I’ve done a bit of testing today and yesterday and it might be helpful for someone if I post results here, also I don’t know about automotive engineering so if you see something that might be a bug please help out and report it as one.
K friction is a quite relevant stat for any vehicle, it seems to adjust how easy it is to control a vehicle, and to emphasize a lower value is better, there are a few ways to decrease your value that I’ve found, wider wheels (Roller drums > armor wheels > wide wheels > wheels > all other wheel types basically), larger diameter wheels (most wheels say their diameter in the name and a few in the description roller drums come out on top again), and more wheels wheels of a lower diameter than your maximum will remove a lot less friction, but they will still change the values I haven’t noted any other problems with different sizes but I’ve been adding my wheels after symmetry.
If roller drums are so great why does not every vehicle have them, well first there is the issue of placement, if you have a roller drum in a spot that spot is not usable for anything else, that will mean larger vehicles that have a harder time moving about in many places, then there is the fact that roller drums are heavy, unless you have a really overpowered vehicle design your vehicle will slow to a crawl whether or not it is overpowered it will be a fuel hog on account of the extra strain on the engine. But then again no other wheel can take as much of a beating as the roller drums so it’s really up to you.
A K friction of 70% is very easy to handle at 40% you can use the handbrake at speeds up to 350 km/h or 220 mph, unless the other values interferes with that.
I haven’t been able to figure exactly what K mass does but I do know that more engine per weight decreases that stat, and for K aerodynamics I havent been able to even change that stat so it does something or other.
For vehicle design I recommend having curtains on every window and non opaque door to be able to use your vehicle as an emergency/travel sleeping spot, hatches however cant have them.
For placement of doors I recommend having two non opaque doors near the drivers seat on each side of the vehicle, any extra doors should be further from the drivers seat (and each other) and preferably be of the opaque variety, the motivation for this is that you always want to exit and enter near the drivers seat in the case that you need to leave quickly, the other doors are emergency doors, to be able to enter and exit when stuck or hunted or in any other case where it is dangerous to leave or enter through the main doors.
Vision in vehicles is also aided greatly by wing mirrors if you see a wing mirror on the vehicle you currently use directly you see everything around it as if you were standing at the tile of the wing mirror however seeing one through another does not give this, anything outside the vehicle will not be able to see in this way either so close to the doors is a good option here too.
Onto cargo, living space and utility placement.
If you have curtains on all your windows and doors and do not use hatches a reclining seat is as good as a bed if not better as it doesn’t take up extra space.
Cabin doors and boards inside the vehicle just take up precious space usable for other things, removing a bed and boards etc can sum you 4 extra tiles to use if you use an RV for example.
Cargo is definitely best placed in cargo spaces (the item needed to construct them is called cargo carriers) and these have a good extra of not hindering your movement inside your vehicle and are a fit replacement for aisles, lit aisles can be replaced by using flash lights as aisle lights on the cargo spaces, it nets a little more weight than a lit aisle but for the space is definitely worth it.
If you can’t craft or find cargo carriers yet, it is a choice between more storage (trunk) or easier access (floor trunk) they can be placed with frame and sheet metal respectively, and it is up to your needs to make the decision.
If you have a controllable vehicle by now I recommend having your side and rear boards into stow boards, they do not hold much damage nor do they hold much cargo, that’s not the point though, they are there to shield eyes from seeing you when you sleep and to be able to move massive amounts of loot without walking back and forward into your vehicle, or lining your car up perfectly.
If you do not have full control of your vehicle though, the side boards are better replaced with heavy boards that can take a beating, if you are worried about zombies breaking your car when it’s standing still, replace all boards with heavy duty ones… but a car should be driving away from a beating or driving head on into it.
Utilities as forges kitchen stuff and welders can all hold cargo which is good to remember but don’t overuse it, it gets much harder to sort stuff if placed in other places than cargo designated areas (which does not include the stow boards they’re there to provide easy access for the most part), but be mindful of placing these so you can still access all boards from the inside (a stow board do not help with ease of access if you can’t access it).
I’d suggest not having wheels at the absolute front unless it is roller drums or armored wheels because they will take a beating, likewise put something in front of your wing mirrors to lower the chance that they run into zombies and have spare mirrors just in case.
And a last note, mufflers are good but from my experience of testing one will do for any number of gas and diesel engines and don’t help at all with electrical ones. More will just add weight.
Feel free to add, ask or just show off. 
And if I need to clarify anything, shorten the sentences or fix my grammar to be understood, please point that out.
My current 5x1 bike has 2 small electric motors and the pedals fit very well there. You’re right on really heavy vehicles the pedals aren’t so useful.