Speedometer way off

I suppose that you could get a gear to re-calibrate your speedo if they make one with twice as many teeth as the one you have. Otherwise just get a 87-89 140mph speedo. You can get the whole cluster if you like or you could just get the speedo and swap it in. The 90-93 have different cuts at the bottom of the gauges and I actually think they will work too but the 87-89 won't work in the 90-93. I'd have to check but I've done a bit of swapping in the past.
 
Yes the bottom is different, the ones on the 90-93 have a bridge type gap made for the steering column. What it looks like is i'm going to have to get a new gauge cluster. I might just go with one of the kits from Florida50.com unless i can find a cheap used one, because i'm not paying 350 for a new gauge cluster thats same as stock.
 
well if the old face is still on underneath, then most likely the person left all of them on. You might be able to remove all the overlays and just go back to the stock black gauges for the time being, if the 85 mph speedo is something you can live with.
 
You can get your speedometer recalibrated somewhere if you want to keep the overlay on it. If not, you can either buy a whole 1989 cluster on eBay or you can buy just the 140mph speedo on www.50resto.com

I suggest finding a whole cluster on eBay though because the speedo on 5.0 Resto sells for $160.