Odometer on Hyundai Accent.

I purchased a Hyundai Accent in June 2018. I quickly noticed that the odometer was way out, under reading by between 6 and 8 %. The local Port Elizabeth branch tested the odometer , but only against a similar car. They found nothing wrong,To me it is obvious that if one Accent is under reading, then simi**** Accents will also be out. I left it at that, but have become more and more frustrated, suddenly all other towns seem to be much closer than ever before. I am wondering if my speedometer is not also out, under reading. I contacted Hyundai Head Office 2 weeks ago, reported the matter, and asked for assistance in correcting the problem. No response. Contacted again, no response. I now need to change my tyres, and if I change the size , the odometer might be more accurate. But I need advice from an expert. This is not happening, Hyundai stays quiet. I am not trusting the local branch to sort out the problem, so any advice will be appreciated. I do not intend to just forget about this issue.