Partial exemption for aero-tyres limits excise duty to a capped ad valorem rate, restricting levy above that cap. Exempts aero-tyres under sub-heading 4011.91 from excise duty to the extent that such duty exceeds the amount calculated at the rate of 30% ad valorem, exercised under the statutory power in section 5A of the Central Excises and Salt Act on public interest grounds.