Sure, the diameter is fine. But my 255's only just fit in width-ways. 305's will probably rub dangerously if they can turn at all.
Terry is correct, while the diameter is close to factory, the width is way too large and will not fit on a stock Discovery.
305s will be 12" wide which means it will definitely rub the front strut/brake housing as well as the rear wheel inside covers.
I know, because I went up in width from stock 245 (9") to winter 255 (10") increasing width by 1" and it is very close to rubbing.
My tire shop pre-fitted the 255s on one wheel and tested it around all 4 rotors - finding they just cleared - even turning the steering wheel all the way left and right. The diameter remained the same because the aspect ratio went down from 60 to 55.
The only way to put on 12" wide tires would be to lift the car, cut out the wheel well areas, and have it look like a F350 lift kit truck from the good ole boys....
And why would you put 22" wheels on a Discovery ?
Gang looks ?
If you want 22" on a Land Rover, look at getting the full sized Discovery/LR4 or the Range Rovers - they have much larger wheel wells and look better with the large rims.