Speedo issues after 3.73 install - Mustang Evolution

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Old 01-10-2016, 09:54 PM   #1
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Speedo issues after 3.73 install

So I recently went to 3.73 gears and I'm having an apparent issue with getting the speedo to be near +/- 3 or so mph from actual speed. I have 285/40/18 nitto nt555 on the back and I know it's supposed to be around 740-745 rev per mile.

Issue is at that setting I'm 12 or so off. I've done 20rev per mile intervals and no difference. Between 700-800 I keep getting 10-15 over actual speed. I'm quite confused.


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Old 01-10-2016, 10:29 PM   #2
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You'll need a tuner to calibrate your speedo for the new gears and tire size. If you have a tuner and still aren't getting it right then I dunno.
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Old 01-10-2016, 10:54 PM   #3
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Yeah I have a tuner. I'm using my sct x4. Unless my actual needle is off?


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Old 01-11-2016, 05:33 AM   #4
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When I had my 3.73s done I had Bama give me a new tune. After my first highway drive I could tell the speedo was still way off, so I contacted Bama again and told them the adjustment was off in the new version, they sent me a new tune and everything was fine.
Maybe request another tune.

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Old 01-11-2016, 07:30 AM   #5
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Pretty sure with the X4 you can adjust the gear ratio and revs per mile by yourself.

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Old 01-11-2016, 07:44 AM   #6
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Very true. Op that's what I would try.

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Old 01-11-2016, 08:42 AM   #7
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So I recently went to 3.73 gears and I'm having an apparent issue with getting the speedo to be near +/- 3 or so mph from actual speed. I have 285/40/18 nitto nt555 on the back and I know it's supposed to be around 740-745 rev per mile.

Issue is at that setting I'm 12 or so off. I've done 20rev per mile intervals and no difference. Between 700-800 I keep getting 10-15 over actual speed. I'm quite confused.


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Just looking at your numbers and trying to follow.
When you say your 12 off or 10-15 over actual speed, at what speed are you talking? I try looking at the % of error to see if it relates to difference in gearing ratio, etc.
Did you change tire size at the same time you did gear change? Otherwise why messing with that?
What was your gear ratio before the change?
That's a substantial speed discrepancy, depending on what MPH it was at.
So you changed the tuner settings for gear ratio and tire size???
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Old 01-11-2016, 12:49 PM   #8
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Tired stayed the same. 285/40/18
I had the setting originally on stock value. Changed the gear ratio to 3.73 and that's when I noticed a difference according to my gps app. At that point is where I started playing with different tire sizes as I assumed my rev/mile went up due to cold weather and usage but that didn't affect it.


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Old 01-11-2016, 08:33 PM   #9
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If you have a straight shift stang you will need to purchase a speedometer calibration tool. I bought mine from Late Model Restoration. You can also find one at American Muscle. But if you have an automatic a Bama tune should do the trick. I hope this was helpful.
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Old 01-11-2016, 09:19 PM   #10
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Tired stayed the same. 285/40/18
I had the setting originally on stock value. Changed the gear ratio to 3.73 and that's when I noticed a difference according to my gps app. At that point is where I started playing with different tire sizes as I assumed my rev/mile went up due to cold weather and usage but that didn't affect it.


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Who is your tuner?

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Old 01-11-2016, 10:20 PM   #11
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If you have a straight shift stang you will need to purchase a speedometer calibration tool. I bought mine from Late Model Restoration. You can also find one at American Muscle. But if you have an automatic a Bama tune should do the trick. I hope this was helpful.
He has a tuner. There is no way he should have to do that. I didn't have to do that with my 2001 GT.
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Old 01-12-2016, 01:09 AM   #12
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AED tunes mine. He said the option are opened to change it; that's why I've been tinkering as per OP.


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Old 01-12-2016, 05:13 PM   #13
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AED tunes mine. He said the option are opened to change it; that's why I've been tinkering as per OP.


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At what speed(s) were your 10-15 MPH discrepancies at???

What gears did you change from???
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Old 01-12-2016, 05:34 PM   #14
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At 60-65mph that's where the discrepancies are. And I did 3.31 to 3.73.


