I bought a set of 2007-2009 Shelby GT500 wheels. The rear tires are Goodyear Eagle F1 285/40-18. Diameter is 27". I have 4.10's now. Is there a way to determine the new rear drive ratio? The speedometer is now about 5 mph slow. My Marauder wheels have the Nitto NT555 265/50-18 which has a tire height of 28.4"