Heresy! Smooth Wheel Covers- Myth Busted!!!

[COcyclist]

At least on the VW Golf

I have tried various forms of wheel covers for over 10 years on my diesel Golf 5 speed manual with no real improvement in highly mpg. I have had several entire tanks testing them where the mpg seems to go down. Wheel covers are one of the first mods many people try on this forum. There is a historical tradition of using Moon wheel covers on salt flats race cars and intuitively they “look” like they would work much better than the blender blade mess of most alloy rims. I recently purchased Julian Edgar’s book and he quotes several studies of airflow and drag on car wheels. These studies show that totally smooth wheel covers are not the lowest drag option in modern automobiles. This would fit with my personal experience and motivated me to do some further testing in an A-B-A or even A-B-A-C-D options. I will post my test data and observations in a following post.

[COcyclist]

The VW GTI alloy 9 spoke rim gave me the best mpg in 2 days of testing and previous extended road trips. The alloy rims alone were better than with flat coroplast wheel covers.

Edit- It turns out this statement may be slightly misleading. Read on

[Chonk_Master]

Which option worked best?

[COcyclist]

Originally Posted by Chonk_Master

Which option worked best?

The GTI alloy rims with summer tires. It turns out tire choice was more important than any wheel covering.

[cowmeat]

It turns out tire choice was more important than any wheel covering

That's pretty much a given

When I put the mirrored moon discs on my Insight I already had excellent LRR Bridgestone Potenzas tires on it, I did it mostly for the look - the moon discs did make the car look way cooler and they got random strangers to stop and ask about it, which in turn got me talking to people about ecomodding

In reality they also got me to work even harder at hypermiling - the car looked like it should do better with them on it so I drove it better if that makes sense

[redneck]

.

I’m curious.

Did you use the same rim and tire combination with and without a flat covering ?

Because if you didn’t, the difference noticed could have been from the overall weight of the combination used. Not the wheel covering.

Lighter wheels make a BIG difference.

Lighter wheels and a good LRR tires will make a even BIGGER difference.



>

[COcyclist]

Originally Posted by redneck

I’m curious. Did you use the same rim and tire combination with and without a flat covering?

The short answer is Yes. I tried multiple combinations and the best of all were the alloy rims bare followed by the same tire and rim with a flat coroplast cover which was consistently worse.

More to come.

Edit- If this were a newspaper I would be printing a retraction. I posted the quick reply to the question above before looking at my notes. As I was entering the test data last night I realized that the coroplast wheel covers edged out the bare alloys by 1 mpg.

[COcyclist]

Kudos to anyone who decides to do A-B-A testing. To do it right takes time and ideal test conditions to get good results. This testing was done over 2 days. I chose a long straight road with very little traffic and open fields on both sides. The runs were almost 2 miles in each direction at about 62 mph or 100 kph. Day one the temperature was 44 degrees with a 7 mph wind from the west. The road goes east west. I averaged the east and west runs together. I used cruise control and left the car running while I added or removed wheel covers and used the same start and stop points for all runs. I used the ScanGuage set to current MPG and reset it at the start point with the car up to speed and cruise resumed.

My A runs were with factory steel rims and the factory wheel covers with worn snow tires. The B runs were the same wheel and tire combination but with the wheel covers off.

A1 east/west average mpg- 54.7
B1 east/west average mpg- 57.85 (this was just the black steel rims)
B2 east/west average mpg- 53.6
A2 east/west average mpg- 55.85

I averaged all the A runs and all the B runs and got 55 mpg and a few decimal points for both. There seemed to be more variation with changing wind perhaps, than the variation of wheel covers on or off.

[Tahoe_Hybrid]

Originally Posted by COcyclist

The GTI alloy rims with summer tires. It turns out tire choice was more important than any wheel covering.

I put Eco tires on the drive wheel my MPG shot up on the same highway route it went from 27 to 31.2 15.55% increase

20 pounds less of rotating mass on both tires total

[COcyclist]

My Day One results were inconclusive so I decided to do another round of testing. Day 2 was 59 degrees with the wind averaging 7 mph but occasionally gusting to 10-15 mph (as per weather radio from the airport just north of the test road). I increased the speed for all runs to 65 mph to try to highlight aerodynamic differences.

A1 runs Day 2 east/west average mpg- 48.45
B1 runs Day 2 east/west average mpg- 48.6
A2 runs Day 2 east/west average mpg- 50.65

The testing was still somewhat inconclusive. The differences could have been from wind changes or other traffic on the road.

I went home and changed to my summer tires and rims. My summer tires are replacements of the stock Goodyears which are getting thin. They are mounted on VW Mk III GTI 9-spoke replica Minilite rims. I also have coroplast wheel covers held onto the spokes with zip ties. Again I left the car running while I removed or installed the wheel covers to keep the same cruise control setting for all these runs. The C runs are the naked alloy rims and the D runs have the smooth wheel covers on over them.

C1 runs east/west average mpg- 56.45
D1 runs east/west average mpg- 54.1 (coroplast covers on)
D2 runs east/west average mpg- 56.1
C2 runs east/west average mpg- 51.75

On my last run of the day my west bound data may have been corrupted. I encountered some slower traffic and had to brake before the end of the run. I got only 47 mpg for that leg. I averaged the C runs with the spinning alloy spokes open to the air and got 54.1 mpg. I averaged the D runs with smooth wheel covers and got 55.1 mpg. There is not a huge difference but the advantage goes to the alloy rims with the coroplast cover by 1 mpg. I have some thoughts on this but will update more tomorrow.

The biggest difference was the change from snow tires on steel rims, covered or not, to lighter rims and summer tires- almost 5 mpg or roughly a 10% improvement!