11's on 235/60/15 drag radials

I recently bought a 73 dart and I am trying to get it ready to drag race this spring. The previous owner ran 12.6's with 235/60/15 M/T drag radials. I'm curious how fast anyone else has run on a tire this skinny. His 60' times seemed pretty bad ranging from 2 flat to a best of 1.8. How much better would a 275/60/15 be. I'm looking into better weight transfer with some adjustable shocks and caltracks. Any advice would be helpful
I ran 12.82 at 108,60ft.was 1.8,yes it could be better.Your sayin that,s pretty bad?What motor are you running?What combo?Gears?Iwas running on 275/50/15 M/T.