90 metro speedometer/milage

I was wondering what I should do about my speedometer. I have 13 in tire and I think original speedogear. I believe the car came with 12 inch tires? Anyway When I drive to work I know its 38.5 miles. my geo convertable shows I only went 35 miles. Whats the answer? get 12 in wheels or have speedogear changed out. I can change the gear at end of cable but the gear in tranny maybe difficult. tks moe

Open your door check the door frame there should be a sticker w/the tire size and other info. Tire size on my 91 Geo it requires P145/80R12. There are shops around that will reset your speedometer. Or drive it until you need new tires and purchase the correct size.