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Do you have a portable GPS? They will have a very accurate speed rating on straight roads.
If you use the GPS, you can confirm if the speed off more the faster you go. If so, you can calculate what you have. If not, you can use the GPS to fix your gauge.
If its a gear problem, yes. But you should confirm the problem with the GPS first. For a gauge face fix, its as easy as repositioning the needle on the face. (If the difference is constant at any speed.