Hi all, just wanted some advice I have 2011 Audi A7 3.0 tdi Quattro sportback and have owned it for about 2 months, I just purchased a set of used tyres on Saturday and ever since I've had all sorts of messages come up on my dashboard but the most serious one was transmission based. The messages said no reverse gear but still continue to drive and I made it home, but the gears that would only engage were 1st,3rd & 6th, I used carly to clear all the fault codes and now the car drives ok-ish but nowhere near how it used to perform, all the gears are now engaging but when bringing the car to a fault, it almost tries to bolt forward which is obviously unnerving. I've noticed the tyres fitted to my car are slightly bigger than what was on the car and I noticed the car started acting peculiarly after the tyres were fitted......would this be  causing the gearbox issues or does tyre size bot matter at all....please help as I'm not a mechanic so don't know which way to go in terms of getting the car back to good health, if it's gearbox then is it cheaper to get it repaired or replaced.

Welcome Darren,

Sorry, but oh blinking heck - to be polite- I don’t know anyone who would fit part worn tyres to such a performance vehicle, but I do now! 
Basics first:- 

Do all four tyres have near enough matching tread depths? 
Are all four tyres the same brand and tread pattern, and load and speed ratings? 
Kind regards,

Gareth. 

Let's be clear about one thing straight away. The change of tyres has nothing to do with the error messages and gear changes. Those are faults in the car which could be rather expensive to fix.

Fitting secondhand tyres to a performance car like yours is frankly foolish. Add the cost of fitting new tyres to the cost of fixing the car and you are looking at an enormous repair bill. Is that something you are ready for?