New tyres are different from old tyres, so fitting ANY pair of new tyres will introduce a front-back difference.

THE HORROR

SOME people (not all of whom work for tyre companies) will conclude that you must always fit a full set of new tyres. These people presumably have a lot of money.

But, but...front tyres wear differently from rear tyres, so a little time will introduce a front-back difference...THE HORROR...therefore you should replace ALL your tyres after each use.

How would you feel if your penny-pinching caused the death of a blue-eyed child?

It'll be fine

I agree with GB - the data from the car's handbook and/or on the door plate will indicate what tyres, both the size and the loading, are acceptable for the car. Just because an XL tyre was fitted as the OEM does NOT mean a non-XL tyre is not allowed.

My Mazda 3 was fitted with 205/55 R16 91V tyres because it was a TS2 model, but the almost identical (with the same engine, suspension and brakes) TS model mostly comes with 195/65 R15 91H tyres but both will also accept the other, plus the V rated R15s which are, in all-season tyres at least, XL rated 95 loading tyres, as are the 16in equivalents.

I think that my car (according to the handbook) also can accept equivalent lower profile 17in tyres, which, if I recall correctly, have a load index of 89, which are lower than the 91 of the OEMs. I changed out my OEM 16in alloys for 15in ones (corrosion causing leaks in the tyres) and added the matching 195/65 R15 91H tyres which have a lower speed rating than the OEMs, but again is fine because the handbook and door plate say they are acceptable.

It was the same on my old K11 Nissan Micra 1.0 S, which came with 175/60 R13 77H tyres, whereas most models with that engine came with 155/70 R13 75T, but were perfectly acceptable by the plate/handbook, especially as the car could not officially crack the 100 mark (T = 115mph).