Put the car on a dynamometer and point a short shotgun at the exhaust pipe?
Bob.

I had to Google Dynamometer. haha Guessing he doesn't have one.

If you had an extra person helping, could they possibly hold a long boom pole out of the side window with the mic aimed at the exhaust pipes? They have booms that will bend at different angles that would probably get the mic pretty close. I'm hoping you are in a remote area for this idea. That is my disclaimer.

Any place that does serious car tuning will have one.
Alterntaively get on of those mic clips used to hold mics for drums, clip mic to rear bumper near exhaust pipe. Not too close or you might cook the mic.
Bob.

If you drive on sand, grass or snow the "road" noise should be manageable.
Tor

I suppose it all boils down to what kind of noise you want. Maybe if you stuck the mic in the engine compartment to add another track to blend with the outside noise. Wish I could have done this with my old L-88 Corvette at the drag strip. 1969 was the year.

JJK

I like the idea of using a mechical pickup except exhaust pipe heat
could melt most piezo devices. You will get some reasonant
noise from the muffler and road vibration.

Could a sound effects CD get you the sound or is this one of those
high performance vehicles where someone going "b-r-b-r-b-r"
in the background would be just as bad?

Could you use two identical mics one to pick up exhaust and
the other to pick up road noise and then try cancelling by
the old method of inverse phasing and adding?

I stick a mic in the trunk, you lose some high freq, but road noise and wind noise are reduced tons.

Thanks for the ideas all. SFX are definitely out. The video is intended to convey what my roadster experience, so a high value's being placed on authenticity. The dyno idea came up on another forum, and I like it as a fallback. It'd still be difficult though, because I'd need to sample all kinds of sounds (lift off, neutral throttle, full throttle, off throttle, etc. at different revs). I'm trying to stay stubborn when it comes to certain compromises. :)

Unfortunately, it'd be very difficult to clip anything to my bumper due to the car's underside being flat at the back, but that sparked a few "creative" ideas that might accomplish the same thing. Using phase is an interesting concept though I have a feeling it'll be hard to do without noticable artifacts. Still might try it though.

Orcatek, funny seeing you here. I'm actually on Z4um. What do you use as a mic, and where in the trunk do you put it? I'd like to prominently feature the car's "voice" as we hear it... only better. The trunk might actually be very appropriate for that provided it doesn't sound too "muffled."

In place of a clip on mic holder you could use one of the larger super magnets. Use gaff tape around it to hold the mic and so you can get it off, they have an incredible grip. If that will not work because the car's fibreglass might be a job for a sticky pod.
The trick will be getting the mic placement right. You should be able to find a spot at the rear of the vehicle where there's no wind, a sort of dead zone. You don't want to get too far from the actual axhaust pipe though or as the speed increases the wind will literaly blow the sound away from the mic. I'd also watch out for the mic's max SPL rating!
Bob.