As Bob mentions, there has been much discussion on the Forum about SU carbs. I have used both 1" and 1 1/8" on my 1931 saloon and have found the 1 1/8" to be best. The 1 1/4" is probably too large for a road going car. The beauty of the SU is that it is easily adjustable and bits are interchangeable and readily available. You also have a big needle constantly poking through the jet so you are unlikely to suffer from blocked jets. With my set-up the car will do 40 to 45 mpg driven quite hard up hill and down dale.