Modifying As-Built Data Guide

[Spart]

If I'm doing the calculations right, my hex value should be 858...username checks out. :headbonk:

Yup, that's the factory value for GT PP's.

ETA: page 65 ownage, check out my tire size as-built calculator for correcting speedometer error: https://docs.google.com/spreadsheets/d/1rNzj0Ibm6pGJZbg-J2szZ0Su0RlTr7FhwR6JxwTo0v0/edit?usp=sharing

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[jody858]

Yup, that's the factory value for GT PP's.

ETA: page 65 ownage, check out my tire size as-built calculator for correcting speedometer error: https://docs.google.com/spreadsheets/d/1rNzj0Ibm6pGJZbg-J2szZ0Su0RlTr7FhwR6JxwTo0v0/edit?usp=sharing

Hex value 858...username is jody858...hello, is this mic on? ;)

BTW thanks for that spreadsheet, nice work! I did it both ways just to verify I was was doing it right. Even went to Tire Rack and looked for the specs for the stock rear tire.

I'll try tonight to get what my hex value is set at now and report back.

 

[Spart]

Hex value 858...username is jody858...hello, is this mic on? ;)

BTW thanks for that spreadsheet, nice work! I did it both ways just to verify I was was doing it right. Even went to Tire Rack and looked for the specs for the stock rear tire.

I'll try tonight to get what my hex value is set at now and report back.

I bet its already 0858. Every GT PP as-built I've seen uses that value.

Did you see my research spreadsheet? There's a GT PP in there: https://docs.google.com/spreadsheets/d/1T1vRZtu-1Y-8Bv4nItTektboTOXYjk5rnZzWBlac8b4/edit?usp=sharing

 

[Redraven571]

I bet its already 0858. Every GT PP as-built I've seen uses that value.

Did you see my research spreadsheet? There's a GT PP in there: https://docs.google.com/spreadsheets/d/1T1vRZtu-1Y-8Bv4nItTektboTOXYjk5rnZzWBlac8b4/edit?usp=sharing

My hex is 0839 for 235/50R18 on my 2015 V6 vert with 51A package. I'll have to see what Forscan says; my speedo is about -2 MPH from GPS when running around 70 MPH.

Likewise, I did it both ways and came up with the same value.

 

[jody858]

I bet its already 0858. Every GT PP as-built I've seen uses that value.

Did you see my research spreadsheet? There's a GT PP in there: https://docs.google.com/spreadsheets/d/1T1vRZtu-1Y-8Bv4nItTektboTOXYjk5rnZzWBlac8b4/edit?usp=sharing

I'm going to double check mine. I feel like the speedo is off at higher speeds. Haven't bothered to check it against my speedo app on my phone yet.

 

[Redraven571]

My hex is 0839 for 235/50R18 on my 2015 V6 vert with 51A package. I'll have to see what Forscan says; my speedo is about -2 MPH from GPS when running around 70 MPH.

Likewise, I did it both ways and came up with the same value.

Base value in Forscan was 0838, I changed to 0839, now to see if there is a difference in speedo vs. GPS.

 

[Spart]

My hex is 0839 for 235/50R18 on my 2015 V6 vert with 51A package. I'll have to see what Forscan says; my speedo is about -2 MPH from GPS when running around 70 MPH.

Likewise, I did it both ways and came up with the same value.

If you know your error from GPS, you can do some math to figure out a corrected value. You're saying your speedometer is claiming 68mph when your GPS says 70?

If your speedo is reading slow, it's because the car is programmed for a larger tire than you actually have.

So the factory value - 0838 - is 2104 when converted to decimal. 68 is 97.14% of 70, so applying that percentage to 2104 you get 2044 (rounded.)

2044 in hex is 07FC - give that a shot and report back.

 

[Agent_S550]

I would like to adjust my speedo to read like a planes altitude gauge in analog when it starts spinning and spinning and spinning. I would feel pretty fast at that point

 

[Redraven571]

If you know your error from GPS, you can do some math to figure out a corrected value. You're saying your speedometer is claiming 68mph when your GPS says 70?

If your speedo is reading slow, it's because the car is programmed for a larger tire than you actually have.

