EGT have been used for years for fuel tuning, especially on gasoline powered airplanes
My airplane had a single probe EGT, some have two and some have one for each cylinder that displayed all of them on the same screen
Mine had a single gauge that did not read low exhaust temps. below 900 F I was taught in flight training to lean it down in flight once I was above 3000 ft. slowly until it reached peak EGT and then richen it up to 25 F to 75 F rich of peak, depending on the brand of motor and if it was normal induction or turbo charged, you would run the turbo charged engines lean of peak by 25 to 50 F
On the rental airplanes that did not have the EGT you could do the same thing on leaning them down in flight by leaning it down slowly until it started to miss or run rough and them turn the mixture control in 1/4 to 1/2 turn
I got taught the hard way to do that on my second flight in my airplane, I fouled both spark plug out on one cylinder in the second flight shortly after take off about ten miles west climbing from the airport in Bull Head City, AZ and I had left the mixture in at full rich above 3000 FT