Before Ashley started a three-hour drive, her car’s odometer reading was $29792$, a palindrome. At her destination, the odometer reading was another palindrome. If Ashley never exceeded the speed limit of $75$ miles per hour, what was her greatest possible average speed? The original answer is in the form of \frac{m}{n}, please provide the value of m + n.

Let's think step by step and output the final answer within \boxed{}.
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