Another speedometer question

[g-man]

I installed a speedometer in my s13 240sx that has a s13 sr20det engine. The new speedometer goes up to 280kmh and was a direct plug in replacement for the stock one that also read in kmh. The stock one worked fine but the new one when you turn on the engine it immediately reads 20kmh before I even move. Any ideas what the problem could be since I was told this speedo was a direct plug and play deal thanks.