Are you sure your resistor in series method is accurate? What's your multimeter's resolution/accuracy when measuring voltage and resistance.
Seems to me you are multiplying error bars of several different measurements (battery voltage, voltage accross resistor, resistance, etc) and your R2 (the resistance of the whole car) is an unknown? Have a go at calculating your error bars for fun
The meter I bough claims 0.01A resolution and +/-2% and 5 units precision, for my measurement of 0.18A I work out the error to be +/-0.054A a bit high, true, but to get a much more accurate current reading you will need to spend several hundred pounds on a accurate meter.
http://www.maplin.co.uk/p/uni-trend-...FezHtAodmRIAhg