If you are only replacing a pair of tyres, Michelin recommends that you have the new (or least worn) tyres fitted to the rear of the vehicle. This is for safety in difficult driving conditions, such as hard braking or cornering, especially on wet or slippery road surfaces.
New tyre fitted in front:
The vehicle's handling characteristics will change compared to before the tyre change, and the driver could be taken by surprise. Whereas previously there could have been a tendency to lose grip with the front tyres first (under steer), the initial loss of grip could now be with the more worn rear tyres (over steer), which may be difficult to control, especially on slippery roads.
The photo above hows a rear end slide (over steer) on a wet surface.
New tyres fitted at the rear:
The vehicles handling characteristics will be similar to those which the driver has got used to before the tyre change. There will now be optimum grip on the rear tyres, meaning that if the car does slide, the front tyres will tend to lose grip first (under steer), a safer and more stable condition than if the rear tyres lose grip first (over steer).
This advice applies to both front and rear wheel drive vehicles. If all four tyres are quite worn, consider replacing the full set.
When replacing a pair of front tyres, Michelin recommends fitting the new tyres to the rear, and where necessary, moving the part worn rear tyres to the front. The tyre inflation pressures should be adjusted to suit the new positions.
In the case of different front and rear tyre sizes, the tyres cannot be moved from one axle to another. However, having the least worn tyres on the rear axle will still tend to produce more stable handling in slipery conditions.
In the case of four wheel drive vehicles, Michelin strongly recommends that tyres are used in identical sets of four. The vehicle manufacturers recommendations should also be followed.