Some 4,264 new cars were registered on July 1, the first day of the new 142 registration plate.
This compares to 1,924 on the first day last year.
SIMI had previously hoped that more than 4,000 cars would be registered in the first week of the new 142 year.
In addition, 292 new vans were sold, compared to 138 on the same day last year.
Director general Alan Nolan knew July 1 was going to be one of the busiest days of the year for car sales, with additional cars being added to the car hire fleets.
However, he said, he did not expect “such a good start compared to last year, which was the first year for the new split-registration plate”.
“Yesterday has turned out to be the highest day for new car registrations this year and nobody expected that. Although it has to be put in the context that we are starting from a low base, it is clear that an increasing number of consumers are returning to the market.
“Dealers are currently competing for customers with strong new car offers and a wider range of finance options and it’s evident that consumers are seeing the benefit of the new mid-year registration period. Overall, it is a sign that we're seeing an improvement in the economy."