True Story? It's a good one for sure...

[Grandpa Weatherbie]

When Albert Einstein was making the rounds of his speakers circuit, he usually found himself eagerly longing to get back to his laboratory work. One night as they were driving to another rubber-chicken dinner, Einstein mentioned to his chauffeur ( a man who somewhat resembled Einstein in looks and manners) that he was tired of speech making.

"I have an idea boss, his chauffeur said." I've heard you give this speech so many times, I'll bet I could give it for you." Einstein laughed loudly and said "Why not! Let's do it!"
When they arrived at the dinner, Einstein donned the chauffeurs cap and jacket and sat in the back of the room. The chauffeur gave a beautiful rendition of Einstein's speech and answered a few questions expertly.

Then a supremely pompous professor asked an extremely esoteric question about anti-matter formation, digressing here and there to let everyone in the audience know that he was nobody's fool.
Without missing a beat, the chauffeur fixed the professor with a steely stare and said, "Sir, the answer to that question is so simple that I will let my chauffeur, who is sitting in the back, answer it for me."

[Sir Toony Van Dukes]

Snopes doesn't think it is true. snopes.com: Chauffeur Switch

[Harpoetta]

I've heard this before, but I don't think it was Einstein when I heard it. It's still a funny story.

[Wowza]

The Snopes article was actually pretty funny in it's own right.