What car do you drive? Is it FWD, AWD or RWD? Are your tyres staggered (ie. BMW M Sport cars)

For normal FWD cars, Your options from most expensive to cheapest, assuming your 80% is indeed only 80% used.

Option 1 ) Buy 2 tyres and fit them on the front
Option 2 ) Buy 1 new tyre and fit them on the back

And if you are really cheap, just use the spare tyre on the back. Buy a cheap second hand tyre for the spare. But unless you are a really cheap bastard, quite likely 1 and 2 are ok.