It is really a matter of what do you dim as off. Car manufactures usually make speedometers that show speed higher than the actual one (sometimes up to 10%, one of the reasons why you are not getting stoped going 5-10 mph above speed limit, is in fact you might be actually going just above or below one.). That means now with larger tires speedometer may show a correct speed. For example after going from 31 to 33" according to GPS my speed is only 2-3 miles above shown on the speedometer at 70-80 mph.
I believe that in your case by going from 235s to 31s difference in measurements of speedometer will be about 5% which translates into only 4 mph of difference at 80 mph (I hope you usually don't go much faster than that). How it translates into actual speed only comparison with something like GPS can tell, (or a cop with radar, for the sake of it). Anyway I wouldn't worry much about changing your speedometer gear just adjust you habits slightly.