So I have 86 z28 that was H vin base line. Automatic tires 215/65R15 stock. I bought the car 2 years ago from an older lady who’s had it since new. The front tires are correct size rears are 225/70R15. The speedo runs approx 7mph slow above 45. The gear in tranny is red 37 tooth. Will those way over size tires throw of speedo? Or what should I be looking at? I am in the process of looking at new tires in the correct overall diameter.

 

The tire size you have on the back won't throw the speedo off that much (1-2mph at MOST). It's possible someone has put the wrong gear(s) in. I think there is a sticky about this, and you can forum search it as well. I used the sticky to correct issues with my wife's speedo. It was off and I used the sticky to order the correct new plastic gear, but later ended up just having to order an Ebay kit with a bunch of different size gears and experiment until the speedo was accurate. These speedometers get old and worn and don't necessarily accurately conform to the correct gear using just a mathematical chart. Trial and error worked better for me. As long as the transmission shaft gear is about correct the other gear (the one the cable plugs into on the side of transmission) is pretty easy to swap in and out.

Edit: here is a link for example of an entire kit. You probably don't need this whole kit and could get by with a cheaper kit with fewer parts, just go over the sticky and search Ebay. I ended up needing a kit with all the driven gears (the one with the long nose that the cable plugs into) AND a new metal gear housing (the GM housing was not compatible with the replacement driven gears). I found each driven gear swap resulted in about a 2mph change in the speedo, but this would depend on which drive gear you have.

https://www.ebay.com/itm/GM-700R4-35...MAAOSwhgJZY6Jj