I have oversize tyres because I have ground clearance problems with access to and from my home here In wales approximately a mile from the tarmac road and is all of road i also use off road biased tyres most of the year. I have gone up from 255/45R18 to 255/55/R18 109. My cruise control works fine with it though i tend only to use it in 20mph zones, Wales is festooned with them and can be difficult keep under 20mph otherwise and if one venture into North Wales the even 1mph can render one with a speeding ticket. Of course they have a greater circumference and will tend to rotate at a slower axial rate than the manufacturers fit but most speedometers overestimate a small amount ( France is considering Law to make overestimating speedometers mandatory) A Tyre Comparison site, many available online, will show the differences between tyre sizes by putting original size on one box and then the alternative choice on another, the site computes the differences and a suggested real speed also. though as I said few speedo are accurate except Police cars or used Police Cars ha ha. Also one can after fitting use ones GPS and speed electronic sighs but to be safe many have the speedo recalibrated. Easy and simple on the T6 but the T6.1 be aware as far as I know once recalibrated can not be re set back, though I heard somewhere someone may have found a way? As long as the difference in rotational speed is not large should not be a huge problem. My change appears to come out exact by using references but be aware that is it most people are lazy and travel with an indicated speed well over the correct speed limit and having large tyres on an uncalibrated speedo exasperates that so any likely fine will be higher also if speeding. So if it said 30, 20 or what ever I never exceed it on the speedo.
I use oversize tyres to clear opticals rocks, deep mud etc but also to reduce ground pressure off-road but it also dramatically reduces my tyre pressures. The tyre comparison site will also produce new tyre pressures which will be correct for almost any tyre size and is easy to do by filling in provided tables for each size manufacturers fit and any new size press and it computes for you. It is a legal requirement to have correct tyre pressures.
