1998 mustang gt 18" staggered wheels/3.73 gear combo speedometer calibration help!

Anarkist

New Member
Aug 8, 2019
2
0
1
El Paso
hello fellow stangers! im new new to stangnet but lurked for a little bit, glad to be on here, and part of the stangnet community. Although I've ready many post and threads regarding the speedometer calibration for our 96-98 gt's, most are not specific and leave a lot of room for interpretation. I own a 98 gt "manual" with 18" rims running a 305-30-18 rear and hesitant about installing my 3.73's. It seems that the DM speed cal only calibrates for either rear gears or tire size but not both, I have read reviews on american muscle from 96-98 gt owners using handheld tuners to calibrate speedos. Called LMR and their rep just read me the description of functions the tuner can do but does not assure me, and that is a 400$ gamble you cant return ha ha!. The DM speedcal at a little over 200$ is a bit pricey as well considering you would still need a tuner anyway in the future for your upgrades, rather spend the money on a tuner and have wider range of use. Furthermore since 96-98 gt's have a speedo gear and VSS sensor could it be possible to use the tuner to calibrate the electronic portion of speedometer and only buy a speed gear to compensate the wheel/tire change? thank your for reading and all of your knowledge is appreciated!
 
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08GT500

Active Member
Jul 12, 2018
715
100
53
Massachusetts
Hi, Apologize my battery is about dead. Call American Muscle & Ask BAMA about your request, they’ll take care of your final output ratio, you can combine both app’s mentioned, the Math is quite easy, easiest to get the Digital take from an SCTX4 Tuner, tunes for life- free. The meter will easily be able to compensate for the difference.
I’ll pop back on with the equation if you require it, just ask. Should be powered up by morning, already 5AM. A few hours, happy to help. Call AM first, then we can work it out further if need be.
Good luck!
-John
 

wmburns

SN Certified Technician
Aug 14, 2009
5,807
499
164
Houston Texas
There's a fundamental difference between the 1996-1998 and the 1999+ that IMO is important that will help you to understand WHY there's confusion.

The 1996-1998 Mustang uses a VSS speed sensor. The OUTPUT of the VSS sensors goes to both the PCM AND the cluster.

Whereas the 1999+ uses an OSS sensor. The OUTPUT of the OSS sensor goes to ONLY the PCM. The PCM decodes the sensor output and "learns" the car's speed. The PCM then "tells" the cluster what speed to display.

Regarding why there is an apparent contradiction in what the forums say. Think about what would happen if just a tuner were used on a 1996-1998. This would create a situation where the PCM had the correct speed but the cluster would not.

But for the 1999+, if the PCM is altered using a turner then the PCM has the correct speed. But since the PCM now "knows" the correct speed it "tells" the cluster the correct speed.

Soooooooooo. This means for the 1996-1998, a turner alone can't do the whole job. Either a different VSS gear or an external converter box is needed just downstream of the VSS sensor. This allows the PCM and the cluster to benefit from the altered signal.

For the 1996-1998 model year a single solution isn't really possible. Just consider the rear end correction and tuning for performance different problems and work from there.
 

Anarkist

New Member
Aug 8, 2019
2
0
1
El Paso
Thanks for looking and for your knowledge guys, it seems a bit weird the way the setup is on those yr model gt's but then again i guess that's why it was corrected on the new edge models huh? i think for now i'm going to install my gears and go with the tuner and well go from there.
 
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