But you can change outside noises by moving the car. If you had a poor voice test, then no moving the car is going to help and the recording technique may not be valid. Good to know that right up front.
For sure, I'm not saying that a car is a fantastic space to record but, given the OP's lack of a quiet space, it may be preferable.
Of course positioning will make a difference, and one can plan for that, however, it's the random noises that are the unknown.
The test I did will give the OP and others, a good idea of the kind of attenuation they can expect.
Too much glass relative to the padded surfaces...
Glass will certainly add to the reverb time and color the sound, it will be dependent on surface area and angle of the glass, ratio of glass to padded surfaces, interior volume of the car, etc
It's easy to test though, no fancy RT60 tests required, just get in and belt out a couple of lines or riffs and see how it sounds.
It's not all bad though, a bit of reverb and color is often nice on vocals and guitar.
I don't need any excuse to visit a park for lunch.
Looks a bit hazy today, best stay indoors.
I've never measured the noise up there.
Where, Mount Lee?
My guess is that noise will be the least of your worries.
It's all the hot air from the pompous celebs you have to be careful of.