Palindromic Milometer

Answer: 70$\frac13$


Maximum possible distance: 3$\times$75 = 225

Maximum possible milometer reading: 29792 + 225 = 30017

Largest palindromic number before 30017 will have first digit 3 $\therefore$ last digit 3
second and third digits 0 so fourth digit 0
Final milometer reading: 30003

Distance travelled: 30003 $-$ 29792 = 211     (or 14 less than maximum)
Speed: 211$\div$3 = 70$\frac{1}{3}$                                  (or 14$\div$3 = 4$\frac23$ less than speed limit)