lewellyn10

i bought a mustang gt and the speedometer is off. i put a gps in the car and found out that when i run 60 it says 80 on the speedometer. when i run 30 it says 40, and so on. later i was planning on checking the rear end to see if it was stamped differently than stock or i was even going to tear it apart to check the gears. i was wondering if anyone knew exactly what the problem was?

Sxynerd

It would depend on if your 96 had stock 2.73's or optional 3.27's but I would assume you would need the 3.73 speedo tooth gear. What RPM are you at when going 70mph?

You can also search google for "Gear calculator" and type in the MPH and RPM and it they would tell you.

cliffyk

Here's my Speed in Gear calculator, there's also a pop-up beta version of a What gears do I have? tool I have been working on, look for the yellow New icon in the top right...

Plug in a speed in 4th gear (because 4th is 1:1 with all Mustang manual trannys), around 60 is best, and what the rpm was at that speed, then click Calculate Axle Ratio. The tool will make a "best guess" effort to figure out what rear-end gears you have, and display the actual calculated ratio.

Oh, change the rear tire size too if needed...

Obsequious1

What gear ratio do I have?
Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

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What gear ratio do I need?
Differential Gear Ratio determines the number of times the drive shaft (or pinion) will rotate for each turn of the wheels (or ring gear). So if you have a 3.73:1 gear ratio the drive shaft turns 3.73 times for every turn of the wheel.

Gear ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. The higher the number, the lower the ratio: a 5.29 gear has a lower ratio than a 4.10 gear. With a lower gear ratio the drive shaft (and thus the engine) turns more for each revolution of the wheel, delivering more power and torque to the wheel for any given speed. Lower ratios are generally desirable when going off-road. Higher ratios are better for freeway driving since they run at lower RPM’s and offer better fuel economy.

Changing tire size affects the final drive ratio. Switching from a 30" tire to a 35" tire changes the final drive ratio by about 17%. This may drop the engine out of its' "power band" and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.

lewellyn10

okay so i used the calculator and it looks as if the rpm's match up with the actual speed as 4.10 gears. but what is the cause of the speedometer being off? the tach reads right with the ACTUAL speed, but the speedometer is still off.

cliffyk

The speedometer is driven by a pulse generator mounted on the transmission output shaft. The pulses are scaled by the PCM according to what it knows about the rear end gears and the tire size. If either of these change the PCM needs to be reprogrammed so that it knows the new values--in you case it sounds as thought that has not been done.

You will need to have someone create and load a tune with the new gear ratio (I understand Ford dealers can do this), buy a "chip" loaded with a proper tune, or buy a handheld or other programmer and do it yourself.

A fourth option, which I have never used so I am certain someone else could speak to with more authority, would be to purchase a "Speed-Cal" device from Dallas Mustang or one of their vendors. It costs $99 and works by scaling the pulse count from the speed sensor before it gets to the PCM.