Forget a diesel if someone is interested in the sportscar aspect, it'd throw off the weight distribution. Just swap one of those cylinder deactivation head setups on and manually actuate it for good ~40-50mpg@55-65mph. Anyhoo, the only danger a driver going 10mph under the limit presents is to the other drivers who shouldn't be driving in the first place. It's not as if noticing another vehicle is driving slower is rocket surgery, and if someone can't deal with another car going ten mph below the limit, what happens when there's a wreck up ahead and they have to deal with a few vehicles going 55-70mph under the limit?
Edit- I suppose
this is what can happen.