H temp gauge and thermostat

So i checked the thermostat and it was stuck wide open.

I replaced the themostat with one from the dealer. Now it runs a bit warmer but the gauge never makes it into the green.

I was mowing in 70 degree weather at full throttle in 1st gear chopping some thick stuff, there was not that much load on the engine, but i still think it should come into the green.

Now when i bought the gauge the dealer said i had my choice of 2, the original type or one that was about 20 cheaper that if i was not worried about retoration detail, would work just fine.

I am thinking maybe the new gauge is made to work on newer sealed systems with coolant and run at a hotter temp.

I would be a lot easier to figure out if they would just mark the degrees on the gauge insted of cold, run, hot....

and ideas?

I want the gauge to be correct for 2 main reasons. I am not always the one running the tractor. Two I think an engine that runs at operating tempature is more effciant and produces less sludge in the crankcase.