I have the same misgivings as Richard F, Dan; I know your picture has perhaps some perspective anomalies but the obvious visible difference in diameter of these tyres is very large and wind-up transmission loads will be horrendous and immediate.

As Richard has said, there is no give in the Ferret transmission between front and rear since the wheels are irrevocably and solidly geared together being driven at the same speed.

The apparent large difference in circumference will likely make the vehicle initially very sluggish as huge amounts of power will be lost forcing the wheels to slip on the road and probably followed in short order by catastrophic bevel box and/or hub reduction failure; possibly both.

FWIW the 2" max circumference difference allowed on FV600 chassis vehicles represents only 1.32% circumference difference, I do not know of the Ferret tyre matching spec but I'm sure one of us can find it somewhere.

The best I can do presently is from my chum Doug Greville in Oz and his website for Ferret owners: http://www.users.zetnet.co.uk/lsm/dhmg/fc-005.html
Again we see apparently 2" is the absolute maximum difference between front and rear Ferret tyres with 1" or less being the aiming point.

I think neither of us Richards that have commented want to be a wet blanket or decry your efforts to keep your Ferret on the road, but rather that you don't have a very large, dirty and expensive job replacing some heavyweight mechanicals when a critical assembly self-destructs through avoidable gross overload.

R.