I just purchased a 1986 K5. It has 33in tires and 373 gears.  When I am going 50mph the speedo reads 65. Usually with bigger tires the speed reads slower! Why is mine reading faster and what can I do to correct this problem.  I appreciate the help  

Have you changed axle gears or speed or Tcase? It is controlled by a gear in Tcase that can have from 34 to 46 teeth. I have several here that I have used over the years to tweak my 89's speedo.  

I am not sure what the previous owner changed or not changed.  I am wondering if there was a speedo adapter on the transfer case and he forgot to put it on when he built the truck?  

If I put a adapter on the transfer case instead of changing the gear inside would this correct the problem  

There is absolutely NO NEED to use a adapter. Gears are available for this from GM and I bought one last year for 6 bucks. You simply pull existing gear and count teeth and go from there. You can also have speed itself re-calibrated if need be though this is rarely needed. That body style does not use a electronic speedo.  

Where can u get these gears for the transfer case and how easy are these to change?  

At any GM dealer parts counter. Just follow speedo cable to Tcase and where it screws on there is a plug held in with one bolt and a little clamp retian plug that cable is attached to. Remove bolt and pull cable and plug out and count teeth on gear and get back with me.(plug is easy to remove and replace though it might be stuck a bit on first removal after many years)  

What would I need to do after I counted the teeth? Like can you run the whole process by me please. I plan on fixing the speedometer in my truck soon.  

If you tell me the number of teeth on it I will tell you were you are at and what you need to do.  

I just found out my vehicle was made for Canada! I think someone put in a american speedo and thats why its running 20+% fast.

Amazing it was in Canada from 1886-1996 and there is no rust,  the bottom is still like new.  

the speedo gears are color coded( brown,blue,red etc.) and if your vin # starts w/a 2 then its canadian  

My Vin# 1G8EK18BLXGF177676.  Before I bought it  I did a Car fax and there was no information until it was 8 years old with 58,000 miles at a car auction in California and then came Utah then  florida and now SC.
I got the original invoice from GM and it said it was built in Michigan and shipped directly to a dealer in Canada.  See the more I dig the more confusing it gets??  

MISTAKE: THERE IS NO B IN THE VIN  

It is and american truck w/the "1" meaning american made and If you had a "2" as the beginning it would be canadian ay! and if it had "3" as the beginning it would be mexican si.  

I have the original invoice from GM that says it was built in Michigan and was sent to a Dealer in canada.  If you give me your email I can send it to you. Thanks Phil