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Old 07-05-2014, 04:23 PM   #1
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Rear gears

Okay, so what do I need to do to get my speedometer working. I have a 1997 auto v6. Installed 3.73 (from 2.73). I have the purple 21 tooth gear installed and my speedometer is still off (9mph fast).

Before the gear change at 70mph I was dead at 2000rpms, at 70mph after the gear change I am at 2250 rpms, but my actual speed is 60. At 70mph, my car thinks I'm going 80, and my rpms reflect that. I used to shift into overdrive around 42/43mph. Now it's around 35 (actual speed). I talked to DallasMustang and they said a 23 tooth gear would fix my problem (got it today and was about to install, they didn't send me a retaining clip). I will try their gear but I doubt it will work.

If I buy a speed calibrator will it calibrate my shift points and my speedometer readings? If I buy the speedcal, what would the settings be on it?

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Old 07-05-2014, 05:10 PM   #2
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Go online and there should be a calculator you can find where you plug in your gears, drive gear, and tire size to get the driven gear you need.


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Old 07-05-2014, 05:26 PM   #3
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I would prefer to not have to change the driven gear. That requires dropping the transmission correct?

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Old 07-05-2014, 05:36 PM   #4
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There's a bunch of different ways you can go about getting the right tooth set up.
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Old 07-05-2014, 05:43 PM   #5
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I don't think the tooth is the problem though. I had a transmission shop (I trust) look into it and the guy said that the problem is in the computer.

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Old 07-08-2014, 09:58 AM   #6
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There isn't a gear you can run to calibrate for 3.73s, all SN95s with VSS use an 8 tooth drive gear and the most you can calibrate for with a Ford speedo gear is 3.27s which is the purple 21 tooth you have. You actually should have had a grey 18 tooth gear for 2.73s but w/e. Anyway, you'll need to get a tune in the computer or a speedcal. The 97 trans uses two sensors, an OSS for the shifting and a VSS for the speedo which is why your car is still shifting ok.

You do not have to drop the trans to do a speedo gear anyway, its right on the left side of the tailshaft but you don't need that. Since you have a 21 tooth purple gear in there your computer thinks you have 3.27s. Buy a Dallas Speedcal, install it and then use their chart to calibrate it for 3.73s with your starting gears as 3.27.
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Old 07-08-2014, 11:49 AM   #7
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That's exactly what I'm planning on doing. I have the speedcal coming in tomorrow or whenever. Where do I find their chart?

Thanks for the help Scott.

Edit: found their chart on the speedcal faq website.

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Old 07-08-2014, 01:53 PM   #8
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Yep, just remember that 1 is up and 0 is down. The Dallas Speedcals are great and the new ones are all "extended range" units so if you ever put a newer OSS only trans in the car you don't have to worry about your speedometer. I've got a OSS trans in my VSS car and the speedo is dead nuts accurate with the Dallas unit.
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Old 07-08-2014, 01:56 PM   #9
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I don't think I'm doing anything that far in depth but that's good to know. Thanks!

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Old 07-08-2014, 02:00 PM   #10
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Also, make sure you actually install it properly. Half the ppl who cry about them not working have their heads up their asses and can't follow simple instructions. If the unit is defective, just call Dallas and get it replaced.
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Old 07-08-2014, 02:02 PM   #11
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Alright good to know lol. I'll be sure to take my time.

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