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Discussion Starter · #1 ·
Okay so I've owned my 02 GT since about june of last year. About a month ago my speedometer stoped working. I found the problem was a dallas mustang calibrator. So today I removed the calibrator and correctly rewired the car harness back to the vehicle speed sensor. It works agian! But its reading the speed wrong. Obviously it needs the calibrator. But it no longer works. The previous owner of his car is the one who did this stuff so I don't know what to say I have to the calibrator/tune company.

This is what I know so far.
1. Need a calibrator/tune
2. It has stock rear gears
3. Same rim size and tire size as stock.
4. And a couple of other stuff that I can't/ shouldn't make me need a calibrator.

And this is what i need to know.
1. Can a more flowing exhuast make me need a calibrator?
2. What else besides what I've mentioned can make me need a calibrator or tune excluding engine work.

I need to know what to check so I know what the calibrator was for so I can get a functional one or a tune so I caan have a functional speedometer.
 

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The only things I know of that would throw your speedo off noticeably are gears or a huge difference in wheel size. I have 35 sidewalls, different than factory, and there's no change in speed. I did have to install a new gear when I put in 4.10s though.
No, exhaust will not affect your speedometer.
 

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How much error is there in the speedometer reading vs. actual speed, in what direction--does the speedometer read fast or slow?

Is it a fixed mph amount (ex. always +5 mph), or a percentage (ex. always fast by 10%)?

You can verify the operation of the speedometer gauge by using the instrument cluster diagnostic mode to observe vehicle speed digitally on the odometer--the analog and digital displays should be very close, withing +/- 2 mph. If not the speedometer gauge may be bad, or the indicator placement is incorrect. Here is how to access and use the cluster diagnostic mode.

If the analog and digital displays match then there are just two common possibilities:

The final drive gears are not the stock 3.27:1 ratio;
The tires are not the same outside diameter as the stock 245/45-17 tires (25.6") ;

Another possibility is that the PCM has a custom tune that was modified for some final drive ratio other than that actually installed.

A final possibility, in which the speedometer would be "way off", would be that the transmission had been changed from an auto to a manual or vice versa. The auto tranny has a 6 notch reluctor ring, the manual a 12 notch ring. The manual therefore produces twice as many pulses per mile...
 

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Discussion Starter · #4 ·
How much error is there in the speedometer reading vs. actual speed, in what direction--does the speedometer read fast or slow?

Is it a fixed mph amount (ex. always +5 mph), or a percentage (ex. always fast by 10%)?

You can verify the operation of the speedometer gauge by using the instrument cluster diagnostic mode to observe vehicle speed digitally on the odometer--the analog and digital displays should be very close, withing +/- 2 mph. If not the speedometer gauge may be bad, or the indicator placement is incorrect. Here is how to access and use the cluster diagnostic mode.

If the analog and digital displays match then there are just two common possibilities:

The final drive gears are not the stock 3.27:1 ratio;
The tires are not the same outside diameter as the stock 245/45-17 tires (25.6") ;

Another possibility is that the PCM has a custom tune that was modified for some final drive ratio other than that actually installed.

A final possibility, in which the speedometer would be "way off", would be that the transmission had been changed from an auto to a manual or vice versa. The auto tranny has a 6 notch reluctor ring, the manual a 12 notch ring. The manual therefore produces twice as many pulses per mile...
It's fast by 5 mph till about 50 ex. On my gps speedo it reads 45, the cars speedo reads 50.
It's fast by 10 mph 50-90 ex. On my gps speedo it reads 80, the cars speedo read 90.
And fast by 15 mph 95+ ex. On my gps speedo it reads 95, the cars speedo reads 110.
 

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It sounds as though the speedometer is reading around 12 to 15% fast--this would indicate that perhaps the rear end has 3.73 gears and the Speedcal was there to accommodate that. You will need a working Speedcal or similar device (the universal SpeedoDRD works quite well), or a tune/tuner to correct the drive ratio in the tune.

You might want to jack up the rear end and count how many turns the driveshaft makes for one turn of a tire--to determine just what the final drive ratio really is...
 
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