We propose a framework to learn a conditional diffusion model for high-performance vehicle control using an unlabelled dataset containing trajectories from distinct vehicles in different environments. By conditioning the generation process on online measurements, we integrate the diffusion model into a real-time model predictive control framework for driving at the limits, and test it on a Toyota GR Supra and Lexus LC 500 in various environments. We show that the model can adapt on the fly to a given vehicle and environment, paving the way towards a general, reliable method for autonomous driving at the limits of handling.