Yes, your tire diameter is 28.9”.
I use
THIS TIRE SIZE CALCULATOR which makes it simple to plan ahead.
To pick the correct DRIVE gear and DRIVEN gear combination, I use
THIS CALCULATOR. . Part way down that page is the SPEEDOMETER GEAR CALCULATOR section. A Chevy T5 can use a blue 7 tooth or a red 9 tooth DRIVE gear.
Since DRIVEN gears only come with limited choices of tooth count, you might need to use a blue 7 tooth DRIVE gear in order to find a matching DRIVEN gear.
Example:
Your current red 9 tooth DRIVE gear with a 3.90 axle and 28.9” tire will require a 24.47 tooth DRIVEN gear. (So either a 24 or 25 tooth.) Unfortunately, there is no DRIVEN gear available for a T5 with that tooth count. Top tooth count is 23 teeth.
Recalculate using a blue 7 tooth DRIVE gear. Result is 19 teeth for the DRIVEN gear, which IS available.
DRIVE gears and DRIVEN gears are available from
LA SPEEDOMETER GEAR. 
