i have a 2001ml 320 with 16" rims n tires on it and my son keep asking me to put some 20" or so on my ml i was told if i put 20" rims n tires on it it might throw off my computer is this true

Not sure on your laws in the US regarding tyre specs, however here in Australia it is illegal to decrease your overall diameter by more than 26mm and can only increase by 15mm to the maximum Optional tyre for that vehicle, to retain compliance and a reasonably accurate speedo. If for instance you have 225/60 X 16's fitted your rolling diameter would be 676mm and you wanted an equivelent diameter in a 20" rim, you would need to fit 225x 40 to give you a rolling dia of 687mm. Basically the larger the rim, the lower the profile tyre you must fit, in this example from a 60 seiries down to a 40 series tyre to compensate for the larger dia rim. If you've never driven on 40 series tyres, it is an experience, terrific on a track that is flat and free of potholes, rocks and roadkill. No roll, no bumps to absorb. Drive these suckers on a normal road and you will feel every stone. Hit a decent pothole and look at a new rim. Hope that helps.