iPhone GPS reliable?

is it worth me using my phone for a accurate speedo reading using a gps app of some sort, or are they junk? I've tried two and both the speedo readings seem to be a bit slow compered to my cars speedo. Do any of you guys use your phone for this or do you use a separate GPS device?

I use my phone GPS (android, not iPhone) and it is going to be the most accurate compared to speedometer. Any GPS app will be just as accurate as a standalone GPS unit. The only benefit to standalone is that it might be waterproof and won't need a cell signal to keep map updated. You can get offline GPS and map apps for the phone too.

I use Waze for most driving/nav. If I want to plan out a route and have my phone follow it, I plot the course using Furkot.com and then export the track to my phone and use Osmand app (offline) to navigate the track.

Car speedo are typically a few mph overstated on purpose.

It's a known issue that the speedometer on these bikes displays 7.7% lower than actual from the factory.  (55 MPH displayed means you're only travelling 50 MPH.)  While the speedo is off from the factory, the odometer is accurate.  Changes to stock gearing and/or tires affect accuracy of both further.

GPS apps will indicate the correct speed and distance travelled.  Here are a few speedometer apps for the iPhone. I'm an Android guy so I can't recommend one in particular for iOS.

The easiest way to correct the speed displayed on the WRR instrument cluster is to install a Speedo DRD speedometer calibrator from 12 O'Clock Labs.  Just bear in mind that 100% speed accuracy on the instrument cluster will result in the odometer indicating 7.7% more miles than actual.  There's no way to make both accurate at the same time, as they share the same sensor in the transmission and the displayed calculations are hard-coded into the cluster.  My personal preference is for accurate speedometer.

When I added a SpeedoHealer to my Stock WR, I had to program it -13% to get it to match my Garmin.

I use an iPhone and a Garmin Montana on my WRR with powered mounts for both. The Garmin is (as others have said) waterproof, dirt proof, etc. The maps and interface are cumbersome and rudimentary at best but I know it will work in very severe weather. My iPhone is in an Otter Box Defender and for the most part is impervious to mild weather - heavy rain/snow and all bets are off. The iPhone and its apps are far more user friendly and feature rich. The GPS speeds on my phone are very accurate. There are a number of great apps that provide routing, mapping and a speedometer. Here's a top list -

GPS MotionX - numerous features, easy to use and you can cache map tiles for out of cel range riding.

GPS Drive MotionX - as above but with navigational features.

ScenicMap - the entire map is on your phone, no streaming. This app has absolutely saved my ass in the back country. No speedo feature.

Avenza PDF Maps - geotagged pdf map reader. Excellent for MVUM's and the like.

Hope this helps!