Tyres question (proper question, honest!)

Basically, I either want 235 85s or 265 75s insa turbo dakars. I've read from loads of posts that both will fit with a bit of trimming etc, but what is the latter like on the road? Would it really slow the car down or make no difference? Just due to the width of it like.
Cheers.

Width doesn't slow the vehicle down.

Diameter does. If it's too large it can affect the gearing and thus potentially slow the vehicle down on acceleration, though higher top speed might result. In fact it's rare that you'd get good enough conditions to exploit any top speed gains 'cos you're driving a brick, a very large brick and air resistance comes into play over certain (around about 50-60 mph ish) speeds, which you haven't enough power to overcome, so the net effect is that the whole thing becomes slower!

Thought as much, since the 265 75s are only 31" tall so there won't be a massive difference in rolling radius.
What size tyres you on anyway paul?

i've got a disco with 2" lift and 235 85s on ........its great on lanes and ****e on the road...like trying to steer a bouncy castle now !

I'm on 31 x 10.5 R15 ... but yes, the difference (on a Disco) is noticeable, especially when towing. Standard size tyres are geared correctly ... and feel all the better for it. But, for me, the towing is a compromise as I tow so I can get to places to green-lane that are different. (And to please the missus, of course)

If yours are going on a Defender ISTR they are not far off 31" diameter anyway.

Also, as it aint a standard size, will i need to tell the insurance chaps about the change and will they care or knock the price up?

Also, will 265s fit my standard alloys that came with 205s?