the stock tires on my 2000 suburban 1500 4wd were 265/75 on 16" rims
per the spare tire and metal plate inside the door.

10 years ago a tire shop replaced the 265/75's with 245/75's.
Recently had a flat tire and noticed the spare 265/75 was 1' taller than other tires.

checking all the tire sites they show stock tires for 2000 suburban 1500 4wd as
245/75 or 265/70. No one shows the 265/75's I had.

So I am a little confused ... it appears I may have been driving on the
wrong size tires for the last 10 years, my speedometer is probably off
and I have a spare that is too big.

Any one ever heard of this problem ??

 

Yes. I like the 265/75 tire size, ran it on my '97 Suburban 2500 diesel. 245's give better mpg's, but I liked the rugged look, traction, and lower highway rpm on the 265's. Run the tire size you like, easy to calculate the speedo error and just keep it in mind when you are driving.

 

Specs give your OEM overall diameter to be between 30.5" & 30.6".
Variations in Tire Manufacturers castings, cause this, and is inconsequential as far as Speedometer Reading. Additionally, one will find that his spare is of a different size, but, if OEM, the diameter will be near the same as what is on the Placard.

Quite frequently, one will find that a previous owner has changed sizes, and frequently doing more harm than good. One of the most common mistakes, is not increasing the Rim Width, when a wider Tire is installed. Breaking, Power and Control issues arise from this as well.

GM puts a lot of thought and testing into what size works best for your model, and when changing sizes, one need to do considerable research to achieve the best results.

My recommendation is, has and always will be, that one abides by the GM recommendation when replacing.

 

Definitely good advice, but then again 10's of thousands of truck and SUV owners have changed their tires sizes. You can identify the rims you have and look at specs of the tires you have, or are looking to buy, to see if the rims are in the right size range for those tires.

Your door placard is the stock tire size, so 265/75 is what your speedo and odometer are calibrated to. Running the 245's will make your speedo read high and odometer spin more miles than are actually traveled. If you switch to the 265/75 size I think you'll like most things about them.