Re: @John Brown - having a "feel" for your speed
Although I understand and agree with pretty much everything you say, my point was in reference to "constantly" watching the speedo and as the driver, you have much more feedback than a passenger. You know which gear you are in and you know where the accelerator is. Any sudden change is obvious since the driver caused it. Gradual change can happen but that's why the driver glances at the speedo for a fraction of a second every now and then.
This might feel different in an automatic, which I understand is the norm in the US, but here in the UK a manual gear box is the norm. I've driven an automatic and it does feel very different, but I only had it for one day so not long enough to acclimatise to the new experience.
For reference, I average about 1000 miles per week so my car is my "office". People who drive a few miles to work, then back at the end of the day and barely manage 100 miles per week might not be able to judge things so well.