Re: Speedo difference on 18's???
My car's computer was set to 18 inch wheels and I had the standard 17 incg wheels fitted, with this combination my speedo read about 10-11% high (verified with my GPS across the nullabor). When I got to Perth I had Ford reset my computer to 17 inch wheels and now it reads about 3-4% high.
If you are going the other way your speedo is likely to read low, be careful of speeding fines or get Ford to reset the wheel setting.