^^ Lets assume OEM wheels are 185-65-R14
14" Rim: 35.56 cm
Side wall: 18.5 x 0.65 = 12.25 x 2 = 24.05cm
Wheel: 35.56 + 24.05 = 59.61 cm
Circumference: 3.14 x 59.61 = 187.175 cm = 1.872m
Lets suppose the car originally does 10km/l, but with the new wheel W, it only shows 8km/l on odo.
Hence, the old setup: 1.872 meter x 10 km/l should be equal to the new setup: W meter x 8 km/l.
so
1.872 x 10 = W x 8
W = 18.72/8
W = 2.34m
Circumference of new wheel = 234 cm
Diameter of new wheel = 234/3.14 = 74.5 cm
Diameter of new wheel = 29.3 inch
The new tyre has to be something like:
315-50 R17 or 325-35 R20
Possible?
However, if he's running rather modest:
205-50 R17
Rim size: 43.18cm
Tyre walls: 20.5 cm
Wheel size: 43.18 + 20.5 = 63.68 cm
Circumference = 3.14 x 63.68 = 199.955 cm = 2m
Using old equation, with new variable R i.e odo reading
1.872 x 10 = 2 x R
R = 18.72/2
R = 9.36 km
So if he's using 205-50-R17 his odo would only show 9.36 km for every 10 km