Regarding the speedo reading, There are web sites where you can match different size wheel/Tyre configurations to get the equivalent rolling circumference. When I changed my 19" wheels for 18" I used one of these sites to select the tyres. The result was that with the 19" wheels the speedo always showed 4mph plus at 70 mph according to GPS, with the 18" wheel and tyre selection the speedometer now shows exactly the same speed at 70mph as GPS.