HP tire size speedo adjustment with forscan

[AZ_Ryan]

Can any one here definitively say what the correct value is for reprogramming the speedo in a non-HP car to HP size tires with forscan? My car was scanned once already using a circumference of 2108 and is still reading off by 2 mph at highway speed. After doing some searching on the forum, there doesn't seem to be a consensus.

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[K4fxd]

You can input the circumference and/or rev's per mile but I doubt it will do anything. Factory the speedo is off 1 to 2 Mph.

 

[Crew4991]

They are all pretty much off slightly like what @K4fxd said. Don't worry about it.
Most don't ever realize the speedo it off anyway.
If it says you are going faster than you actually are - take that as a blessing next time you get a speeding ticket and it isn't as pricey as you expected.

 

[AZ_Ryan]

They are all pretty much off slightly like what @K4fxd said. Don't worry about it.
Most don't ever realize the speedo it off anyway.
If it says you are going faster than you actually are - take that as a blessing next time you get a speeding ticket and it isn't as pricey as you expected.

I've never heard of the speedo on these cars being off from the factory by that much before. My last GT was spot on. Where are you getting this info from?

 

[Crew4991]

I've never heard of the speedo on these cars being off from the factory by that much before. My last GT was spot on. Where are you getting this info from?

Every single one of my Fords I’ve had were off. Thank them for crap quality control. Maybe you got lucky but it’s normal for it to be off slightly.
My info is coming from personal experience: 2 mustangs, explorer, focus and f150. All off 1-2mph except for the explorer which was off by 4 for some reason lol

 

[K4fxd]

I've never heard of the speedo on these cars being off from the factory by that much before.

There are a coupla threads on it. Mine usually reads the same as the road side radar signs but when I compare it with my dragy or other sat devices the car reads 1 mph fast.

 

[AZ_Ryan]

Sooo does anyone actually know the answer to my question? Lol

Surely someone has found the right circumference for the 315/30 to use with Forscan.

 

[K4fxd]

If you read the forscan thread I do believe it says what lines to change for different sizes. it dosen't work on my car. No matter what I enter the speedo reads the same.

I swapped rear gears and the speedo self corrected for the gear.

When I put my racing tires on the speedo self corrects also.

Some say the forscan changes work, you will just have to try it.

 

[AZ_Ryan]

If you read the forscan thread I do believe it says what lines to change for different sizes. it dosen't work on my car. No matter what I enter the speedo reads the same.

Some say the forscan changes work, you will just have to try it.

No offense, but did you read my first post?

I'm not asking how, I'm asking if anyone knows the correct circumference to input, since 2108 is still off a little.

 

[K4fxd]

What size tire?
https://tiresize.com/calculator/

Google is your friend

 

[ihasnostang]

idk change the 4 digit number circumference # slightly based on whether its over/under and then do the pcm/tcm relearn and go test it?

 

[D Bergstrom]

idk change the 4 digit number circumference # slightly based on whether its over/under and then do the pcm/tcm relearn and go test it?

This is the correct answer. No two cars or tires are the same, what works for one combo may be off for something else. Try different values and test until your speedo is correct.

Doug

 

[DRAGOON]

Can any one here definitively say what the correct value is for reprogramming the speedo in a non-HP car to HP size tires with forscan? My car was scanned once already using a circumference of 2108 and is still reading off by 2 mph at highway speed. After doing some searching on the forum, there doesn't seem to be a consensus.

I have a HP with HP 315/30/19 rear tires. The diameter of the stock cup 2’s are 26.5”. My speedometer reads spot on with the cops, GPS & the local speed trap roadside in my area while I use the digital gauge. Your value (2108) comes from the wrong diameter used to obtain that number.
26.5 x Pi = 83.2524 x 25.4 = 2,114.61096
Increase your value to 2,114 in your recalibration & go see if that’s closer to what you’re looking for.

 

[AZ_Ryan]

What size tire?
https://tiresize.com/calculator/

Google is your friend

And so is reading. I already said the tire size. And I said the value (2108) that calculator gives is incorrect. Try to keep up.

 

[AZ_Ryan]

I have a HP with HP 315/30/19 rear tires. The diameter of the stock cup 2’s are 26.5”. My speedometer reads spot on with the cops, GPS & the local speed trap roadside in my area while I use the digital gauge. Your value (2108) comes from the wrong diameter used to obtain that number.
26.5 x Pi = 83.2524 x 25.4 = 2,114.61096
Increase your value to 2,114 in your recalibration & go see if that’s closer to what you’re looking for.

Thank for this.

So Forscan changed the stock diameter value of 2178 (275/40-19 tires) to 2108 (315/30-19 tires).

Problem is, its still reading that I'm going 2 mph faster than I am. If I change to 2114, that's going the wrong way.

The generic tire calculator gives the diameter of 2108 for all 315/30-19 tires which believe is incorrect. I figure it should be slightly less. Another wrinkle here is Im running ps4s, not cup 2s. I know for a fact ps4s are slightly larger round due to the tread pattern, but thats probably not worth taking into account.

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