2006 Voyager 2.4 manual speedo calibration
 

Hi All.
I am new to the forum and to Chrysler ownership. we recently got a 2006 model Voyager 2.6 with manual box. From the start the gears felt extremely long( eg on the hi way driving at 120km/h gps speed in 4th gear rpms are around 4000)
about a week ago noticed the speedo is much too slow compared to gps speed. for example where the gps reads 100km/h the speedo sits just below 80km/h

The question that I have is: will the gearbox from a manual diesel vehicle fit onto the 2.4 petrol engine without modifications? What else could cause the speedo to be that far off?

Just a note: These base models were never sold in South Africa where I am but some imports from neighboring countries are available second hand as people bring them along when returning from work contracts or similar situations, also we have some very shady mechanics wielding handtools at vehicles which is why I suspect a gearbox swap.

Any info will be greatly appreciated
Thanks

Does anyone maybe know the correct codes that should be on the gearbox and where I can look for it to compare to what it should be?