First question I have is why does my speedometer not start reading until after I get over 15 mph? Reads fine after that and I can slow down below fifteen and it reads fine but if I stop I have to get over 15 for it to start reading again. Second question is about putting larger tires on. Is there a way to recalibrate your Speedo with the new tires? Any help would be appreciated. thanks

both are the speedo gear (at least likely on the sketchy speedo).

 

theres a small plastic gear in your transmission that hooks to the speedo with a cable. its probably worn,slipping, or has fluid and oil in it which is messing up your speedo.

 

if you take it to a speedo shop they will calibrate with the larger tires and put a new speedo gear in to make your spedometer accurate. might fix 2 birds.

 

that said you really have to go waaaayyy larger before it really matters. my tires are 3 inches bigger diameter and it puts my speedo maybe 5 miles off at 55.

If you have to replace a speedometer gear, the vehicle is most likely totalled before you drive into the shop. They will probably want to get paid in full, up front. But a used one at the wrecking yard and replace it yourself.