Careful: while the number of miles driven correlates with GDP, it doesn't mean that a reduction in one will result in a reduction in the latter.

While a reduction in GDP would mean less disposable income, it's easy to reason how that would result in less miles driven.  However, since some of the driving is for pleasure, it doesn't seem unreasonable that one could drop the amount of miles without directly effecting GDP.

Unless you want to do a write up showing that GDP is directly related to miles driven, as Stuart's done this excellent example of the correlation.

Even frivolous driving gives the economy a boost. You need to service your car, buy new tires etc much more often if you drive more. Also, road-side restaurants get better business if people drive more. Cars will also wear down earlier, forcing the purchase of new cars more often if people drive for fun, and not just for profit.