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MetroMPG 11-28-2007 09:55 AM

Side mirror drag & effect on fuel economy - quantified
(Originally written August 11/06)

You often hear people talking about removing side mirrors to reduce drag. Is it effective or worthwhile? That's what this thread is about.

Still that thread seemed to run out of steam. I'd like this one to perhaps lead to more concrete numbers, maybe encourage an experiment or 2 to confirm...)
  • Both the Civic VX and Metro XFi eschewed outside passenger side mirrors. Cost savings could have easily been the reason on both cars (mirrors on both sides weren't as commonplace in the 80's and early 90's as they are now).
  • Phil Knox ran his aero-modded CRX at Bonneville, and says removing his side mirrors made no measurable difference in his top speed (radar-measurement)
    Yahoo! Groups
Yet so many aero concept cars omit the mirrors. For example...

GM's PNGV concept car


Instead of outside side mirrors, the car uses tiny cameras — a set-up similar to that seen on the Cadillac Evoq concept car unveiled at the 1999 North American International Auto Show. "We knew we couldn't afford the 30 counts of drag, so we went with camera mirrors," said George Claypole, Vehicle Integration Engineer for GM's Advanced Technology Vehicles. - Automotive Engineering International Online
So there we have mirror drag quantified: "30 counts of drag" - which if I'm not mistaken means Cd ".03". Seems overly large to me, but there it is.

Anyone have their Cd formula handy for their vehicle want to translate that to FE savings at various speeds?

MetroMPG 11-28-2007 09:57 AM

From the Chalmers University site:


According the the following article, the rear view mirrors contribute 3% to 6% of the total vehicle drag.

The following article also has a lot of basic information about drag reduction in cars.

.03-.06, is 3-6% of total drag.

MetroMPG 11-28-2007 09:58 AM

According to those two aero sources, it's a pretty significan effect (at least in my books).

3 - 6% of total Cd for a typical car is low-hanging fruit, ripe for the picking.

MetroMPG 11-28-2007 09:59 AM


It has been estimated that a 10 percent reduction in Cd can lead to a 5 - 6 percent improvement in fuel economy at highway speeds and a 1 - 2 percent improvement at urban speeds. (source: llon%22&hl=en&ct=clnk&cd=2)
Though: the base Cd isn't mentioned, and highway speeds aren't defined.

MetroMPG 11-28-2007 09:59 AM

If it turns out it's really worth taking mirrors off, I may go for a combination approach...

- Probably a set of folding mirrors so I can retain full mirrors for city driving (my current pair are fixed, and I have a set of folders from the red Forkenswift)

- with one of those round convex mirrors stuck on the aft end of the driver's side mirror for use when folded.

- Probably an addition to the inside rear view mirror

- and more shoulder checks for the passenger side

MetroMPG 11-28-2007 09:59 AM

I de-passenger side mirrored the Flea this afternoon, and picked up the folding driver's side mirror from the red Forkenswift. It's not a direct swap - I'm going to have to re-engineer things a little.

I would like to do a SG1 mirrors vs no mirrors experiment on my test course, but I have other things I still want to test too, and only so much time.

MetroMPG 11-28-2007 10:00 AM

Using the aero/rr web tool I made based on Dan's spreadsheet, my car theoretically gets 56.7 mpg (US) @55 mph (Cd: 0.34, frontal area sq.M: 1.858, @ 25C, default fuel & engine eff. values). That's actually not far off my actual FE at that steady state speed.

Removing one mirror: Theoretically, using the more conservative GM numbers, taking off a mirror reduces the Cd by 0.015 and the frontal area by about 0.0375 sq. M.

That improves my 55 mph highway FE to 59.64 mpg (US), or +2.94 mpg (US) / +5.19%.

That's a gigantic improvement in my books, if it's true.

EDIT- I mistakenly used the least conservative numbers. The Chalmers report has the more conservative range. So I'm guessing the actual effect will be less than 5.19%

MetroMPG 11-28-2007 10:01 AM

More tidbits from the Chalmers PDF:

Other notables from the PDF (it's definitely worth a read for anyone who hasn't seen it yet):
  • "A very well designed mirror increases the drag of a car of approximately ΔCd=0.012. But this value can reach 0.025 to 0.030 for the worth [worst] designs. Actually it seems reasonable to think that the average value for ΔCd is of around 0.015 (ref. 9)."
  • "...[the mirror's] combination with the A-pillar contributes to the creation of trailing vortices at the side front of the car, which are highly drag-consuming."

MetroMPG 11-28-2007 10:01 AM

Undertook a little project this afternoon: replaced my stock non-folding but otherwise aerodynamically shaped driver's side mirror with a folding but otherwise not very aerodynamic mirror from the red Forkenswift car.

Above image: before - the stock mirror. (click to zoom)

Above: after - the folding mirror. It took a little cutting and drilling to make it fit, and I had to cut a piece of black plastic (left over krazy karpet) to fill the gap since the mirror base is a different shape than the OEM mirror). But all the modifications are hidden & reversible. Should I want to revert to original condition, I'll be able to no problem.

Above: it became clear pretty quickly that the folded mirror might present less area to the wind, but the open hinge was an aerodynamic disaster. So I drilled a hole on the hinge to hold a shaped gap filler made from a piece of sheetmetal contoured with bondo. A "pin" (bolt) fits in the new hole on the hinge. I can easily install/remove this from the mirror while sitting in the car.

The plan is to fold the mirror for highway use. I'm going to stick half of one of those round convex mirrors to the trailing edge of the folded mirror, so I'll still have some functionality.

Is it better than the stock mirror? Only if I fold it - it's clearly less aerodynamic than the stock mirror was.

Am I crazy? The evidence is mounting, isn't it.

Also, I have no way to definitively say that folded, this mirror will create less drag than the stock mirror. There's no question frontal area is reduced. But it will create its own unique turbulence, and I won't pretend to know that it will automatically be less drag inducing than the stock turbulence. I'm just guessing that it's going to be better.

MetroMPG 11-28-2007 10:02 AM

While on the side-view mirror topic, we all know how to set them, right? No, probably not.

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