I don't know. It wouldn't be too hard to try and work one out yourself.
Grab some blueprints of new cars (of whatever type you're after, sports/saloon/MPV etc), measure stuff like the diameter of the wheel, then find out how high/wide/long the cars are in relation to the wheel.
Do that to a few cars of the same type and you may see a pattern emerge.
Follow the lines front to back, they rarely break the curve and usually end up at a similar point (lower window, back/front bumper etc.) Also, every surface has curvature on atleast one axis.
I saw something once that used tires to measure a car's spacing kind of like the pictures we see using the human head for body proportions. But I'll be damned if I can remember where I saw it.
Positions and size of the engine, gearbox, radiator, wheels. Driver and passangers positions also consider visibility. Think which lights it needs and where. How the doors, bonnet and boot open. Exhaust layout (if the car is rear engined theres no point to have side exhaust for example). Brake, radiator, oilcooler, intercooler etc. cooling (where it gets the air and where will it dump the air) same for air intake for the engine. Make sure the wheel wells are large enough for some suspension travel and wheel turning. It wouldnt hurt to consider what kind of suspension setup it uses an where and how it is mounted.
If you want to go one step further you should check if the position of the lights etc. comply with regulations, if its viable to manufacture the car the way you designed it or perhaps you should cut the body up to create more panels, can the passangers get in and out of the car safely.
Also it wouldnt hurt to get little insight of how air works, but you cant take it very far unless you have accsess to wind tunnel as its mostly trial and error method.
Only proportion that matters is how the driver and/or passengers will fit in it, Engine and Movement related (often wheels but depends on what the theme is)
I mean you should go to the sketchbook website they had some good car concepts on there a while back. Might be worth checking out. There's also that Frankfurt Autoshow that's on right now, again another good place to checkout.
funny, I looked into this a couple of months ago by doing what mop suggested. From the side profile, current cars appear to have the A pillar (windscreen) pointing to the front axle, the B pillar (central column in a four-seater) pretty much inline with and on roughly the same angle as the driver's seat. The C pillar is usually where the design styles differ. i.e. Sedan / SUV = passenger door terminates over the rear axle in family models. (here are a few shots of mine for example.) The tail of the C pillar (and rear window) tends to terminate at roughly the same place the rubber does. Sports cars have varying mergings of B+C pillar which are usually delineated by follow-through from the apex of the roof line (driver's head) and predominantly has the leading edge pointing to around the rear axle, and trailing edge well above the rear axle - this is what makes a sports car look sleek or agressive i.e. Lamborghini countach / gallardo.. Ferrari 360, Lotus Elise / Exige. Small Hatch = Door cut points to rear axle, C pillar commonly starts over rear axle and lends support for the rear door. As Erol said check the Blueprints at www.suurland.com for the mercedes A class, C class, and Mclaren F1 (the leMans car - not the Formula-1 ..) they're examples of what I'm talking about. Of course the further away you move from the norm is where the signature style comes into play for each brand.
If you're trying to do something like Harald Belker's red minority report car, note that the driver's seat is so far forward that the vehicle lacks front mudguards to accomodate driver access which would not make it practical for public consumption. The newer egg-shaped designs kicked off by Belker's influence on Porsche in the '90s look like attempts to push out and round off the A and C pillars at their respective wheel and integrate with the bumpers. Start from there if you're aiming for the futuristic look.
The "Sketches and rendering" and Techniques -> "2D" subfora at www.productdesignforums.com seem to attract artists that are specialising in industrial design and implementation. Stuff like this .. and this.
and at www.cardesignnews.com under the portfolio tab you'll find some futuristic automotive student designs.
I'd start with making a wheels + seats template from some of the suurland.com pics and try sketching your own overlays.
the proportiojn are usually 2.5 to three wheel widths between the front and back wheel )Wheel including rims and tires. Car design news is a good place to look as well.
Replies
Grab some blueprints of new cars (of whatever type you're after, sports/saloon/MPV etc), measure stuff like the diameter of the wheel, then find out how high/wide/long the cars are in relation to the wheel.
Do that to a few cars of the same type and you may see a pattern emerge.
Then again, you may not.
In the meantime, try taking these blueprints into photoshop and do some measuring of your own:
http://blueprints.onnovanbraam.com/blueprints/cars/
http://www.suurland.com/blueprints_archive.php/
If you want to go one step further you should check if the position of the lights etc. comply with regulations, if its viable to manufacture the car the way you designed it or perhaps you should cut the body up to create more panels, can the passangers get in and out of the car safely.
Also it wouldnt hurt to get little insight of how air works, but you cant take it very far unless you have accsess to wind tunnel as its mostly trial and error method.
If you're trying to do something like Harald Belker's red minority report car, note that the driver's seat is so far forward that the vehicle lacks front mudguards to accomodate driver access which would not make it practical for public consumption. The newer egg-shaped designs kicked off by Belker's influence on Porsche in the '90s look like attempts to push out and round off the A and C pillars at their respective wheel and integrate with the bumpers. Start from there if you're aiming for the futuristic look.
The "Sketches and rendering" and Techniques -> "2D" subfora at www.productdesignforums.com seem to attract artists that are specialising in industrial design and implementation. Stuff like this .. and this.
and at www.cardesignnews.com under the portfolio tab you'll find some futuristic automotive student designs.
I'd start with making a wheels + seats template from some of the suurland.com pics and try sketching your own overlays.
Scott