OK I was playing this some more and its now unlocked for me. PLEASE NOTE this is glitched or was for me anyway as I had all the mods for all the cars. For some reason when I was using the porsche 918 and went over 50 miles the achievement popped! Really weird. So it seems you need do a minimum of 50 miles in each Velocity Car as well as get all the 3 mods.
slight correction: I was having the same problem with the achievement and found this and dreaded the thought of having to drive the fiesta for another 40 or so miles, good thing I didn't have to!
After you unlock all 3 mods for each of the 5 cars you just have to hit one of the 5, 10, 25, etc. mile marker challenges for only one of the cars. I got all of the mods unlocked then drove the audi up to 25 miles and the achievement unlocked and I know for a fact I don't even have half of 25 miles on a couple of the other cars.
