Speeding with the Garmin

So I was on a long drive, and while the speed limit was 65 mph, I was going 75.  My arrival time said 10:15pm, and I wanted to know how long I would have to drive for that 10:15pm to decrease by 1 minute.  This is how I figured it out, with the result being

"Driving Time" = "Speed Limit" / ("Speed Limit" - "My Speed") * "Change in ETA"

So given my circumstances, 65 / (65 - 75) * (-1) = 65 / 10 = 6.5 minutes.  So after 13 minutes of driving, my ETA would be at 10:13pm, for example.  (By the way, this is actually only valid for a single road's speed limit.  Once I left the highway, I would have to recalculate.)  If I were going 85 mph, then it would have decreased every 65 / 20 = 3.25 minutes.  

The easiest way to calculate it in your head is as follows:

1. Take the speed limit (e.g., 40mph)
2. Divide by how much faster you are going (If you are going 50mph, do 40 / 10.  For 60mph, do 40 / 20.  etc.)
3. That's how long (in minutes) it will take for your GPS's arrival time to decrease by 1 minute.

To see how much of an effect traffic has, you can do the same thing:

1. Take the speed limit (e.g. 40mph)
2. Divide by how much slower you are going (If you are going 30mph, do 40 / 10.  For 20mph, do 40 / 20.  etc.)
3. That's how long (in minutes) it will take for your GPS's arrival time to increase by 1 minute.

So for people who get annoyed at drivers going 35mph in a 40mph speed limit zone (me included), you would have to be behind that person for 8 minutes before your arrival time is affected by a single minute.

Just some food for thought.