3:73 or 4:10?

those are 4.10's.
i didnt change my speedo immeadatley and it was just that.

get it changed asap. it counts your miles on the car about 15% more than what it actually is

As far as determining the rear gear ratio, I read this in a 4x4 magazine(could be somewhat difficult on our Stangs since they sit much lower to the ground- on a lift would be perfect)...Mark your tire at the very center where the tread meets the ground, attach a string to your driveshaft so it hangs straight down(matches the tire mark), rotate your tire for 1.0 revolution, count how many times the string rotated around the driveshaft. that will be approximate ratio. If the string wraps 3 3/4 times around the driveshaft you have 3.73s, if just over 4 times around driveshaft you have 4.10s.
I didn't have my tuner with me when I went from the factory 3.31s to the 4.10s so I had to drive 35miles home to get it. My speedo was WAY off!!!!! It was showing 85-90 MPH at around 2500 RPM in O/D and people were flying by me on the interstate!!!!! My guess was I was doing around 65-70 MPH. I tracked my odometer over an 8.0 mile stretch according to the roadside markers...my odometer showed 14.8 miles. After the change with the tuner, I went back out to see the difference. At 2500 RPM in O/D my car now showed 68-70 MPH and was dead on over a 5.0 mile stretch.