I just bought a Diablo Tuner for my mustang 98 GT.

I was wondering how to fix the Speedo since I have 4.10 gears?

Mods are in the Sig
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It is under modify parameters.

 

Yes Ive tried that, but no worky.

I went to Parameters and then another screen shows up with a list of options. So I chose "Axle Ratio"(or something like that) and then another screen comes up and it says "Traction" So I selected it and it just asks you if you want it to turn on or off. Im pretty sure thats for traction control or something. Is there another way?
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You need to get a new speedo gear for your transmission. Pre-99 mustangs have mechanical speedos, the computer has no control over it.

 

I wasn't aware of this.

Can this be done at the Dealer? Or Where? Or can this be be done by me? If so where can I buy the gear? And is this the gear thats in the trans?

Its an Automatic BTW.
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tbird is right, the Predator will not adjust speedo for 98s.

http://steeda.com/products/speedomet...tion_gears.php

You will need one of these.

 

Simply put, you have a cable that goes from your cluster down into the side of your transmission towards the rear (speedometer cable). You have to replace the plastic gear in the side of you transmission that hooks up to the cable. Changing the gear for a different ratio gear can compensate for a change in tire size, rear end gears, and such.