Phone Lap Timer MPH Accurate?

So at yesterday's time trial I used Track Addict; it was super easy to use and worked no problem. 

I used a phone mount clamped to the passenger seat cross bar (no passenger seat in my car).

The one thing I did notice is the MPH the files displayed at the end of the straightaways to not jibe with the MPH I'm getting based on the tachometer readings (I have an extensive gearing chart). 

I'm seeing a 2-4 mph difference. Is the phone GPS not able to keep up or is there some other reason for the descrepency?

Tom1200 said:

Is the phone GPS not able to keep up 

Yes, phone GPS only updates about once a second.

I use an external GPS antenna to address this. It's bluetooth to the phone and updates at a faster rate (0.10sec.).

Dual Electronics XGPS160

In reply to Tom1200 :

I don't understand how you can drive all out and worry about speed differences?
I have no speedometer in my race car.  I scan the  Tach, oil pressure and water temp. I don't care if I'm doing 150 or 155 as long as I'm gaining on whoever is next.      

In reply to frenchyd :

Because there is nobody to race but yourself in a time trial. So knowing where you are gaining or losing speed is important to be able to combine the fastest parts of your laps into 1 even better lap

I don't know that I would trust either of those numbers all that far.  The ECU knows the engine speed pretty accurately, but there are often errors when driving the needle.  Also, translating engine RPM into vehicle speed is made more complicated by tire slip and the tire diameter increasing slightly as it spins faster.  GPS combined with accelerometer data has the potential to be more accurate at measuring speed, but the phone hardware isn't really designed for it and so has other problems.

The short answer is that if you really want to know you need to buy a logging system such as an AiM.

In reply to frenchyd :

It was more of a curiosity than anything else.

I use the tach as the JG once called the redneck Motech.

My 80whp is all about RPM management; I'm watching the tach everywhere..........100 rpm in my car is worth like 2-3 tenths.

I had one lap where I got a slight draft and it was 4 tenths faster then any other  lap.

In reply to Tom1200 :

in our small underpowered cars little things add up. 2-4mph may not be bad, depends on percentage. 2-4mph at 110 is a lot different than 2-4mph at 60.

The bigger thing is consistency. if its always the same you can use the data. 

red_stapler said:

Tom1200 said:

Is the phone GPS not able to keep up 

Yes, phone GPS only updates about once a second.

Yep the speed and heading readings really suffer from this and are very laggy, track position readings (and lap times) are surprisingly good though. I did a little TrackAddict review here:

https://grassrootsmotorsports.com/forum/grm/trackaddict-users-best-phone-orientation/208168/page1/