Calibrate Speedo?

what is it i need one for my 94 cobra with 390's

MacDiesel said:

i know it has 18in wheels and i am pretty sure it has 3:73 gears in it.

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We can figure out your rear end gear ratio with a little math. What size are the tires (width/profile/rim), and what rpm does your engine turn at 45 mph in 4th gear?

MacDiesel said:

since my speedo is off this might not be acurate but we can give it a whirl. when my needle says 45mph in 4th gear the tach is at 2000rpm and when i am going what i think is actually 45mph the tach is at 2500rpm.

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2500 @ 45 mph with those wheels is a 4.30 rear. Congratulations.
2400 @ 45 would be 4.10's and 3.73's would be about 2200.

i drove beside a buddy and i figured that my speedo is off about 12 or 13mph. so when my speedo says 57mph i am really going 45mph. my rear tires are
295/35 ZR18. the width of the rims are 10 inches.

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For your wheels, uncorrected 4.30's would put your speedo at about 58 when you're really going 45. 4.10's would be about 55, and 3.73's would only read around 50 mph.

Another way to check is to set your cruise control at 60 mph and time how many seconds it takes to get from one mile marker to the next. With 4.30's it should take you about 77 seconds. 4.10's would be 74 seconds. 3.73 would be 67 seconds.

There's only one way of knowing exactly what gears you have and that's to jack up the rear end of the car, mark the driveshaft and one wheel and spin the wheel one full revolution, counting how many times the driveshaft spins...I put 4 equidistant marks on my driveshaft to make it easier to count...

As for that electronic box you posted, that's an Abbott ERA speedo calibrator...there's better ones out there than that...go to here: http://www.brothersperformance.com/shop/mustang/brospeedcal.html and get that one...it's much easier to install, no splicing of wires, and it's much easier to set with the dipswitches inside it...and since you have 18" wheels it can also account for that using this calculator: http://www.dallasmustang.com/index.mvc?Session_ID=fd1f5aa12601961f7a5c8c785914d220&s=speedcal (click where it says "Click Here to use the Dallas Mustang SpeedCal Calculator")