So my friend and I were debating about having 3.73's will affect the speedometer or not. He argued that it wouldn't affect my speedometer and I said it would. I called my gf's step dad who is an aerospace engineer and he said that having a different gear ratio would not affect my speedometer being off. But when I look at lethal performances it says this....
Superchips performance programming along with increases in horsepower and torque, removes most of the torque management. The Max MicroTuner allows the end user to custom tailor the rev limiter, WOT shift points and shift firmness level to the specific driving task at hand. You can also calibrate the speedo for rear gear changes in the Mustang and Marauder. You can raise the top speed limiter on most models*. Also, while we do not require the installation of a colder thermostat, the 1724 & 1725 provides the ability to set the electric fans to come on sooner if equipped.....So now I ask you my fellow mustang enthusiasts, having 3.73's, would that affect my speedometer being off or no?

 

yes it will, mine was about 10mph off, 4.10s are now b/w 8-20mph off depending on speed.

 

I've noticed about 3-5 mph off under ~50mph and it escalates to 10 mph off around ~60mph. Now my next question is how does a gear ratio change affect the calibration of a speedometer?

 

not sure what you mean by that..? but you can fix it by getting a tuner or computer that will fix it.

 

How do 3.73's change how the speedometer reads?

 

Gears are making everything rotate faster, such the speedometer will run higher. Depends on the type you have, not a wizz at all of that, some a gear drivin, etc, depends on your application.

 

its read off your tail-shaft of the transmission, it is past the gears so engine rpm doesn't matter but rpm and gear. Now that the last gear is shorter the rpms have to be higher to make up for the shorter gear. thus higher rpm in the tail-shaft and the speed reads higher. from my 2.73's to 4.10's my speedo was 50% higher then normal, but going from a 18tooth to 23 tooth gear it dropped to 17.5%

18/8=2.25 speedo ratio for 2.73's is correct for me and 23/8=2.875 for 4.10s is 17.5% error

4.10/2.73=1.5
2.875/2.25=1.27777777777

1.5/1.27777=1.1739
so 17.4% fast

now if i switch to a 21/6 which i have bought and plan to do one day it will be with in 1% error which is what you want. However you wont care about this since you are electronic speed sensor. they changed in 99, but pre 99 will have to do this.

you can find out what your error is by replacing my 4.10's with your gear, my 2.73's by your original gear and my 18 with your original speedo gear tooth size. the 8 tooth drive gear is common for gt's and the older cobras have 7 i think.

 

This^^

And if that was too complicated.... yes it will affect your speedometer, your mustang has an electronic speedo, correct using a handheld or Dallas Speed calibrator. Most who sale gears also sale calibrators.

 

changing any gear ratio will affect the speedo reading. the car's computer reads speed by how fast the tail of the tranny is moving (as said above). the whole point of raising the gear ratio is so that the tranny doesn't have to work as hard to rotate the gears in the diff, but at the same time, it must rotate more to move the tires one full rotation.

for example, with your gears, a 3.27:1, your driveshaft must rotate 3.27 to rotate the tires one full rotation. a 3.73:1 must rotate 3.73 times, 4.10 must rotate 4.10 times, etc. so by changing the gears, you are fooling the car into thinking that it is going faster than it really is. a handheld tuner or speed cal intersects the signal going to the car's computer and recalibrates it to make it right.

next time, don't ask an aerospace engineer, as mustang forums :lol:

 

Aerospace engineer vs mechanic. Mechanic wins