tire size and trip meter computation

[impulzz]

my 2nd hand vitara came with mikey thompson tires 225/75r15

when i read the owner's manual OEM tires are 205/75r15

according to their websites

1. the Mikey's are 28.2" diameter = 88.59" circumference

2. the bridgestone OEM tires 205/75r15 has 27.1" diameter = 85.13" circumference

my friend said that my trip meter is not accurate anymore since my current tires are bigger. so when i compute for my fuel consumption, i'm getting a false answer.

how do i compute for the difference in kilometer reading? so i can compute for a more accurate km/L reading.

my last computation was 222.9kms/39.8L = 5.6kms/L