So the factory value - 0838 - is 2104 when converted to decimal. 68 is 97.14% of 70, so applying that percentage to 2104 you get 2044 (rounded.)

2044 in hex is 07FC - give that a shot and report back.

I used your As-Built tire calculator and when I plugged in a diameter of 693, it says Forscan value should be xxxx 0839 xxxx; the default on the car was 0838, so I changed to 0839. seems like 1 digit difference may make the difference in speedo reading. Using the 4 separate links yielded the same answer; I'll see if there's a difference and post back.

 

[Spart]

I used your As-Built tire calculator and when I plugged in a diameter of 693, it says Forscan value should be xxxx 0839 xxxx; the default on the car was 0838, so I changed to 0839. seems like 1 digit difference may make the difference in speedo reading. Using the 4 separate links yielded the same answer; I'll see if there's a difference and post back.

I understand, but if you're truly reading 2mph slow, not only are you changing the value in the wrong direction, you're not changing it enough. Give the value I calculated a shot.

The nominal diameter you calculate using that online tool is just that - a value in name only. Sort of like the nominal dimension of a 2x4 is 2"x4". That's not the real world measurement.

Because you've compared your speedometer against GPS, you've taken a real-world measurement and we can use that to calculate a corrected value for your particular car and tire setup.

 

[Spart]

I would like to adjust my speedo to read like a planes altitude gauge in analog when it starts spinning and spinning and spinning. I would feel pretty fast at that point

Set the as-built value to FFFF and see what happens. :headbonk:

 

[Redraven571]

I understand, but if you're truly reading 2mph slow, not only are you changing the value in the wrong direction, you're not changing it enough. Give the value I calculated a shot.

The nominal diameter you calculate using that online tool is just that - a value in name only. Sort of like the nominal dimension of a 2x4 is 2"x4". That's not the real world measurement.

Because you've compared your speedometer against GPS, you've taken a real-world measurement and we can use that to calculate a corrected value for your particular car and tire setup.

Are you ready for this? I changed it and it didn't seem to make a visible difference. I changed it back to the baseline value (I'm going on a trip tomorrow morning and figured I'd set it back to base, so no surprises) and now the speedo, cruise and GPS now all align? WTF?

 

[Spart]

No clue what's going on there. My speedo has always been dead on, I really wanted to figure this out so I could keep my options open for wheels/tires down the road.

 

[TexasRebel]

Too bad odometer fraud is a felony.

It could be 1,003,000 miles... you don't know!


My C10 has an odo that rolls over at 100,000. I bought it when I was 17 and the odo had rolled at least once. I reset it by hand to 0 so I could track the miles I put on the newly rebuilt engine and my mom freaked out thinking the black helicopters were coming down. Back before the days of household internet it took a while to track down enough literature to prove that misrepresenting the mileage on a vehicle for sale is a felony, but once an odometer is past it's mechanical limits the numbers no longer matter. I also showed her that I could go to LMC and buy a new one that reads 00000.0. We then took a step back and I asked her if anyone would ever believe that this pickup has fewer than 100,000 miles on it.:lol:

I think that's why just about everything goes to 1,000,000 these days.

 

[Agent_S550]

It could be 1,003,000 miles... you don't know!


My C10 has an odo that rolls over at 100,000. I bought it when I was 17 and the odo had rolled at least once. I reset it by hand to 0 so I could track the miles I put on the newly rebuilt engine and my mom freaked out thinking the black helicopters were coming down. Back before the days of household internet it took a while to track down enough literature to prove that misrepresenting the mileage on a vehicle for sale is a felony, but once an odometer is past it's mechanical limits the numbers no longer matter. I also showed her that I could go to LMC and buy a new one that reads 00000.0. We then took a step back and I asked her if anyone would ever believe that this pickup has fewer than 100,000 miles on it.:lol:

I think that's why just about everything goes to 1,000,000 these days.

My daily is an old GMT400 short bed single cab. Great truck. Stupidly the original owner (who I bought it from) marked mileage exempt while also writing the mileage down. So now the truck is exempt unless I take it back to the DMV and have it redacted. It only has 115k original miles.

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