Using my mental calculator and rough numbers...
The difference in tires is a factor of 1.07, so divide your current gear ratio by 1.07 and you should have your new "effective" gear ratio.
meaning a 3.73 with 28 inch tires will now act like a car with 26 tires and a 3.50 ratio etc..
Slower? Maybe, it's just like using the next number lower gear ratio I guess.