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Old 01-12-2016, 07:05 PM   #15
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At 60-65mph that's where the discrepancies are. And I did 3.31 to 3.73.
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The difference in gear ratios from 3.31 to 3.73 would cause the car to travel 11.3% slower than your speedometer is reading! For example, if speedometer is reading say 75 mph it is actually traveling at 66.5 mph. So the speedometer would only be reading 8.5 mph faster than your actual speed. So if you are getting readings in the range of 10 to 15 mph over your actual speed, that's a hell of a lot more than what it should be even if you never touched the tune for corrections.
Slightly different tire sizes would make very little difference.
Sounds like the info was not entered correctly into the tuner and transferred to the car. OR the unit is screwed up somehow.
Strongly suggest you go back to your tuner and have them make the adjustments.
Oh ya, you may want to verify the accuracy of your GPS while you're at it.
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Old 01-12-2016, 07:43 PM   #16
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Well right now I have it at around 7-8 mph differences with the setting at 745 rev/mile at 65 mph. The gps is accurate for all other cars but not mine. Question. What's 2krpm supposed to be on 3.73?


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Old 01-12-2016, 09:05 PM   #17
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Well right now I have it at around 7-8 mph differences with the setting at 745 rev/mile at 65 mph. The gps is accurate for all other cars but not mine. Question. What's 2krpm supposed to be on 3.73?


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With your tires and 6th gear of a manual...
Click image for larger version

Name:	2000RPM.jpg
Views:	69
Size:	54.5 KB
ID:	195756

Approx. 61MPH
Here you go...one of many similar sites.
Speedometer, Gear Ratio, and Tire Size Calculator
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Old 01-12-2016, 09:51 PM   #18
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Well I'm doing 72 issh


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Old 01-12-2016, 09:52 PM   #19
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I don't think anyone's doing that? I remember when I checked it said it was supposed to be 66


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Old 01-13-2016, 03:33 PM   #20
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I don't think anyone's doing that? I remember when I checked it said it was supposed to be 66

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Don't know what exactly you checked to get 66 but output is only as good as the input. If you use the same tire size and gear ratios, you won't get 66 for 2000 RPM.

Be sure to let us know what it took to resolve this problem.
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Old 01-13-2016, 03:40 PM   #21
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I guess a better question is what rev/mile should you enter to make the speedo read slower. Ex. Speedo says 70 while GPS says 61


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Old 01-13-2016, 03:49 PM   #22
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I guess a better question is what rev/mile should you enter to make the speedo read slower. Ex. Speedo says 70 while GPS says 61


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Sorry but I can't advise you on those matters. I've never had a tuner. Don't you have a function to change the gear ratios? and tire size?
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Old 01-13-2016, 04:00 PM   #23
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Yeah. Gear ratio were changed but I guess the 3.73!have exacerbated the speed differences. Then again I don't know if it's always been wrong. There's a setting to change rev/mile on xcal4


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Old 01-13-2016, 04:22 PM   #24
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Yeah. Gear ratio were changed but I guess the 3.73!have exacerbated the speed differences. Then again I don't know if it's always been wrong. There's a setting to change rev/mile on xcal4


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What does the "rev/mile" currently read?
I was going to ask if it was obvious what rev's they were referring to, but I guess it will be obvious when I see it.
I'd be a bit reluctant to advise you on any change without knowing what the available options are, and what they represent exactly.
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Old 01-13-2016, 05:10 PM   #25
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Stock value is 757 rev/ mile. You can only change rev/mile and axle ratio


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Old 01-13-2016, 06:36 PM   #26
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Stock value is 757 rev/ mile. You can only change rev/mile and axle ratio


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Well it looks like the stock value of 757 rev/mile was based on 26.64 diameter tires. Not very far off from the calculated 748 rev/mile for your 285/40-18 tires.
Although the problem is elsewhere, I assume you could compensate by reducing the rev/mile. It would appear that reducing it to about 658 Rev/mile would make the difference but I'm just proportioning the numbers and still suggest checking with a tuner.
Essentially what we would be doing is saying that there are 30.7" dia tires on it to compensate for what appears to be a problem with it not taking the gear change criteria.
I believe someone else on this forum had a similar problem but ended up getting it fixed by a revised tune.
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Old 01-13-2016, 07:40 PM   #27
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He has a tuner. There is no way he should have to do that. I didn't have to do that with my 2001 GT.
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Old 01-28-2016, 04:11 PM   #28
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Stock wheels/tires, Brembo package with 3:73's.

In 6th gear, 80mph is exactly 2500 RPM's.
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Old 01-28-2016, 09:28 PM   #29
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He has a 2013 GT, just a little bit different than yours.
I get that. You're missing the point. I also have a 2012 GT/CS. My point is tuners clear back to that time had them, and you didn't need to mess around with a calibration tool. My current tuner does as well. Just tell it the revolutions per mile of the tire, and the gearing.
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