Thanks for that clarification, Chris. One more for you then... If my truck thinks it's going 61 when really it's going 59 will the odometer be off too? I read that the odometer isn't affected by the speedo, but if the truck thinks I'm going 61 it seems like the odometer would be racking miles up quicker too.

 

The ODO & trip meter all work off of the VSS.

 

For sure, and so lrudduck, yes the odometer is off by that much. Sorta. You see, due to several previously mentioned factors, at different speeds and different temps those readings will vary.
Say your average speed is 42. That would be very high. Sure you go 70 on the freeway, but in the life of the truck most will spend a lot more time on city streets, traffic jams, around your neighborhood at low speeds.

So at 42 it may be much closer. If you adjust it to be perfect at 59 you may be moving it off at 24 where it is on the money on a cold day.
Most people start obsessing about how they might loose 3 % of their 36000 mile warranty. In actual fact, if you drove it mostly around town you might find that you are gaining distance on the warranty.
Cold tires, low speed and the error usually goes the other way.
Now if you wanted to fiddle with your tire size to match your edge to your GPS you could probably get it to under 1 mph, which would be very good.
At one certain speed that is...

Why don't you do a check at exactly 20-30-70 on the GPS.
Results might be interesting.
I've done it with my ScanGauge. Interesting curve. My speedo is on at 36 mph, tires cold. By 70 tires hot it's off 3 mph on my Scangage, 2 on my needle.
Chris

 

Well with the Edge tuning in...it and my GPS showed the speedometer reading about 2-3mph faster. I'm pretty sure I've got the Edge settings right, but mine likes to reset my gear ratio to 3:73 from 3:55. But I remember using my GPS on a prior trip with stock tuning and it read 2-3 mph slower than my speedo, so I think I'm really close to where I should be.

 

Cool, thanks for all the help guy's I guess I will use the Edge's MPH display as the more accurate measure of velocity.

 

That's what I do, I find myself not looking at all at my cluster. My edge shows my speed, engine oil and coolant temp and transmission temp. No real need to look at the cluster anymore.

 

I spoke to Bill at Edge regarding this some time ago and he said that their truck and every truck they tested on had the speedo read +2-3 MPH over what the PCM and the Evolution said the speed was. Just the nature of the beast, I guess. Anyway he said the PCM and Evo were always within 1 MPH of each other as long as the Evo is correctly calibrated with the actual tire size and the correct gear ration for the truck.