Speedo incorrect

[Highwood Malibu]

So I changed out my speedo gears in my transmission and according to the maths I should have a correct speedo, but I don't. I am still around twelve kilometers too slow on the dial. I was talking to a shop the other day which specializes in speedo's and they said that I could get the speedometer calibrated because they work on magnets and springs. Any idea how I could get this done at home? There looks to be an adjustment on the spring but I have no idea how it works. Ideas?

 

[Intragration]

12 km/h too slow at what speed? Are you closer now than you were with the old gear? What's your rear axle ratio? Speedo gear in the trans is the typical way of getting the speedo to read correctly.

 

[Bonnewagon]

Math only gets you in the ballpark. Best way to know is to get behind a friend in a new car going 60 mph and see how off you are. Have him put his flashers on so you know. Then adjust the trans speedo gear accordingly.

 

[Jim H.]

Just use your GPS the one I use in my car is right on the money with my Pontiac GTP speedometer

 

[Highwood Malibu]

I have a red drive gear (17) and a grey driven gear (44), rear end 3.30, tires are 25.75 inches in diameter. I fooled around with the tension adjustments today again and managed to get the speedo to read 30 km which was actually 50 on the road and around 60 km was actually 100 km. SO essentially my Km/h speedo is reading in miles per hour, but it still makes little sense as to why.