auto_da_fe
I have a speaker in my car that rattles when the bottom (kick or bass) is too much.  (No decent commercial recordings make it rattle )
 
I was going to try to fix it but I decided to leave it alone.
 
 
JR

That was a good decision.

I listen with laptop speakers. If it sounds good there, it'll sound good anywhere.

That's very true. However, I just can't bring myself to do it because everything sounds bad on my laptop speakers.
 
That's why I travel with proper headphones, so I can watch a movie on the laptop if I get bored. As it happens, I am leaving within the hour on a business trip and had to make the agonizing decision not to take the good cans...compromised for the Shure IEMs, which work much better on airplanes anyway. Those stay in the laptop bag all the time - I would never insert those nasty little plastic things they hand out on airplanes into my ears! Even if they might serve well as the ultimate "bad" reference system.

I can't wear earbuds. They don't fit or stay in my ears. Can't wear safety ear plugs either. Have to wear over or on ear muffs/headphones.
 
I've used cheapy ($5-$10) headphones at work. Some actually sound OK for general listening but after breaking so many I finally got a pair of $49 Skull Candy wireless for work. They sound good and give a good clue to the mix.
 
For the car I load up a few tracks on the smart phone and play them thru Bluetooth on the car stereo - a Rosen in-dash factory fit unit with 6" screen I bought on Amazon for $600. Car speakers are factory "premium" JBL 6 speaker set - 4 in front, 2 in rear. I'll include a few commercial tunes of similar genre to compare with my tunes. Works for me.

I've been driving a Jetta with the Fender sound system. It's head and shoulders above what most cars are equipped with, but I would never judge a mix even on a better than average car audio system. Like you guys mentioned, it's an inherent acoustic nightmare. One thing I didn't see mentioned elsewhere is road noise. Even with a solid car and a decent audio system, there is the rumble and whoosh of driving that has to be overcome either through volume or EQ. What sounds good at a stoplight will sound a bit empty or lackluster at 65.

I'd say the car test is fine to make sure the most important sounds in a track are heard, but if you're concerned about subtleties being lost, you're fighting a losing battle.

I've been using my 2000 Honda Accord to test my CDs for years.  I know the sound of nearly all my commercial CDs in that car, so I can compare my own mixes to them.  Things got easier about 4 years ago when I started using the speaker emulations in ARC 2, and more recently, Mixchecker which CW offered earlier this year.  These software programs get you in the ballpark quickly, and have allowed me to get better mixes on laptops, cellphones, and boomboxes in addition to my car.  When used in conjunction with a Focusrite VRM box with headphones, you have nearly all bases covered.  Now, when I take a CD to my car, I know what to expect, and those nasty surprises don't occur as often.  Nevertheless, I still start all my mixes in mono on a single Auratone (ca. 1979), and that may be the best method for me in the long run.  Still, I can't say enough for ARC 2 and Mixchecker.

P=no road noise.

My car sound system is actually really good for a stock system (Mark Levinson) and I listen to so much music on it I "know it" it so to speak. I really hear the difference when I listen to SiriusXM vs a CD - the satellite signal just sucks in comparison. 
 
I listen in the car, at the office, and in the studio. In fact I'm doing that today with a mastering candidate. I really know I'm on to something when it sounds good on all three. For me, I've found the Avatntone Mix Cube was a breakthrough. If it sounds good on that, it sounds good most places. I just added a second so I can mix on them in stereo, or hit a button and just hear a mono signal out of one of them.

it's reassuring to see that we're all having the same experience :)