On Sep 14, 2013, at 8:37 AM, Don Hanson <dhanson928@GMAIL.COM> wrote:

> That is why we encounter very slow trucks on the downhills
> as well as the climbs....They've shifted to a low enough to hold their
> trucks from gaining speed...and thereby they can control the heat they
> create with their brakes.....

If this statement is about very large trucks such as diesel rigs, this would be less than an accurate statement. Excerpt taken for Wikipedia which is accurate enough for this discussion.

"Diesel engines do not have engine braking in the above sense. Unlike petrol (gasoline) engines, diesel engines vary fuel flow to control power rather than throttling air intake and maintaining a constant fuel ratio as petrol engines do. As they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects."

So, NO. Those trucks are not downshifting to slow down. They are using alternative mechanisms such as jake brakes. Ever notice that the diesel in your Vanagon & other VW's have an auxiliary pump to create vacuum for your brake booster? Same deal. 

BenT