The rolling 40 year tax exemption starts on the 1 April, each year. So, for example, on 1 April 2016 - all vehicles built before 1 January 1976 are tax exempt. If you need to tax your car now, contact British Motor Heritage to obtain the build date for your car - there's a good chance it was was built in 1975 and is therefore already tax exempt. Failing that, declare it off-road with a SORN notice and on 1 April, your car will qualify for free road tax. To re-classify the vehicle's tax class as 'historic' you'll need to complete section seven of you V5C. You can read more about tax exemption for historic vehicles here: classics.honestjohn.co.uk/classic-car-road-tax/cla.../