21%
 
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