I am using the smartphone app MapMyWalk on my iPhone. I just took a walk today I mapped out here first, and arranged to meet a friend at the other end. Because gmaps-pedometer said it would be about 3.3 miles, I figured 45-50 minutes (I'm 6'2", so a 4mph is normal for me). However, it ended up being 80 minutes. And guess what? MapMyWalk, using GPS, tracked me as going 4.8 miles. That is a HUGE, and completely UNACCEPTABLE, difference. http://www.gmap-pedometer.com/?r=6211135. Even just near my house, gmap-pedometer tells me it is 0.43 miles to my nearest busstop, but on MapMyWalk, it is *consistently* 0.93mi ... nearly TWICE the gmap-pedometer estimate. [Interestingly, a walk of a distance between those two that I do regularly, clocks her at 1.1 miles and on MapMyWalk it is abou 1.2 miles, so very close.] Can anyone tell me what is going on? Bear in mind this is San Diego, so "overcast" days are rare, so it's not satellite interference. And, as I noted, simply because of the time it took me to do my walk today, the GPS app is far more reliable. There is absolutely NO WAY that a 3.3-mile walk took me 80 minutes. I could to that all uphill in less time.
I'm confident this site is not off to that degree. I don't get other complaints of mileage being off by close to 50%, and I'm pretty sure that I would if many users were experiencing that. The way this site calculates the distance is fundamentally different from how a device using GPS does, and are actually more likely to be accurate. The GPS points used in the calculation here are chosen by you when you enter the route, and the math is not subject to change as long as the points are basically accurate.
The points used to calculate distance in a device come from its GPS receiver, which can be highly variable in their accuracy. I have had the experience of using gps devices that will create a point randomly a good tenth of a mile off my route, despite the pointe before and after it being in the right place. From that one point I had a route that was off by .2 of a mile. And that was with a dedicated device designed to use GPS for fitness tracking -- the GPS accuracy of a phone is likely to be lower than that of a dedicated GPS device.
This is a pretty well-known problem with fitness tracking using GPS. This article contains detailed step-by-step instructions showing how to repair a route a cyclist recorded with his GPS trip computer, and has some graphics that illustrate the problem nicely. http://mechturk.wordpress.com/2012/11/20/howto-fixing-dodgy-gps-data-for-strava/ And this message board thread suggests that people get very different results from different apps; some find mapmyrun to be more accurate, some less: http://www.myfitnesspal.com/topics/show/567387-anyone-find-the-gps-on-mapmyrun-inaccurate
Interestingly, I used the online mapping tool that is available at mapmywalk.com, and the results were much more in line with those from milermeter: mapmywalk got 3.45 where your route here on milermeter was 3.28. http://www.mapmywalk.com/routes/view/361006407 -- Milermeter did come out shorter, but the two distances are a lot closer.
I know it's frustrating that the results vary so much, but I hope with this information you'll feel that milermeter's calculations are a bit more acceptable.
It has to be your phone app. I've never been off that far either. In fact, I compare the Milemeter maps to my Garmin 320XT and we're both happy, as well as the thousands of runners I've prescribed my courses to. It has to be something within the phone app. I'd be curious to see how your results are at distances upwards of 12-15 miles. I wonder if they'd "clean up" more?