If your new rolling radius is bigger than standard you'll be moving faster than your speedo says you are if the speedo is dead accurate with standard wheels and tyres. Likewise, you will actually cover slightly more miles on a journey than your odometer says you travelled.
Example:
Changing stock LR defender wheels and tyres;
205/80/16, rolling radius = 367.2mm
To;
235/85/16, rolling radius = 402.95mm
You end up with a new rolling radius 9.74% bigger.
Put another way, for every revolution of the wheel you travel 9.74% further than before. Only way to travel further in the same amount of time is to go a little bit faster but as your speedo is calibrated for the smaller radius it under-reads by 9.74% (i.e. when your speedo says 70mph you're doing 76ish)
Trick is, very very few speedos are accurate at speeds above 30mph anyway and usually have between a 5% to 10% over-read. Only way to be absolutely sure how fast you're going is with a GPS unit in the car as Griff said.
Since relative economy isn't affected by such dubious things as mechanical speedos, you can expect a mathematical 9.74% increase in economy with the bigger wheels, but this being the real world I'd guess you're only likely to see up to about 5%.
Cheers,
