[aerohead]

For those who have,or will be doing aero mods,and want to know what effect changes might make,or have already seen mpg gains from mods,and would like to know what their new Cd is,the following is a form of "Shade-Tree Windtunnel" to help with the quanta.

The source is SAE Paper#649A,"Aerodynamics for Body Engineers,"by Kent B.Kelly and Harry J.Holcombe,General Motors Styling Staff,General Motors Corp.,presented,January 1963 at the Automotive Engineering Congress.

"--- for our typical passenger vehicle------,a 10% reduction in aerodynamic drag makes a 5% reduction in fuel consumption possible at 55 mph,and a 6% reduction of fuel consumption possible at at(sic) 70 mph."

The relationship is very straight forward and can be used in forward or reverse.One major stipulation applies to its use.

Values vary in an additive, not a cumulative fashion.I'll explain,using my T-100 as an illustration.

From my vehicle log-book I determine that over its life in standard form,it returned 25-mpg,at 70 mph.

Suppose I want to know what the truck would do at 70-mph,with a 10% drag reduction,all else remaining constant.

From the relationship,I multiply the original drag coefficient,Cd0.44 by 90%,or 0.90 to get Cd 0.396( 10% less than original ).

For the mpg portion,the 10% drag reduction should net me a 6% improvement,so multiplying the original 25-mpg by 1.06 yields 26.5-mpg,or,a gain of 1.5 mpg with the 10% drag reduction.

From my 5,000+ mile trek out to Battle Mountain,NV,for the Human-Powered Vehicle competition in 2005,for the 70-mph portion of the trip,the T-100 returned an average 32.035-mpg over 1,300+ miles of 70-mph travel.

Noodling with the formula,I divide 32.035 (the new mpg) by 25 ( the original mpg ),which shows a 28.14% increase in mpg.

At 6% delta-mpg per 10% delta Cd, dividing 28.14% by 6% yields a change in Cd of 46.9% necessary to explain the mpg increase.

With drag reduced by 46.9%,only 53.1% of the original drag remains.

Multiplying 0.44 ( original Cd ) by 0.531 (53.1%),yields 0.2336 for the new drag coefficient.

So far I haven't needed to do a coastdown or hire out the use of a full-scale windtunnel.

You can use the formula serially,adding 6% mpg( 1.5 mpg in my case ) for every 10% drag reduction.Note: the percentage changes must be made from the base vehicle baseline data,not the accumulating figures.

So to figure mpg at a 20% delta-Cd I reduce the original Cd 0.44 by 20%,not the Cd 0.396 arrived at with the 10% change.

You can quickly build a table,showing the original Cd,and then increments of it at 90%,80%,70%,and so on,until you reach a realistic minimum,say around Cd 0.12-0.15.Across from it,you list the original MPG,increasing 6% with each "jump",as you descend down the column.

With a pocket calculator and a few minutes,you can calculate the entire potential for your vehicle,and also work backwards from your Scan-Gauge or Tank Mileage data to see what you've accomplished.

That's about all there is to it,and you never had to go to Lockheed or Pininfarina.

In the days when I did Bonneville and Chrysler Proving Grounds,you could telephone Glen Sharpf at G.M.'s Aero Lab and bounce numbers off him.He concurred with my appraisal of the CRX mods and the numbers I generated using these relationships,and said the delta-Cd/Delta-mpg ratios are golden,and can be used with a high degree of confidence.

That's it!