"I M" <nowizer@webtv.net> wrote in message
news:18670-48E7E564-1044@storefull-3253.bay.webtv.net...
>
>
> does the EPA actually road drive cars to establish mileage
> estimates....or is it it done in the lab on a dyno?

It is done in the lab on a dyno. I doubt they are not making any
allowance for top up / top down. I assume they do the testing using
the CD for top up. See http://www.fueleconomy.gov/feg/how_tested.shtml
.. For the Mustang there are not separate listings for convertibles and
coupes. Toyota does list the Solara convertible separately from the
coupe, and the convertible gets 1 mpg lower highway mileage. I assume
this is a result of the convertibles heavier weight and higher CD.
Another example would be BMW, they list convertible and sedan models
separately. The 135i sedan gets 1 mpg better city mileage. This is
almost certainly because it is lighter.

Ed