The pKa values in water are 3.25 for 2,6-dimethylbenzoic acid and 4.37 for 4-methylbenzoic acid. The value of $K_a(\text{2,6-dimethyl})/K_a(\text{4-methyl})$ is closest to

$K_a=10^{-p{K}_a}$, so the ratio $=10^{4.37-3.25}=10^{1.12}=13.2$. The two ortho methyl groups twist $\mathrm{COOH}$ out of the ring plane (steric inhibition of resonance), which makes the acid much stronger even though methyl is a donor.