For steering a car, the effect of steering is only experienced as a function of change in position. So when the car moves forward, try altering the direction of travel by a delta multiplied by speed, each frame. If the steering wheel is straight, the delta is 0. If it is left, it is negative, right is positive. This way rotation is dependent on motion. Each frame, rotate the car by delta*speed, then move forward in that new direction.
If you are using just keys for steering, and not the mouse, you can smooth the steering wheel with this:
decayFactor = 0.95
decayScale = 1.0 - decayFactor
delta = delta * decayFactor
delta = delta + decayScale * delta0
...where delta0 is the result of your key presses (-30, 0, +30 for example). The closer to 1 decayFactor is, the slower the wheel will respond. You could apply an identical filter to the speed to give the car some inertia. When you press the up key, it will accelerate forward. When you let it go, it will coast to a stop. In addition, if you wish it to start and stop more accurately with both kinetic and static coefficients of friction, you can switch between two different decayFactors for the speed, depending on the current speed. Your main decayFactor for speed would be close to 1. But any time your speed is below a threshold (near 0), use a lower decayFactor instead that is not close to 1. This would be more for slowing down and then stopping than for accelerating. You could use it just for stopping if you wanted.
Not sure if the BASIC syntax is accurate, but here is the psuedo-code at least.
angleY = angleY + delta * speed
yrotate car, angleY
moveforward car, speed
Or in place of moveforward psuedo-code, you could use Attila's sin and cos as mentioned above to calculate the new positions.
P.S. Your decayFactors should be based on your framerate, since each it's repeated per frame. To convert from it in seconds to what it would be per frame, it's the decayFactorSeconds raised to the power of (1 / FPS):
decayFactorFrame = decayFactorSeconds ^ (1.0 / FPS)
An example of decayFactorSeconds would be that if you set that to 0.2, then at the end of 1 second, your speed would be 0.2 what it was.
Q: ...you mean computer imagery was still based on the paradigm that the world was flat? Even into the 21st century??? Talk about doing something the hard way!
A: Yep! Back then people would render simple shapes with complex meshes of thousands of flat little triangles. Next to the bottleneck processors they used, it's the main reason why their computers were so slow. In the last days of the religious atmosphere of centralization and trade, corporate dogmas had people believing that flat was faster.
