Garmin Nuvi 3597 Doesn't Always Select Fastest Route Even Thought That Is What I Have Selected

This happens to me a lot and I use a 3597, but it happened to me on my 3760 as well. I'll make a left instead of a right sometimes and it recalculates and all of a sudden my arrival time is less than it was before. It makes no sense. I've fiddled with the settings, but nothing seems to chnage this behavior.
It will also give me different routes for the same trip with no rhyme or reason. Sometimes when I go home it tells me to take one exit, sometimes it tells me to take another. TrafficTrends is off and no traffic is on route for either exits. It's odd to me that presumably the same algorithm is calculating the trip differently, with change in variables that I can see.

Do note that Garmin Routing favors the least amount of left turns in it's algorithm.

Yes, but this is the same point A to the same point B. In other words, it will have the same amount of left vs. right turns. As a matter of fact, the exit I normally take to get home (and the GPS usually offers is:
At the end of the off ramp:
Left
Right
Left
Left
Left

The other way the GPS sometimes suggests is:
At the end of the next exit:
Left
Left
Right
Left
Left

So it's the same amount of left turns.

Maybe Garmin taught it's GPSes to flip a coin.

At least in my city, you want to make the least amount of left hand turns as possible. Due to heavy traffic, it is actually quicker to make a right hand turn and go what seems backwards

There are certain things you can do to minimize this erratic routing behavior on your 3597 and other GPS units as well.

Turn off "Traffic Trends"

Turn off "My Trends" (if the option is available)

Uncheck all "Avoidances" and remove "Custom Avoidances" unless you have a specific reason for having them.

Under "Optimize Route" select "On Request"

Clear "Travel History" frequently. This disables "My Trends" on GPS units which do not have a separate option to do so.

As a last resort, you can also uncheck "Travel History" which will disable "My Trends", "Where I've Been" and "Trip Log". I only do this under circumstances where routing is consistently in error.

This information was obtained by experimenting with the help of my brother. We used 5 separate, identically optioned 3597 units all navigating the same route at the same time. Although we were never able to get all 5 units to route identically for longer more complicated routes, the above settings produced the best results.

Two wrongs don't make a right, but three rights make a left.

Here's the deal. Your description is nothing new. Its been told in here over and over in some way or fashion. I can prove what you say is true in most cases where i live also. I come off the highway and go through 2 lights to get home. If i let the gps tell me its takes me to the next exit and i need to go through 7 lights and its also a main thoroughfare. Its also 8 minutes slower the gps way during the day. Thats just an example. I gave up long time ago trying to rely on it as far as the type of routing it does. In the end it gets me there. Its for when i'm traveling, it tells me how to get there. I don't know if its faster or slower. With out it i'm stopping at gas stations like in the olden days or using Maps. In the end 8 minutes isn't a big deal? Getting there is!

When I know where I'm going I don't use the GPS but do find it very handy when I'm in an unknown area.

Sure there's most probably a shoter/faster route but as Speed2 said the importance is getting there.

It's true that that the maps/POIS aren't always up to date even though you have a lifetime subscription & the routing is sometimes questionable.

That being said I certainly can't complain about my hundred or so dollar investment that still works well today & has thankfully always gotten me to destination

Excellent reply. I need to try that.