Mark Foster and
The Dervman has some interesting data about his KA On his website. It compares engine load, and fuel consumption in both 3rd gear, and 4th gear. From what I can remember I think 3rd uses a negligible amount of extra fuel, but 4th uses negligibly less fuel. If the gradient increases just slightly though the load in 4th becomes a lot greater, and I think the fuel consumption becomes a lot higher. So in an ideal world with perfectly flat roads etc 4th would probably be slightly better, but bearing in mind that it's not a perfect world 3rd will probably work out just as good if not better in most situations. That's my understanding anyway, but the Dervman will be able to correct me if I'm wrong.
I did a little (and very unscientific) test in my car yesterday. On a relatively flat road I did 30mph in 4th, and then changed to 3rd without even moving the gas pedal between changes. I found that the speedometer didn't even change by 1mph, but then soon after I can to a hill. It's nothing conclusive, but I suspect that fuel economy is very similar to the Dervmans findings on his KA.
In 3rd gear there obviously are other advantages too, more torque-acceleration, it's easier to slow up, and easier to maintain a steady speed (which is mainly what the IAM were getting at).
