How do you know which gauge is correct? Just because it's a mechanical gauge does not mean it's absolutely correct. A differential of 10F between gauges would probably fall within the reasonable range of tolerances.
Most gauges are not extremely precise. They are intended to give the operator a good idea of how well the engine is operating relative to the values they are reporting, but usually they do not provide extremely fine readings. A 10F error would not be considered out of line, generally speaking. That's why when one sees a gauge getting close to red line it's usually time to stop and find the problem.
Having said that, for really long runs between the electric sender and gauge, a larger diameter wire would be a very good idea. The exception would be if the gauge is intended for rear-engine RVs, in which case it should come with an appropriately sized wire (but I wouldn't depend on it sometimes).
