Speedo shows wrong speed

The speedo in my 2011 Holden Colorado shows 110 km/h when the actual speed is 102 km/h. Apparently this is within the law, but makes it very annoying to drive. According to the dealer nothing can be done to correct the problem, and, as they have parted ways with Isuzu, Isuzu have washed their hands of it. Can the speedo be adjusted at all, or do I have to just put up with it? I have read that some owners are fitting larger diameter tyres to minimise the problem, but this seems an expensive option for a small gain. As a side to this, it could lead to road rage problems, where a driver thinks they are doing the speed limit, but is actually doing well under the shown speed.

Yes, it is within the law, and no, there's not a lot you can do about it. If it's any consolation everyone else is in the same, or a similar boat. The best solution is to estimate the actual speed using the GPS results and drive accordingly.