In the ongoing debate over whether the Detroit Belle Isle Grand Prix's organizers should be allowed to convert the city's one-of-a-kind island park into a private raceway, gearheads typically fall back to one point — the annual event provides a huge economic boost to the city.
This year, billionaire Grand Prix founder Roger Penske is claiming that his 2017 race generated $58 million in spending in the local economy, up $11 million from 2014.…