The T5 has only 2 options for the drive gear, blue or red. Blue has 7 teeth and red has 9. Use the drive and driven gear calculator to find the combo that will work best using available driven gears.
In your situation, switching to the blue drive gear is probably best.
For some folks, using a ratio adapter might work. I have several that I pulled off of older T5s. IIRC the adapter is a 1 to 0.72 ratio change, so I think it would slow down the speedo.
Fewer teeth on the drive gear = faster speedo reading.
Fewer teeth on driven gear = faster speedo reading.
More info
LINK HEREJay
