Here are some sources of measurements of how cars can be a source of magnetic fields:
Magnetisation of tyres
Bioelectromagnetics. 1999 Oct;20(7):440-5.
Magnetic fields from steel-belted radial tires: implications for epidemiologic studies.
Milham S, Hatfield JB, Tell R.
Gravelly Beach Loop NW, Olympia, Washington. firstname.lastname@example.org
Magnetic fields emanate from radial tires due to the presence of reinforcing belts which are made of magnetized steel wire. When these tires spin, they generate alternating magnetic fields of extremely low frequency (ELF), usually below 20 Hz. The fundamental frequency of these fields is determined by tire rotation rate and has a sinusoidal waveform with a high harmonic content. The static field of radial tires can exceed 500 microT at the tread, and the tire-generated alternating fields can exceed 2.0 microT at seat level in the passenger compartment of vehicles. Degaussing the tires reduces both the static and alternating fields to low levels, but the fields increase gradually over time after degaussing. The tire-generated fields are below the frequencies detected by most of the magnetic field meters used in previous studies of power frequency magnetic field health effects. If these fields are biologically active, failure to detect them could compromise exposure assessments associated with epidemiologic studies.
Health Phys. 2006 Feb;90(2):148-53
Low frequency magnetic fields induced by car tire magnetization.
Stankowski S, Kessi A, Bécheiraz O, Meier-Engel K, Meier M.
Berne University of Applied Sciences at Biel, Quellgasse 21, CH-2500 Biel, Switzerland. email@example.com
Alternating magnetic fields have been measured in a variety of different cars, the dominant contribution being from magnetized tires. Magnetic field strengths have been measured as a function of frequency directly at the tires and at different positions in rolling cars. Measurements at the tires showed field strengths up to 100 microtesla (microT). In the interior of rolling cars, close to the wheels at foot regions and at the back seat, field strengths of several microT were obtained in the 10-200 hertz (Hz) frequency domain. In some cases measured field values were considerably higher than those found in previous studies. Purposely magnetizing single tires made it possible to study the influence of various parameters. Degaussed tires retained low field values over prolonged time under conditions of normal use.
Currents between alternator and battery
The WHO Environmental Health Criteria includes the following summary:
"Cars are another source of ELF magnetic field exposure. Vedholm (1996) measured the field in 7 different cars (two of them with the battery underneath the back seat or in the trunk), engines running idle. In the left front seat the magnetic field, at various ELF frequencies ranged from 0.05 to 3.9 μT and in the left back seat from 0.02 to 3.8 μT. The highest values where parts of the body are likely to be were found at the left ankle at the left front seat, 0.24–13 μT. The higher values were found in cars with the battery located underneath the back seat or in the trunk."
Vedholm (1996) is a reference to an unpublished thesis.
Which produces higher fields - petrol or electric?
A study in California has compared gasoline (i.e. petrol) and electric vehicles, in some cases alternative versions of identical models. It measured the field (40 - 1000 Hz) at multiple locations in the vehicles on a standardised test drive. The finding was:
average for 7 electric vehicles: 0.095 µT
average for 4 gasoline vehicles: 0.051 µT
Bioelectromagnetics. 2012 Apr 24. doi: 10.1002/bem.21730. [Epub ahead of print]
ELF magnetic fields in electric and gasoline-powered vehicles.
Tell RA, Sias G, Smith J, Sahl J, Kavet R.
We conducted a pilot study to assess magnetic field levels in electric compared to gasoline-powered vehicles, and established a methodology that would provide valid data for further assessments. The sample consisted of 14 vehicles, all manufactured between January 2000 and April 2009; 6 were gasoline-powered vehicles and 8 were electric vehicles of various types. Of the eight models available, three were represented by a gasoline-powered vehicle and at least one electric vehicle, enabling intra-model comparisons. Vehicles were driven over a 16.3 km test route. Each vehicle was equipped with six EMDEX Lite broadband meters with a 40-1,000 Hz bandwidth programmed to sample every 4 s. Standard statistical testing was based on the fact that the autocorrelation statistic damped quickly with time. For seven electric cars, the geometric mean (GM) of all measurements (N = 18,318) was 0.095 µT with a geometric standard deviation (GSD) of 2.66, compared to 0.051 µT (N = 9,301; GSD = 2.11) for four gasoline-powered cars (P < 0.0001). Using the data from a previous exposure assessment of residential exposure in eight geographic regions in the United States as a basis for comparison (N = 218), the broadband magnetic fields in electric vehicles covered the same range as personal exposure levels recorded in that study. All fields measured in all vehicles were much less than the exposure limits published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Institute of Electrical and Electronics Engineers (IEEE). Future studies should include larger sample sizes representative of a greater cross-section of electric-type vehicles.