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Hello everyone

There are some strange things concerning my GPS. I am doing a task in a project relating to connecting a GPS module Leadtek LR9552 to java software and parting its fields showing them on gauges on java.
But I found 2 strange things:
1-The messages stated in datasheet of NMEA183 : GGA,RMC,GSV,GSA, VTG but what I have found actually was VTG message is missing.
2-When I tested the GPS with the java software to examine speed accuracy, I found out that the speed is not accurate regarding the actual speed of the car (although the DOP was 3.3), and a delay of 30-90 seconds in reading existed too.
I have read that GPS is more accurate than car speed indicator, so I really do not know if the GPS module is of bad quality or what.

I can upload the java program here, so that you can test it on a better GPS.

Can anyone help me?

I looked up the Leadtek device and it seems to be an OEM device. There's not much information about it.
Also I can't tell how you have built the device - the leadtek website indicates it is an OEM module that you
have add power supplies and such.

I use a Garmin Nuvi 255W and the speed is quite accurate, keeping up within a few seconds when my speed changes.

Yes, we added to each pin the configuration assigned to it. And we have added the required power to the module.
Excuse me, but what does OEM stand for?

Can I ask you a favour ? Can you test my java application on your device (if it supports NMEA 183)? It will receive the data from serial port. just tell me the com port no to adjust the program to read from it, and I will attach the application here.

OEM = original equipment manufacturer.

So how are you determining the "actual speeed" of the car? If the comparison is only between the GPS and car speedo then you really have no way of knowing which one is right and which one is not.