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I am working on a 96 Mustang gt that has 4.10 gears and recently had the transmission replaced with a 2001 grand marquis transmission. The connector was repinned for 98+ and it was jmodded at the same time. The speedometer was reading very high, likely due to the 4.10 gears, we think. The main concern is that the car will complete the 1-2 shift at a very low speed (less than 10 mph). As soon as it goes into 2nd it will start bucking because the rpms are too low, I think.

We thought the VSS reading was causing this issue, and just installed a Ford Racing Speed Dial (similar to speedcal). The speedometer is acting more normal now, but the shifts are still happening way too early. What would cause the PCM to command a 1-2 shift at such a low RPM? Really would like to get some input on this one. It is turning into a head scratcher. Thanks in advance!
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Chances are the car came with a 3.27 from the factory; if you go more than 20% off in gearing in one direction from factory you need a tune to adjust the shift points. Basically what you're getting is the shift RPM for a 3.73 (which is the 20% limit of adjustability for a car with a 3.27 program) with a 4.10, hence the lower-than-desired shift RPM.

You need a speedcal for anything higher than a 3.73 in a 4R70W with an 8 tooth output shaft (anything other than FN10 or MN12 transmissions) - so good call on that. It just needs a tune now.
 

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Discussion Starter #3
I think you hit the nail on the head. Stock this car had 3.08s. After driving around for a while with the scanner hooked up watching data we decided the high oss reading was making it command the shifts. I guess the previous owner just drove it like this lol. Thanks for the quick reply. Now to find an xcal and send don lasota an email!
 
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