A quick and simple definition of proportions could be the following: The size of the diferent elements of an object and the relation betweet them.
The latter is my lame deffinition, not the official one.
Up next I´ll explain it in car drawing terms:
Every car has diferent form and sizes and to draw these corectly we need a reference. That reference is the car’s own wheel.
The image bellow shows the car´s wheels in black. The red circles are the same size of the black wheels and we will use these to measure the different car elements, this is what I ment by “reference”, the wheel is our unit of measure.
The car elements we will measure (image bellow) are:
1 Ground clearance: Is the distance between the ground and the floor of the car.
2 Wheelbase: Is the distance between the front wheels and the rear wheels. In this tutorials we’ll use three wheels in every case. Why? Because it works for most car types.
3 Overhangs: This is the part of body that goes beyond the wheels on the front and back of the car.
4 Belt line: This is the overall height of middle part of the car’s body.
5 Roof line: The roof line is the total hight of the car.
VERY IMPORTANT: These proportions are not fixed, they are DIFFERENT IN EVERY CAR AND CAR TYPE, these examples only help illustrate what proportions are all about so you what the idea is. After understanding these ideas you will be able to draw any car by analazing the proportions for that specyfic vehicle.
Four Door Family Car
Here are the general proportions for a family car (the front pinting to the left) :
1 Ground clearance: This is about half a wheel but it can be lower.
2 Wheelbase: Of course three, but since there are many types of four door cars the wheel base can be very long, almost four wheels in some case like in big luxury cars. This is more of a mid size car.
3 Overhangs: At the front is about half a wheel and at the back can go from half up to one full wheel depending on the truck space.
4 Belt line: This is about half a wheel.
Finally, notice the light blue vertical line behind the front wheel. By drawing this line you can have reference of where the winshield starts, or rather the base of it. Of course, it´s diferent for all cars but I put it at about one third of a wheel because I like the long hood look but you can put it closer.
Big Truck/SUV Proportions
Prfoportions for pick up trucks and SUVs are very similar. In fact, many SUVs share platform with a Pick-up truck.
1 Ground clearance: Three quarters of a wheel.
2 Wheelbase: Three wheels work fine unless you’re drawinf a crew cab truck (four doors ar big cabin), if that’s the case the wheelbase can grow to four wheels.
3 Overhangs: Half a wheel at the front and up to one wheel at the back. For SUVs the rear overhang isnt wuite as long, maybe three quarters at most.
4 Belt line: Two thirds of a wheel.
5 roof line: One wheels and a half but it can be a little more if you want.
The light blue line for the wind shield is closer to the front wheel. Pick-up trucks don’t have long hoods (or bonnet).
Sports Car Proportions
1 Ground clearance: Since this cars are very short the ground clearance is minimal. About one quarter or less. Actually, the lower, the cooler the car looks, most of the time.
2 Wheelbase: In this case, this three wheel length is quite acurate for the average sports car. There are just a small bunch of cars that have a longer wheel base, most of them super high performance cars, like the Venom GT.
3 Overhangs: In sports cars, the front overhang is usually longer. About one wheel. The rear overhang is short, about half a wheel or less. But take a look at some cars, like the Jag XJ220 or the Saleen S7 and you’ll notice that the rear is very, very long. This is to accommodate a big difuser.
4 Belt line: This is about one quarter of a wheel, more than that and the car looks very weird. Low belt lines look better in sports cars.
5 Roof line: The roof isn’t usually more than one wheel.
In this case there´s no windshield line. Because the cabin in sports cars are very forward in the body, the best reference is the top of the front wheel. So draw the wind shield from that point.
That´s basically what proportions are all about.
But I know what you’re thinking: Hey! How come a pick-up truck is almost the same height as a family car?!
The answer is, of course, that they aren´t. The key here is the size of the wheel. In the image below you can see a small hatchback, with it´s references in green, and in the background there´s a faded SUV truck with it´s proportions references. The wheels on an SUV are slightly bigger (sometimes a lot bigger), you can see this at the rear of both cars. This diference in wheel size makes the hatchbak evidently smaller.
This is a very important thing to take in to account.