What an Engine Needs to Idle Smoothly. Your 500’s engine needs air, fuel, and spark to achieve optimum ignition . When one or more of these variables is off, it can lead to a rough idling condition. Fuel – The ideal air-to-fuel ratio is 14.7 parts air to one part fuel. Air – The air entering your car’s engine must be metered by the MAF 17. May 7, 2006. #4. exactly what he said,,, 70mph. i do about 300 motorway miles a week in my car. and i see sooo many being pulled over from cops in normal cars and cops on the lil slop you get on the harsh shoulder. so yeah 70mph, i never go any faster,, would rather keep my license then waste petrol/speed and get done!! lol. KG2o.