I haven't been able to get very good pix while in a car thats driving at about 75-95 MPH -any advice what kind of settings to use?
How fast should I set the shutter speed in proportion to MPH?
This is a bit of a tough question, as it depends on several factors. I would in fact say that there is no possible general answer to this question.
When moving at typical vehicular speeds, the shutter speed is selected to stop motion, in this case, the photographer's motion. When you're moving at these speeds, objects closer to you appear to be moving faster than further away objects. This phenomenon is known as parallax. In addition, it would depend on whether the objects are moving to your side (like the landscape), or are more or less moving towards you (like cars in front of you).
In addition, the higher your camera's resolution, the more noticeable shake and/or motion blur will be when viewing the image at full resolution. However, for a print of a given size, shake and/or motion blur should be the same regardless of sensor resolution.
It is possible to trigonometrically calculate the apparent motion of an object at a given distance and thereby select the shutter speed based on the acceptable amount of motion blur for a given photograph, but given that you're probably going to be well past the object by the time you finish calculating, there isn't a lot of point.
So in conclusion, for close or medium distance objects, I'd try to take photos with high (1/500 seconds or less) shutter speeds and keep going higher or lower till you find an acceptable setting. For far away objects, you can go pretty low, possibly all the way down to 1/50 seconds. Eventually, you'll learn to judge the shutter speed fairly accurately depending on your sense of motion; but it will require practice and deliberate experimentation to develop that judgement.
It has been 7 years since I wrote this but I think it still applies. The main point is to crank up the shutter-speed as high as possible and take lots of shots, probably in continuous drive mode, to increase the chances of getting something usable.
There is no direct correlation between shutter-speed and movement speed because it depends on relative velocity and angle-of-view. In other words you will need a faster shutter-speed to shoot at a close-by subject than at one that is far away. It also depends the direction of movement and the focal-length you are using. The longer, the lens, the higher the shutter-speed will be required to freeze motion.
Regardless, by my estimate at 60 MPH+, you should be using close to the top shutter-speed of your camera. Most DSLRs do 1/4000s and some go to 1/8000s. Smaller cameras can reach even higher shutter-speeds, up to 1/16000s for mirrorless and 1/40000s (yes fourty-thousandth) for compact cameras but chances are you cannot compensate enough with aperture and ISO sensitivity to get a well exposed image.
Two things are influencing your need for shutter speed.
First, there's the increase in shake induced by the motion of the vehicle over the surface of the road. If your lens has additional features for stability in moving vehicles (more recent Nikon lenses have this), then you want to activate that.
Second, the motion of the vehicle. So, this really, really, depends on your relationship to the subject:
Far away and large like a mountain way off in the distance is unlikely to change much in appearance during the exposure, at least from the perspective of camera. You could be as low as 1/30 for that, experiment a little.
Close to you like a person at the side of the road is going to see a position change fairly quickly relative to the car. Your options there are to push up to a really fast shutter speed (say 1/800 to start, maybe 1/1200 or more) or pan your shot as your vehicle moves, keeping the subject in place. A lot will depend on your distance, the movement of the subject, and the light.
There's a lot of variance between the two relationships with the subject distance, so I don't think there's a good hard and fast rule there. I would start with these and experiment a bit. I think with that you'll start to generate a feel for what works with your camera and skills.
Honestly, the biggest thing is probably that shooting from a moving car not only means that other things are moving, but that the vehicle is shaking, probably quite a lot. We get used to traveling in cars a lot so we tend to not notice it too much, but you are bouncing around a lot. A stabilization rig or a lens with lateral image stabilization would do a lot for helping with the jostling a camera gets in a car. It is worth point out that a while a typical Image Stabilizing lens may help a little, they are designed for camera shake from holding which tends to be angular (tilting the camera) more than movement up/down/left/right.
When you factor it out, at any kind of significant distance, the amount of camera shake induced by a car is going to be comparable to quite a bit of speed of movement even though it doesn't have to move all that far.
Either way, without Image Stabilization (and even with it really) the general answer is still the same. Shoot as fast as the available light will allow. If changes to distance are an issue, then either set to manual focus and take the picture as it moves in to focus or try to minimally close the aperture to allow for adequate focusing. It's going to take a bit of guess and check work since the distance and speed and quality of your AF are going to alter how good of a job your camera does at holding focus on a wide open lens.
Also, make sure that you are using the feature of your camera that will follow the target being focused on if you have one (AI Focus or AI Servo on Canon bodies, don't know the name on others.)
The requirements for shooting from a moving car/train/motorcycle vary depending on which direction you are shooting. If you are shooting straight forward through the windshield, or straight backwards through the rear window, then the majority of items in your viewfinder are relatively motionless relative to your camera/sensor. If you are shooting out the side windows, or even out the front or back at a large angle to the direction of travel, then things very close to you are moving at very high speeds.
When you are shooting straight ahead, the distant scenery is essentially motionless. As is the car or truck going the same direction and speed as you. Traffic going the other way will be traveling at twice your speed, and will not stay in the frame very long.
When you are shooting at right angles to your direction of travel, the far away distant scenery is essentially motionless, but items (telephone poles, bushes, parked cars) are moving past you at extremely high relative speed. No amount of panning will keep up with nearby things on the side of the road.
Obviously, with digital cameras, you can experiment as you go. I would not have high expectations that image stabilization will help a lot.