Big-truck manufacturers in the U.S. had their best May since 2006, as medium- and heavy-duty truck sales rose 16.5% to 37,225 units, compared to 33,185 units the previous year, according to WardsAuto. This was the 21st consecutive month of year-over-year sales increases, WardsAuto said.Class 8 sales continued to drive the overall market – up 24.1% on sales of 21,501 units compared to 17,990 a year ago – with all companies posting gains. Daimler’s Freightliner saw a 36% sales increase, PACCAR’s Kenworth jumped 29%, and Peterbilt was up 26.9%, WardsAuto said. In addition, medium-duty sales were up 7.5% compared to 2014 with 15,724 units delivered in May. All weight classes saw year-over-year increases as well, according to the report.The sales streak continued in Canada as it saw its ninth consecutive month of year-over-year increases in medium- and heavy-duty trucks. Class 8 sales were up slightly by 1.8%, with Daimler’s Freightliner gaining 24.9% and International delivering the best results among all competitors, jumping 48.8% on volume of 374 units, according to WardsAuto. Class 7 saw the highest surge – 61.4% – among all big truck groups, with all companies posting double-digit gains.