A large tire will make the speedo read too low. I have a speedo adapter that is designed to reduce the speed and would be used if the speedo was reading too high, not too low. You can find different ones on eBay. Mine is by Stuart Warner and stamped 0.73333 so it reduces speedo reading by 26%. I'm not sure, but it sounds like your speedo reads too low. That's what my mini truck does because I'm running larger diameter tires than stock.
