Demand in Houston was "particularly strong", Ford's USA sales chief Mark LaNeve said on a conference call.
While the hurricane recovery drove sales, "this will be a short-lived party", Michelle Krebs, executive analyst for online sales site Autotrader, told reporters on a conference call.
It is the highest SAAR since the 20.64 million rate recorded in July 2005 behind employee-style discounts.
Ford Motor's September US sales to retail customers were at 164,067 vehicle units, a ~4.4% rise from the corresponding month of the previous year.
It was a particularly strong month for light trucks, with sales advancing 12 percent, while auto demand remained weak and down 3.3 percent.
The "overall strength of the USA economy is the main force driving the market", said GM chief economist Mustafa Mohatarem, with recovery in hurricane-damaged areas "spurring new and used vehicle sales".
Fiat Chrysler Automobiles NV (FCAU) slid 6 cents to $17.89 after the vehicle maker said US auto sales in September declined 10% to 174,266 vehicles compared to 192,883 vehicles in a year ago month and retail sales rose 0.3% to 146,904 vehicles while fleet sales tumbled 41% to 27,362 vehicles.
GM's 12 percent jump marked a second straight strong month after a tepid start to 2017.
Ford retail sales totaled 169,544 vehicles, up 4.4 percent compared to a year ago, while fleet sales totaled 52,704 vehicles, representing a 25.1 percent increase versus 2016.
Ford sold 30,899 vehicles in September, up 2.4 percent from the same month a year ago. Nissan Motor Co. up 9.5 percent, bounced back from a double-digit decline in August. The new Chrysler Pacifica minivan, which has now been on the market for more than a year, saw its sales decrease 11 percent to 444 units. FCA's sales to consumers rose 0.3 per cent.
Ford SUV sales increased by about 2% in September, led by growth from the Escape and Flex and solid sales from Edge and Explorer. Vehicle demand slipped 1.3 percent and utility volume rose 1.8 percent, Ford said.
Deliveries were up 9.3 percent at the Nissan division, behind another strong month for truck sales, and 12 percent at Infiniti.
FCA sold 152 Fiats and 109 Alfa Romeos in the month.
Strong pickup truck and SUV sales once again propelled the September increase.
The Hyundai brand posted modest gains in September, up 2.6 percent to 11,961 vehicles.
In fact, sales are a bit flat.
Of companies reporting early Tuesday, only Fiat Chrysler reported a sales decline.
USA sales have now fallen 1.7 percent this year through September after seven straight annual gains and a record 2016.
Hyundai Canada's total sales, including the Kia brand, were 19,830 in September, up 8.9 percent over previous year with Kia doing most of the heavy lifting. There were 26 selling days last month compared to 25 in September 2016. Toyota division light trucks continue to lead the company's US sales, with September marking a string of eight consecutive monthly sales records, and nine consecutive monthly best-ever sales records for Highlander and RAV4.
September was the first full month of sales of the all-new 2018 Camry, which outsold last year's total by double digits.
Passenger vehicle sales in the month slipped 1.3% to 49,918 units while fleet sales surged 25.1% to 52,704 units while trucks sales soared 19.9% to 103,625 units while Lincoln brand unit sales rose 0.1% to 8,802 units.
"With 6 of the 12 major automakers expected to pay in excess of $4,000 per vehicle in sales incentives, one must question whether we will establish a new normal in terms of spending or whether automakers will collectively pull back and more closely align production with natural consumer demand", said Eric Lyman, ALG's chief analyst.