1997 gmc speedo recalibration

[68c10owner]

My 2wd truck had 31/10.5/15`s on it when I bought it. I`m about to replace the tires and was thinking of going back to the stock 235/75/15 to get better mileage but I know they are shorter and will throw the speedo off. I googled it and read something about resoldering stuff in the VSSB but there has to be a easier way. I dont have alot of tools available and dont have money for a scanner. I have a $50 unit to check codes but I dont believe it will recalibrate the speedo. What can I do? will driving it with the speedo off effect anything? I mean will the computer think its going a certain speed and do something it shouldnt? I dont know much about these newer trucks so I`m still learning.