Just a sillly question (although Sister Kevin in 4th grade used to tell us that no question was silly...damn her)

Recently my runs seem to feel easier even though I have been at this for just about 6 months.  What made me notice was on Wed I ran on the treadmill for 7 miles at a 9:15 pace and that seemed much harder than todays 7 miles on the road at 8:45 pace.  I know there are several factors, one being the temperature (basements are always warmer than outside), and calibration.  My treadmill is about 10 years old and I've never calibrated since purchase (don't even know if you can).

Anyway, just a passing question as to the accuracy/chaning accuracy of the GPS function relative to speed and distance, and could Garmin or Obama (the govt) change the algorithims that drive the data that we all feed on?

Treadmill pace always feels harder to me.  8:40 pace on the treadmill takes a little work.  8:40 pace on the road is pretty effortless.  Assuming my legs are fresh in both cases.

Regardless of what treadmill I am using (gym, fitness center at work, YMCA, a hotel), I use the readings from my footpod and watch, so I know I am getting a consistent reading each time no matter how uncalibrated the treadmill is.

GPS is more likely to be short rather than long, especially if your run has lots of turns (as in a mountain trail).  But even so, it is not going to be off much.

Interesting side note:  I have a Garmin 205 that I used for about 3 years.  When it when belly up I got a 210, but sent the 205 back for rehab.  Once I had both watches, just for chuckles, I used both on a roughly four mile run.  The older 205 read 4.15 miles, and the newer 210 read 4.05 miles.  Go figure.

All other things (rest, hydration, fuel) being equal, running outside always feels easier to me than running on a TM. I haven't yet noticed a discrepancy between the readings given by my 210 and iPhone apps like RunKeeper or MapMyRun.

My only problem is when I"m doing a route I've done 300 times and the same Garmin starts saying I'm 1/10th of a mile behind.  It is minor, but it screws up the pace (makes me slower).

 

I'm no math wizard but my calculator says that's a variation of only 264 feet in 7 miles, or about  7/10ths of a percent. That's close enough for a training run. My measured straight out and back 10k course with a clear view of the sky is usually repeatable within 1 or 2 1/100th's of a mile. I run it as 6.21 miles when I'm running it for time, just to be sure.

Consumer grade GPS watches are good enough for what we need, but are not 100%.  And that's why they will rarely measure a race course for the exact distance.  I actually did some GPS art earlier today on a frozen lake, and I ran in the same exact path to trace out some words several times.  Yet, the finished product showed my path moved by several feet each run.

You might find this article, In GPS We Trust, an illuminating comparison of the relative accuracy of GPS and course certification.

GPS is accurate enough for everyday training but not accurate enough for a race.  People seem to think that a GPS is infallible because it's electronic.  Go figure.

Yeah, don't get me wrong, I love my GPS, but I know it's not 100% accurate.  It does bug me when people are all like "the course was advertised as X but my watch said Y, so the race director LIED!!!" and then they flip out.  A lot of my local courses have twists and turns in them which I know affects it, so if I run a PR on a course that measures 3.07 miles on my GPS, or 3.25 miles on my GPS, I count it.  They're not perfect.