I understand that in AE slang, a "rice grinder" is a Japanese or Korean or Chinese motorcycle or car.
My question is, is this phrase derogatory? Or can I use it in front of anyone, given the context is informal?
The more common slang expression, in my experience, is "rice rocket" - at least for motorcycles. It is derogatory, usually used by bikers who ride big Harley-Davidsons* and want to put down lesser vehicles. I assume "rice grinder" is derogatory too.
*I hasten to add that I have a good friend and neighbor who owns both a Harley and a Ducati. However, I'm quite sure he would never use this expression. Besides, he drives a Mazda.
I've heard "ricer" to refer to Asian vehicles (usually "interestingly" modified ones - try a Google image search), but never "rice grinder"...and yes, slang based on the assumed diet of people of a given race or nationality is generally considered derogatory.
I have never heard "rice grinder" used this way. The Urban Dictionary lists this slang meaning, but calls it an "arcane" term, so most people don't know it. If you use it, people will have to figure out what you mean.
Real "rice grinders" are kitchen appliances: small machines. Calling a car/motorcycle a kitchen appliances is making fun of how small they are (compared to American cars or motorcycles). So they are derogatory in that sense. But I don't think there is any "racism" in the phrase. Some Americans probably make fun of European cars for the same reason (size) but instead of "rice" (which we associate with asia) use some term we associate with Italy or Germany. A pasta-maker?