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Any idea if a similar method would work to correct the difference between the main speedo and the set speed displayed for cruise control?
Some of what you see there is parallax error. Your head has to be in a weird place to correctly read the analog speedometer at highway speed.
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Some of what you see there is parallax error. Your head has to be in a weird place to correctly read the analog speedometer at highway speed.
Maybe some, but only a small fraction. There is no viewing angle through the center of the car that can correct my error.
 

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Can this be done using an SCT X4 tuner ?

Since I installed my 20's, my mileage is off. My office is 18.3 miles away, since I installed the 20's it now reads 19.5. As you can see, Im adding uneeded mileage to my Stang.

Thanks
Hey man go on sct website,it tells u right on the website that the sctx4 corrects for tire/gear changes.Im sure this method works too,but u already have an x4.I did mine one week ago.
 
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So, I finally got new wheels/tires and did this mod on my own car. I updated the OP with info about the DTC that results (it's not a problem.)
 
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Any idea if a similar method would work to correct the difference between the main speedo and the set speed displayed for cruise control?
I want to hit on this topic again.

There is a *large* amount of parallax error in these cars when the needle is pointed up, which it is when you're at highway speeds of around 70-80.

Why? The needle is thick and spaced out from the gauge face, so the perceived "needle" (the illuminated red center section) will give a great deal of parallax error unless you're viewing perfectly perpendicular to the needle.

Unfortunately, the designers chose to put the speedometer in such a position that reading the speedo with your head perfectly perpendicular to the needle means moving your head WAY right, far away from any natural position in the car.

All that academic bullshit people probably won't read, so here's a video that illustrates the effect:

[ame="[MEDIA=youtube]62kWnHu0IrA[/MEDIA]"]
 

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Mine won't correct no matter how far I lean to the right. It's not just parallax.
 
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Mine won't correct no matter how far I lean to the right. It's not just parallax.
I know about the parallax error and I go by the cruise setting anyway, because it's more accurate. BUT, I assume the cruise setting is rounded up or rounded down for display.

So if you set the cruise and the car happens to be going 73.6 MPH at the time, you'll see 74MPH indicated cruise setting. Even though the car will be trying to hold the speed to a tenth.

If you set the cruise and the car happens to be going 74.4MPH at the time, you'll see 74MPH indicated cruise setting. Even though the car will be trying to hold the speed to a tenth.

ALSO, even slight inclines and declines will cause the car to go slower or faster than the indicated setting by half a MPH or more. When I was figuring my speed difference vs GPS (which was reading in tenths) I noticed this and made sure I was on a flat and level stretch of road to figure my correction.

All this stacked together can make for quite a difference. Rounding of the cruise setting for display + the car not quite getting the speed right uphill or downhill + parallax error = differences between the speedo and cruise setting.
 

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Just to add a bit of information to this thread (and bump it back up...where are the mods to stick this when you need them), after installing the Ford Performance tune tonight, I modified the tire size using the ProCal software. Entering the revs per mile in the tuner resulted in the same value that the spreadsheet gave after just entering the tire size.

Thanks again to Spart for figuring out the correction factor and putting this spreadsheet together for all of us to make easy tire size adjustments! :)
 
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Just to add a bit of information to this thread (and bump it back up...where are the mods to stick this when you need them), after installing the Ford Performance tune tonight, I modified the tire size using the ProCal software. Entering the revs per mile in the tuner resulted in the same value that the spreadsheet gave after just entering the tire size.
Good to know! I assume the ProCal software is additionally modifying something in the PCM firmware or PCM as-built so it doesn't throw a DTC.
 

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Just to add a bit of information to this thread (and bump it back up...where are the mods to stick this when you need them), after installing the Ford Performance tune tonight, I modified the tire size using the ProCal software. Entering the revs per mile in the tuner resulted in the same value that the spreadsheet gave after just entering the tire size.

Thanks again to Spart for figuring out the correction factor and putting this spreadsheet together for all of us to make easy tire size adjustments! :)
So I assume with the ProCal I can correct for the size I have. Stock is 235/50/18 with a rev per mile of 765.

I have 245/45/18 with a rev per mile of 780.

So I can change out 765 for 780?

Correct?
 

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So I assume with the ProCal I can correct for the size I have. Stock is 235/50/18 with a rev per mile of 765.

I have 245/45/18 with a rev per mile of 780.

So I can change out 765 for 780?

Correct?
Yep, and it will modify the as-built for you just as described in this thread without the need for using FORScan. Had mine done this way prior to the ProCal, then reverted to stock and let the ProCal handle the update, both accomplished the exact same thing. :headbang:
 

tal

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Yep, and it will modify the as-built for you just as described in this thread without the need for using FORScan. Had mine done this way prior to the ProCal, then reverted to stock and let the ProCal handle the update, both accomplished the exact same thing. :headbang:
I've decided I'd like to keep my Steeda tune and not use the Ford ProCal.

So...with that in mind, I assume I can still use ScanTool? Windows 10 on the laptop OK?

If so, can I use the USB model?

https://www.amazon.com/ScanTool-425...rd_wg=k0aPf&psc=1&refRID=QFVJ17P4THH5Y4X95HMA
 
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That model doesn't support MS-CAN.

For the ODBLink models, only the MX Bluetooth and WiFi ones support MS-CAN.
Got it!

Ordered the BT version.

Thanks!
 
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Got it!

Ordered the BT version.

Thanks!
Hopefully you saw that there are two BT models. The MX is the one that supports MS-CAN (and is more expensive.)
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