5:40 PM PT — Lexi Kramer‘s family claims James Wehr allegedly hid his infidelity and misled Lexi about his divorce.

Car influencer James Wehr lost control of his vehicle before dying in a fiery car crash … and TMZ obtained the first video revealing what happened when his McLaren smashes into a tree and bursts into flames.

The harrowing footage — recorded on another car’s dash cam — shows James passing on the left in his exotic sports car just after the light turns green, and he apparently put the pedal to the metal … but not for long.

James raced over a wet section of the pavement, and immediately hits his brakes … the McLaren swerves to the right and drives smack into a tree, exploding into a massive fireball.

It looks like a bomb went off … there’s a fiery inferno and a mushroom cloud … with debris scattered all over the road.

Kyle Oldoerp of the Irvine Police Department tells us they’ve seen the video and tells TMZ … “Our Major Accident Investigation Team (MAIT) is conducting an extensive investigation into the collision that will take months to complete.” 

Oldoerp continues … “MAIT Investigators will examine all possible contributing factors to the collision, including roadway and vehicle conditions, the vehicle’s speed, and any factors that may have affected the driver’s operation of the McLaren.”

The fatal crash went down early Sunday in Irvine, CA … killing James and his passenger, Hooters calendar girl Alexis “Lexi” Kramer.

Orange County firefighters were called to put out the blaze … and police found the bodies when the smoke cleared.

Our video makes it clear … there was no way James and Lexi were walking away from this crash.

Police said speed was a factor … and the footage drives that point home.