My name is Deysha from Ford Customer Service. Like duckbubbles mentioned, this is an upshift light. It illuminates when it is best to shift to the next highest manual transmission gear to maximize fuel economy. You can find this info in the Shelby GT500 Supplement guide. This is not an option you can disable.
Actually it can be disabled in the program.
08 Shelby GT500 Convertible - Torch Red / White Stripes
#2009 of 2070 verts. - 453 of 463 Torch Red - 5/22/2008
Roush Exhaust, Ford Racing Short Throw Shifter, Ford Lowering Kit,
Light Bar, Sequential Tail Lights, all the GT500 options.
RWHP - who knows, it's fast enough.