Asphalt Overdrive

For this achievement you need to upgrade all 3 stats (Engine, acceleration and Nitro) of a single car to level 5. I think prices differ per car, but the most expensive upgrades are the engine upgrades. For one of my first cars the last levels of nitro and acceleration costed me 4,500 and 4,000 credits, whilst the Engine upgrades costed me at least double as much.
I got this achievement somewhere near the end of episode 1. You will already have upgraded your car a few times, to be able to play some of the Getaway missions. Some of those require a specific "car stat level".