[autotech]

The problem is that vehicle handling is dependent on equal grip from all 4 tires. When you introduce differential in traction front to rear, you can throw the "understeer or oversteer bias" directly out the window. Modern cars do a great job compensating for less than ideal conditions but there is a limit. If you still think the new tire on the rear rule of thumb is inaccurate, watch this:

https://youtu.be/gSz7cm6MwH0

[throwaway0a5e]

Re-read what I said. I'm specifically talking about the ass-heavy FWD crossovers and minivans that dominate the roads today (though I suppose there's a few FWD sedans out there with similiar handling). They all have a very respectable sway bar up front for good "car-like" handling and they all have a lot of weight out back. This results in a class of vehicles that mostly cannot go sideways even if you try (and believe me, I have tried). Someone with one of those cars comes in with four nearly bald tires and wants two good ones put on and you put them on the rear then you have improved their handling by exactly zero because the fuse in the system is still the front traction. Adding more rear traction doesn't help because handling/performance is determined by the weakest link in the chain and that's still the front which is unchanged . If you put the tires up front then sure, there's a 2% chance of going sideways rather than a 1% chance but you've also improved the handling greatly by addressing the weakest link which is end that was lacking in traction. The vehicle will corner and stop much better because the threshold for loss of traction is much higher.