On our 265/70 r16 we were 7.8 % off
On our 275/65 r18 we were 9.9 % off
Now we are on 275/70 R18 and are running 12.1 % off

These numbers were calculated using 10mile road tests, over several years and different tires. There are a few shops out there that can test and build a speedo adjuster (electronic)