The new Soliad Wendover is the best car to modify, tune and upgrade. But there is one problem. The car oversteers at high speeds so much that it's undriveable. There is a lip-type spoiler, but I think it's not enough. A wing would be perfect, because there is a widebody bumper with a splitter that gives so much downforce to the front, but not so much in the back. The part selection window problem got fixed in days, thanks for that. Awesome game, awesome devs...
There is a wing, take a look at the hillclimb configuration, it's the same wing that every hillclimb car has.