I have problem with ambient outside temperature fluctuate, if i drive on freeway around 60mph or 65mph then it display seem o.k but if I drive faster like 75mph or 80mph temperature will increase like 10 degree higher.

I just replaced left side catilyc converter with after market, do you think it may be affect to outside temperature keep move up and down with the speed

The engine temperature stay good at 80

The new converter should be unrelated. You may want to look at replacing the ambient temperature sensor. They're inexpensive and easy to replace.

I think the sensor is near the left front fog light. (inside the bumper)