http://www.popularmechanics.com/cars/how-to/a3121/6-common-tire-myths-debunked-10031440/
http://www.theaa.com/motoring_advice/safety/car-tyres.html
https://www.kwik-fit.com/tyres/information/tyre-rotation
http://kumhotyre.co.uk/kumho-news/should-you-fit-new-tyres-to-the-front-or-rear/
....and pretty much every single reputable source that I could find.
Well done chadr, I'll bow graciously to the experts on this one, far be it from me to argue with them.
However, there's one overriding nonsense that I think shouldn't be overlooked.
IIRC, the OP suggested that the pair of part worn tyres he had were in good serviceable condition, well I assumed that anyway when I gave my advice.
If any tyre is worn/old enough to be prone to aquaplaning, blow-out, high propensity to puncture due to wear/age, or whatever, it shouldn't be on the car in the first place.
That means that all other things being equal, the only difference between the new and the part worn tyres would be tread depth.
On my 80 and we're talking AT tyres here, at half worn, there's probably more tread depth than a brand new asphalt biased tyre, so where's the danger in aquaplaning (which is the only evil left if your tyres are roadworthy)?
I've been driving for over 44 years (which has to be in my case pretty close to 1.5 million miles) and I'm a stickler for regularly rotating tyres, so there's no danger that there's ever much difference between each of the four new ones I fitted in the first place.
I always buy tyres in sets of 4 or 5 (usually 5 even though I might have a new unused spare from the last set of 5). This mainly because there's a big difference between brands of ATs whereas asphalt biased tyres don't vary so much between reputable brands, IMO.
First and foremost, I would always advise bying sets of tyres, not singles or even pairs.
So, on the basis of the above but in the event that I did buy a pair, I would still put my new pair on the front and benefit from the better tread under emergency braking and steering.
IMO, blow-outs on good tyres are a very minor risk, tyre walls fail for reasons of incorrect inflation or damage, rarely anything else.
I won't accept that good undamaged part-worn AT tyres of reputable brand are any more prone to punctures, blow outs, aquaplaning or turning into a pumpkin than the new AT tyres.
Many drivers are incompetent, we see them every day, here especially, driving on bald, damaged, rotted, and cracked things that are illegal and should never be on a car. Those are the most likely to aquaplaning, puncture, or blow-out unexpectedly.
The U.K. Has been spoilt by good education, first the 1mm tread depth rule in the 60s and more recently 1.6mm I think. My AT tyres get changed well before that.
Here they get away with murder, quite literally.
I'm sure there'll be a backlash to the above even though I've admitted losing the case, but it's been a good discussion nevertheless.
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