According to the explicit specifications of the halakhah, the only amount which is required by law to be removed is that of a kezayith. Or, if there are several pieces of hamess that, when gathered together, would equal the amount of a kezayith or more - in the same room - then it is required to remove them. (cf. Rambam, Mishneh Thorah, Hilkhoth Hamess Umassah 2:1ff; Arokh HaShulhan, Hilkhoth Ta^aroveth Hamess wa-Hamess Nuqshah, Siman 442:27ff)
This still leaves two questions:
- What is the size of an olive (kezayith)? - le-^aniyuth da^ati, the shiur of an olive (kezayith) is between 2cc-4cc, as are the "medium-size" (i.e. Suri) olives of today. (cf. Mishnah, Kelim 17:8). The Rambam, although he details the other halakhic measures of Hazal (cf. Mishneh Thorah, Hilkhoth ^Eruvin 1:12), he never gives a definition of a kezayith, which almost certainly indicates that he felt that it's measure was self-evident. [See HERE for a full explanation of all sources and issues involved in this definition]
- Can't a lot of crumbs be gathered into a kezayith? - Although the crumbs on a smooth floor might be able to swept together (and probably should be removed if possible), there are several considerations: a) crumbs embedded in carpet (as in the floor of most cars) are usually not able to be gathered - even by hand, b) Only hamess which is still raui le-akhilah are of concern and must be destroyed (cf. Hilkhoth Hamess Umassah 4:8), and c) when hamess is destroyed the declaration is to have it be considered "like dust" (Ibid. 2:2) and one method which is given for destroying hamess is to porer wa-zoreh la-ruah ("reduce it to crumbs and cast it to the wind" - Ibid. 3:11).
Thus, it seems that the crumbs in the floor and crevices of your car would not be an issue. Nullification in your heart and declaring it ownerless will suffice.
Hope this helps. Kol tuv.