Motorcycle helmets are tested to meet the highest safety standards in the industry .... from the Snell Foundation and are labeled with a sticker inside each helmet with a "Mxxxx" rating which refers to Motorcycle and the year the Snell Foundation update the standard.
Auto Racing helmets carry the Snell Foundation SA2005 ratings which refers to the SA, Special Applications and the year the Snell Foundation updated the standard.
Snell updates its standards every 5 years. You may still see a '95 in the market place but most likely M2000 or SA 2000's. The newest helmets on the market carry the 2005 which is the latest update.
In order to get the Snell rating, both types of helmets must pass a series of vigorous and identical impact tests. In addition to these tests the SA standard must comply with 3 additional variances or tests.
1) The SA helmets must have a Nomex fabric liner and are therefore more fire resistant for auto racing.
2) The SA standard allows a minimum eyeport size that is smaller than the minimum allowed for the M standard
3) The SA helmets must pass a test designed to simulate impact with a rollbar.
Some manufacturers include aerodynamic features on their SA helmets auto racing car helmets that would not make sense to put on the M helmets for motorcycling and many SA helmets are available with provisions for forced air cooling or emergency air supplies that are inappropriate or unnecessary for M helmets. Manufacturing costs being what they are... most manufacturers build their SA and M helmets identically with the exception of the inner liner.
We suggest buying SA 2005 for all racing events. The sanctioning body of each event, club or race will determine the standard accepted . Many clubs and or racing events will require SA ratings and may not accept M standards. Most clubs stop accepting both standards after a certain number of years past the date code. Check with your club, event, venue, prior to buying your helmet to see what is required.
We hope this information was helpful.