2,2-Dimethyloxirane is treated with methanol containing a catalytic amount of $\mathrm{H}{}_2\mathrm{SO}{}_4$. The major product is

Under acid catalysis the protonated epoxide has significant positive character on the more substituted (tertiary) carbon, and methanol attacks there. The methoxy group ends on the tertiary carbon and the OH on the primary carbon, giving 2-methoxy-2-methylpropan-1-ol.