Dale Earnhardt Jr. -- Lures New Crew Chief Amy Reimann ... with Giant Diamond Ring!!

Dale Earnhardt Jr. is taking the ultimate checkered flag ... he's getting married!

NASCAR's most popular driver proposed to longtime girlfriend Amy Reimann while vacationing Wednesday in Germany. Reimann posted a pic -- naturally flashing her engagement rock -- in a Lutheran church Dale says his family attended 300 years ago.

Reimann and Earnhardt have been dating since 2009 ... and you gotta figure she knew the question was coming sooner than later. Dale gave her a shout out in a commercial he shot last year.

You don't mention a run-of-the-mill GF in an ad you know will air constantly during NASCAR races.