Constant 4wd tyres

We have had 2 constant $WD vehciles, a Freelander and a Rangie. With the Freelander, no problems with a mix of old/new tyres. With the Rangie, you need around the same tread depth on front and same on rear - front to rear does not seem to make any difference. If the side to side tread depth is way out, we get a vibration.

There are some tyre dealers that have told me that they have been advised by who knows who, that all 4 tyres should be the same tread depth and they will only sell you 4 tyres - at $400+ each for the Range, no way when we only needed 2 so we changed tyre dealers.

A friend of ours with an xTrail runs the same tread depth side to side but different front to rear with no problems.

Stevesub

I've been driving a Prado with constant 4WD for 11 years, and I like to use the centre diff lock on dirt at times to aid stability. Clearly, this would not be a good idea with different rolling radius front to back, since the RPM would be the same front and back, but the bigger tyres would be trying to push the vehicle faster - not good for stability.
Also, in normal conditions, with the diff lock disengaged, the front axle would be turning at a different speed than the rear all the time, and so the diff would be 'working' all the time. Doesn't seem right to me , but it may not be a problem

You'll get a better answer if you name the vehicle. Lots of different 4wd systems out there now.

Generally, the difference in circumference between a bald tyre and new tyre is about 1-2%. Any 4wd with an open centre diff will have no problem with this.

I had a transfer case bearing fail in one of my cruisers. The gearbox specialist said that he has done heaps over the years of different makes due to different size from front to back.
Side to side doesn't usually cause long term damage exept if you have an LSD or locker fitted. But will cause handling issues.

I always buy 4 at a time and rotate every 10,000km so they wear evenly.

Cheers
Dave