I think a lot of folks discount rider weight a bit. I used to get 5k out of GP4000's regularly. This was riding them down to where the cords started showing through and I never road on dirt roads. I weigh 135-140.

I've switched to the GP5000's and am getting less mileage. Under 3.5k which is a huge decrease, but I have been riding dirt roads a bit. I wasn't able to run them to the cords due to an abundance of little cuts in the tread that got me a bit worried.