Answer
$T\gt T_α$: null hypothesis is not rejected.
There is not enough evidence to conclude that 92 octane has a greater gas mileage than 87 octane.
Work Step by Step
$H_0:M_D=0$ versus $M_D\lt0$
Let the "87 octane" values to be the X and the "92 octane" values to be the Y.
$D_i=X_i-Y_i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Rank$
$D_1=X_1-Y_1=18.0-18.5=-0.5~~~~~~~~~~~~~-4.5$ $D_2=X_2-Y_2=23.2-23.1=0.1~~~~~~~~~~~~~~~~~~+1$ $D_3=X_3-Y_3=31.5-31.9=-0.4~~~~~~~~~~~~~-2.5$ $D_4=X_4-Y_4=24.9-26.7=-1.8~~~~~~~~~~~~~~-10$ $D_5=X_5-Y_5=24.1-25.1=-1~~~~~~~~~~~~~~~~~~-8$ $D_6=X_6-Y_6=23.4-22.8=0.6~~~~~~~~~~~~~~~~~+6.5$ $D_7=X_7-Y_7=23.1-23.5=-0.4~~~~~~~~~~~~~~-2.5$ $D_8=X_8-Y_8=19.0-19.5=-0.5~~~~~~~~~~~~~~-4.5$ $D_9=X_9-Y_9=26.8-26.2=0.6~~~~~~~~~~~~~~~~~+6.5$ $D_{10}=X_{10}-Y_{10}=31.8-30.7=1.1~~~~~~~~~~~~~~+9$
$n=10$
Left-tailed test.
Test statistic: $T=T_+=1+6.5+6.5+9=23$
Critical value: $T_α=10$
(According to table XII, for $n=10$ and $α=0.05$)
Since $T\gt T_α$, we do not reject the null hypothesis.
