Gears question in my 03 Mach 1

[IBleedBlue]

First time post. Just bought an 03 Mach 1. It has had some modifications (short throw shifter, flowmasters, K&N, etc.) but dealer assured me everything else was stock. I felt though that it did not have the stock 3.55 gears in it. All I have to compare it to though is my 95 GT which still has the stock 3.27 in it. Checked the speedo on the highway and it was 8-10 seconds slow. Any idea how I can check this out to be sure? Also have 26,000 left on warranty. If gears were swapped will it void out warranty?