Basically what the title says
Can you check engine health by checking your current mpg??
Just did a check up on mpg and got 26 mpg
In my 1994 celica Gt manual
Wondering Good health or no ? 198k miles

They are related but your driving habits have a bigger influence barring major mechanical faults like excessively low compression. A better gauge of health is a used oil analysis, compression test, and leak down test. All 3 together will give a fairly good picture of whats going on inside your engine and how worn or how it's wearing.

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Other mpg factors include tire friction through pressure and compound, brakes dragging, wheel alignment, and aerodynamics among other things. So as far as engine health goes doing what Bitter mentioned will give a better idea. Honestly 26 mpg seems on the low side, but if it's mostly city driving and you romp it often it's probably about right.