Speedometer stops working after about 10 kms driving, I see the needle slowly dying. Problem started when ABS light came on (after I changed the diff). My digital odometer doesn't work too, but works sometime when I disconnect and reconnect the battery.

Any idea?

 

This has been covered countless times. SEARCH.

 

I've searched...

 

There was a thread about similar speedo issues just a week or two ago, I posted a ton of information about things to check.

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/speedo-issues-1002933/

I also just searched the 3rd gen section for threads with "speedometer" and there were 86 threads, covering just about every problem you could have with the speedometer.