Turning the Prius into a Plug-in Hybrid
Additional Detail on the Michelin/PSI HY-LIGHT Prototype

On the Wings of a Beetle: ~60+ mpg

The wings.

Ernie Rogers, an aerospace engineer, set out on a mission “to promote fuel conservation and reduce greenhouse gas emissions by researching, developing and commercializing novel technologies that simply and cost-effectively improve the efficiency of cars and trucks.”

One of those technologies or approaches is the addition of a simple aerodynamic drag reducer. Adding such a device (picture to the right) and slightly oversized, low rolling-resistance Michelin Energy MXV4 S8 tires to his 2003 VW Beetle TDI diesel resulted in an average 58.8 mpg in extended, high-speed highway driving. The EPA rates the diesel Beetle at 49 mpg highway. Here are his latest specific results:

Fuel Consumption w/ Drag Reducer and Low Resistance Tires
DateMiles TraveledFuel (gal)MPGl/100 km
2 Nov 04 797.6 13.71 58.2 4.04
11 Nov 04 793 13.37 59.3 3.96
24 Nov 04 786.7 13.45 58.5 4.02
8 Dec 04 792.5 13.37 59.3 3.96

Gasolines have freezing points well below even the most severe winter conditions. Diesel fuels, on the other hand,  have both pour points and cloud points (the temperature at which a cloud or haze of wax crystals first appears and separates from the fuel) well within the range of cold temperatures at which they might be used. To guard against this, refiners change blending formulas between summer and winter, working from an ASTM specification for fuel composition based on seasons and latitudes. The cold weather fuels have a lower density and carry slightly less energy—hence winter fuel economy suffers slightly by comparison with a summer blend.

Because the results above are winter results, Rogers estimates he can improve that result by approximately 10% with a summer blend, up to some 64–65 mpg.

Adding the drag reducer lowered the Beetle’s coefficient of drag to about Cd=0.28 from Cd=0.38, and resulted in an increase in fuel economy by 5% to 8% depending on speed. The new tires kicked in another 6% improvement or so. (Rogers says that his baseline highway mileage was approximately 52.5 mpg—higher than EPA figures.)

The story is further evidence that there are numerous different and incremental approaches to reducing fuel consumption that, when aggregated in a vehicle, can provide substantive benefit. I’d say a 12% increase is pretty good. Not everyone can design and install their own drag reducer. But low rolling-resistance tires are definitely an option.




Did you factor in the change in tire size?


As in, what would the change in fuel efficiency have been had he not increased the tire size along with changing to a LRR tire? I don't think he made that calculation, but let’s see.


OK, rather than rephrase it, I’m just going to paste Ernie Rogers’ calculations below. Bottom line: he approximates that two-thirds of the improvement came from the larger diameter of the tire; one-third from the change in type of rubber.

The car was originally supplied with the following tires: Michelin Energy MXV4 Plus, size 205/55R-16 These tires had a diameter of 24.7 inches (new). I replaced these with a newer tire type: Michelin Energy MXV4 S8, size 205/60R-16. The new tires have a slightly higher profile--the diameter is 25.7 inches, about. So, the diameter was increased by one inch, in addition to having a newer rubber formulation.

The rolling resistance (RR) of a tire is directly related to the amount of fuel consumed because it represents the energy required from the engine and drive train:

RR = Wt * RRC (Car weight x rolling resistance coefficient)

Several important factors are lumped together in the rolling resistance coefficient (RRC). These include: tire width, tire profile, rubber softness, tire pressure, and tire diameter. There are other factors not associated with the tire such as temperature, road roughness, and suspension properties. Maybe the complexity of the RRC is a reason why tire manufacturers are reluctant to publish the number. Of all of these factors, one of the easiest to separate out is tire diameter. The RRC has an approximate inverse relationship to tire diameter:

RRC = k /d, ("k" is a constant, "d" is tire diameter)

So, we can use this relationship to ask, what would have been the mileage improvement by changing tires if the "k" is the same, and only "d" is changed. Let’s try dividing the old RRC by the new RRC—

RRC (old) / RRC (new) = 25.7 / 24.7 = 1.04 (change of 4%)

Remember that the improvement with the new tires was 6%. At least as an approximate answer, then, I would say that two-thirds of the mileage improvement was from the change in tire diameter and one-third was from change in tire type, assuming that the change in profile (shape) had little or no effect.

Ernie Rogers


As expected, mileage during this last summer (2005) averaged about 65 miles per gallon, when traveling at a normal highway speed of 65 miles per hour. There is a rule of thumb that a car will get 10% better mileage by slowing down 5 mph on the highway. Or, if you slow down 10 miles per hour, your mileage should improve by a whopping 20%. So, you can calculate how well the car would do at a different driving speed. Remember, when figuring speed, you should include any headwind.

In early September, I traveled from Utah to Wisconsin to attend the annual meeting of the TDI Club, an on-line group of VW diesel car owners. I wanted to see just how good my mileage could be-- I drove all the way to the meeting at 55 miles per hour. Early on Wednesday morning, I topped off my tank (filled with 5% biodiesel blend) at Park City, Utah, waved goodby to the attendants, and headed out determined to see just how far I could get. I was blessed by perfect weather, almost no wind, all the way.

I finally stopped for fuel about noon the next day at Davenport, Iowa, a one-tank trip of 1,209 miles. (All at 55 mph, remember. I was honked at four times.) I filled the tank, taking 15.5 gallons. I had obtained a record --for me-- 78 miles per gallon.

Ernie Rogers

Pete Mitchell

Great mileage. I too am looking to improve my mileage by all and any means..

One thing though, also being an engineer, I must ask the following:

Have you taken into account the fact that your new tires will make 811 revs per mile, while the old ones made 835 revs/mile?
Did you also change your speedometer gear to account for the difference in the outer circumference in the swapped tires?
If not your speedometer, and odometer will be off by 3-5% That will translate into error in your calcs.

Is it possible for you to recalc your results to account for the larger tires used?

BUT. Great work otherwise.

I am performing similar tests with my wife's 93 Civic. The change which has made the most difference for my gas powered car(as opposed to your diesel) is the addition of acetone to my fuel tank. The acetone decreases the surface tension of the fuel thereby decreasing the diameter of the atomized droplets sprayed by the fuel injectors. The smaller the droplets, the more surface area of fuel ignited, and therefore more efficiency. I have see an increase of 34 to 37 mpg in long drives to Maine from Connecticut at 65-67 mph. Once I have organized my data as well as you have yours, I will post it as well.



John W.

Yes, great work with the VW. Pete, thanks for bringing up Acetone: I was very interested in hearing of anyone else's experience with Acetone, whether good or bad. And I thought you could put acetone in diesel fuel too. That would give your Beetle better mileage too.

I looked for another article on acetone in GCC but couldn't find one--if there is can someone point the way?

Pete Mitchell

No problen John, I have been experimenting with acetone for a while now and have found reason to use it in gas powered cars. I have not been able to try it in diesel cars. I have only found a 10-15% improvement.
The info I have found on-line is below. Generally the info expressed in those websites are consistent.


And remember the acetone added to your tank (6 oz/ 20 gal) is at roughly 0.2% concentration in gasoline, and gasoline is made up of 50+ different solvent additives at higher concentrations.



i have purchased a 2001 diesel & very interested in the biofuel, i don't know what i need to check before purchasing. when you talk of difference in summer & winter, is there a difference in the fuel or do i need to add additive. thanks


Now make that spoiler thing out of clear lexan so u dont need to see its ugly ass

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