Let the original tire be 100cm circumference and the new one be 120cm circumference
(that is 20% =bigger)
Say you want to go 1200cm
that would be 120 rotations for the original one and 100 rotations for the new one.
\(\frac{120-100}{120}*100\% =\frac{200}{12} \%=16.\dot6\%\)
The number of tire rotations needed has decreased by 16 and 2/3 %
------------------
Explaining it a different way :)
If the circumference has increased by 20% i.e. it is now 120% of the original
then
the gound covered by the tire will be 120%
so the rtations needed will be 100%/120% = 10/12 = 83.3% of the original.
which is 100-83.3 = 16.6% less than the original. (that is .6repeater)