Jeff Goeke-Smith

November 14, 2021

Road Trip travel time planning.

Having just got done with a trip to Montana, let me inform you of a planning detail.  The map systems will tell you this is a 24 hour drive, and reasonably, you might think that means you can do it in two 12 hour days.  You are most likely wrong.

The trip planning software all assumes that you will be going the speed limit the entire time you are in motion.  And for spaces east of Iowa, that's not even a bad assumption, but starting in South Dakota on my trip, the speed limit is 80 Mph on non-urban interstates.  I was not going to be going 80Mph.  It was just uncomfortable in my truck.

Let's talk about a related subject for a moment, fuel economy.  For most vehicles at highway speeds, the dominate force in consuming energy is aerodynamic drag. All other things being equal, the energy necessary to push through the air is a function of the square of the speed.  So, for example, if I have a truck that gets 21MPG at 65Mph, what would it get at 80MPH?  First, find the additional factor of energy needed.  (80/65)^2= 1.514. So I'm going to be using 51% more energy.  Let's see what that does to my MPG.  1/((1/21)*1.514)= 13.8 MPG at 80Mph.  Which experimentally, is accurate.

Attempting to hold a speed of 80MPH was getting me about 14MPG, and more importantly, very uncomfortable. To get the engine to do this, the engine must be in its power band, which is at about 4000 RPM.  It's loud and roaring.  This reduced my max speed on many of these highways that are marked at 80MPH to something more comfortable, 70-75MPH.  And I wasn't alone.

What this implies is that your 12 hour transit time might be something more like 15-16 hours, and that makes for two very, very long days of driving.  I don't think I'm going to do the drive like that again.  If you are planning this drive, find a spot where you can go 80MPH up hill in your vehicle, and ask yourself if that would feel comfortable for 12 hours straight. 

A supposition..  One thing I noted while I was out in Montana is the prevalence of Class 2b and 3 trucks as being the default.  I wonder if there's a practical value there, where having a truck that large is easier to hold at the speed limit, because the larger engine for hauling, also means higher power output in the efficiency band of the engine.  High enough that holding 80MPH in a truck is practical and comfortable.