Wrong speedometer calibration?
07-16-2011, 12:38 AM
#11
I talked to my dealer Service manager today. I was wondering how much they would charge. He quoted me $70 to program the new ratio.
We did discuss issues with this change. It seems they can only program ratios that were available options on your model. The system has optional ratios that can be selected. It does not allow them to key in ratios that are not in the list. The reason for this, I think, is that it is not just telling the PCM what ratio you are using, it is loading a translation table used to interpret the speed sensor output. The PCM does not calculate this table on it's own. If you need a ratio not listed, they have no table to load.
We did discuss issues with this change. It seems they can only program ratios that were available options on your model. The system has optional ratios that can be selected. It does not allow them to key in ratios that are not in the list. The reason for this, I think, is that it is not just telling the PCM what ratio you are using, it is loading a translation table used to interpret the speed sensor output. The PCM does not calculate this table on it's own. If you need a ratio not listed, they have no table to load.
07-16-2011, 02:15 AM
#12
The Paranoid One
Join Date: Apr 2006
Location: Texas
Posts: 5,320
Has anybody had a problem getting thier speedometer to calibrate right after getting the gears changed?
I just had my gears changed from 3.31 to 3.73 at least thats what I wanted and paid for and thats what was stamped on the gears but,when they (the dealership) recalibrated the speedo for 3.73 gears it was off by about 6 or 7 mph (to fast).
The tech got his GPS unit and we went for a ride to check it to make sure and I was right.
We ended up lying to the computer and calibrated it for a 3.55 gear. That got it to about 1 or 2 MPH off. Close enough but, that doesn't make sense either. If you go from a lower gear to a higher gear it takes more RPM's per MPH or am I wrong on that?
Anyway just curious and if I get my own tuner I may have to know for sure about it.
I just had my gears changed from 3.31 to 3.73 at least thats what I wanted and paid for and thats what was stamped on the gears but,when they (the dealership) recalibrated the speedo for 3.73 gears it was off by about 6 or 7 mph (to fast).
The tech got his GPS unit and we went for a ride to check it to make sure and I was right.
We ended up lying to the computer and calibrated it for a 3.55 gear. That got it to about 1 or 2 MPH off. Close enough but, that doesn't make sense either. If you go from a lower gear to a higher gear it takes more RPM's per MPH or am I wrong on that?
Anyway just curious and if I get my own tuner I may have to know for sure about it.
I believe it takes an adjustment via aftermarket tuner.
Also if you have a different size tire than stock you will need to put that information into the tune due to rotation or it will be off too.
I just saw this last post.
If the dealership says they can do it but charge then pay them and re check it.
If it's still off get your money back and go elsewhere
05-11-2016, 05:48 PM
#13
Join Date: May 2016
Location: OH
Posts: 2
I just bought a 2011 GT and it has the factory 3.71s in it. With my SCT tuner I set the axle ratio for 3.73s and the right rev/mile for the tires. My speedometer reads anywhere from 7-10mph fast depending on the speed. What would be cause this if everything it set to where it should be?
05-11-2016, 07:40 PM
#14
5th Gear Member
Join Date: Mar 2008
Location: Clermont, FL
Posts: 2,952
I just bought a 2011 GT and it has the factory 3.71s in it. With my SCT tuner I set the axle ratio for 3.73s and the right rev/mile for the tires. My speedometer reads anywhere from 7-10mph fast depending on the speed. What would be cause this if everything it set to where it should be?
05-12-2016, 05:21 PM
#15
Join Date: May 2016
Location: OH
Posts: 2
My axle code on my car located in the driver side door sill is double B (BB). If you look up that axle code for this year of mustang gt it is the 3.73s.
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