Mixing tyre sizes

I currently have 205/70 R15 tyres on all four wheels, 2 x Geolandars on the rear (new) and 2 x Duellers on the front (part worn).  When I replace the Duellers would there be any problem in fitting 215/70 R15?  I will probably change the Geolandars to the front and have 215/70 R15 on the rear?

According to a tyre size chart the diameter will go from 26.3" (205/70) to 26.9" (215/70) a 2% increase, so off-road in 4WD there should be enough slip in the tyres to avoid problems with different rotational speeds?  The width will only increase by 5%, will I gain anything?

I wouldn't mix tyres on the same axle.

 

I would avoid using 4wd has the transfer box doesn't have a differential 

As long as you use 4wd sparingly  (e.g. only when your wheels are slipping and you actually need it) then it won't cause any trouble there. 2% difference between axles is the equivalent of doing a gentle turn.
The only thing that might get a bit upset is the ABS if you have it fitted. The difference in wheel speeds may make it more sensitive than before.
As to whether the extra width with gain you anything, I'd say not a lot. A bigger diameter is the best way to gain an advantage off-road as it increases both the footprint and ground clearance.

Thanks guys. Given that I won't gain much I think I will stick with 205/70, perhaps just go for a better tread pattern.

I think I wil look for a spare set of rims, then I can have two sets of tyres general summer use and something more suited to winter/green lanes.