Speedometer error - Where did I go wrong? -UP DATE

[ThumperF150]

I'd try setting both your new ratio and your tire circumference (per their actual specs, x .967) in the BCM and everything should be happy again. 21-23 says axle ratio isn't used (however mine is correct at value 0175 hexD, translates to 373 decimal, also checked a 24s and it is used). However the tire circumference can be used to properly adjust your speed by substituting a value appropriate to the distance you're actually covering vs the distance your truck said you traveled.

The 4 stars of 726-15-01 (positions 5-8 should be changed to 019A) If you had 373s it should read 0175

Go find your tire's exact specs, convert the circumference to mm, multiply by .967 (rolling diameter correction), take that value and enter it into a decimal to hexadecimal converter to find your 4 digit value. Here I chose a random number of 2527.4 mm circumference, * .967
2527.4 * .967 = 2444

As an example, while the gear number is what you need, you didn't post the exact tires you have so I can't provide you the numbers that fill out your BCM values

Thank you.
Forscan site says my 2024 is not supported.

I’m hesitant to buy, Forscan, buy a PC laptop and buy a cable in order to attempt to change coding in my trucks computer when I clearly don’t know what I’m doing.

I’ve talked to a couple of folks now with similar but stock trucks and they are turning 1700-1800 rpm’s at 69-71 actual mph in 10th.

My truck is doing the same. Plus it just feels like it is driving and shifting correctly so I think mechanically my taller tires and lower gear ratio worked as I had hoped.

I still don’t understand why my speedo is so far off but I can correct that with a Hyper tech plug and play box mounted behind the cluster.

The Hypertech can be adjusted for tire size and gear ratio. It recalculates the signal from the truck before it gets to the instrument cluster. Can be changed and or removed so I think that’s the route I will take.

thank you for your help and information.

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[HammaMan]

24s are supported in forscan. You just need a cheap PC, a $40 cable, and the 2mo free license forscan comes with.

Not sure where the speed is coming from in 24s, but if you're still off, it may be the ABS that's counting the exciter speed and that's why you're still wrong. Forscan mods as described is the only fix.

 

[chava0191]

@ThumperF150 im trying to calibrate my speedometer but with the stock 3.73s. can you please let me know if you find a solution?

@HammaMan i checked forscan’s website and it says 2024s are not supported…

 

[Parthery]

'24s are definitely supported. They need to update their website.

 

[jadatis]

Mayby you overlooked that the rolling-circumference is not the circumference of tire.
Most calculators on internet give circumference as rolling circumference.

But rolling-circumference smaller, and dependend on deflection of tire.

I estimated deflection in normall use to be 15% of free flexible part of sidewal , wich is on its turn about section highth minus 1 inch or 25 mm, for the part in rimm and treaththickness.

Then used aproaching formula
( overall diameter minus deflection ) x pi.

So your new tire has higher sectionhight so more 15% deflection is also more, wich makes the difference in rolling-circumference between old and new smaller then you calculated.

 

[ThumperF150]

Mayby you overlooked that the rolling-circumference is not the circumference of tire.
Most calculators on internet give circumference as rolling circumference.

But rolling-circumference smaller, and dependend on deflection of tire.

I estimated deflection in normall use to be 15% of free flexible part of sidewal , wich is on its turn about section highth minus 1 inch or 25 mm, for the part in rimm and treaththickness.

Then used aproaching formula
( overall diameter minus deflection ) x pi.

So your new tire has higher sectionhight so more 15% deflection is also more, wich makes the difference in rolling-circumference between old and new smaller then you calculated.

I hear what you are saying and I think this explains part of my issue.

My axle is turning 10% slower due to gears.

However, truck does not know that. Truck still thinks it is stock and thinks it is moving at 60mph and it shows same rpms as stock.

Because the speed sensor is on the transmission and the truck is sending the speed signal from the trans sensor up to the cluster and obviously my changes are not part of that loop.

So my 10% taller tires SHOULD have me moving 10% faster than indicated. But they don’t, only 8%.

I think that 2% difference is the rolling circumference you are talking about.

 

[jadatis]

When I fill in my made rolling-circumference calculator, it gives with your given sizes in opening post 9.29% lower speed given, so almost 10%.
And mayby even in old situation not exact speed given.

So does realy slower axle-rotation give higher speed given? Where is the sensor situated at gearbox, if at the end , mayby no compensation needed for that

Filled in my calculator with your given sizes in opening post. Attached it again, hope you have excell or similar program.
Then gave as sectionwidth 10.5 x 25.4 mm
And section hight 35-17 inch/ 2 = 9 inch
Then calculated highth width division (9/ 10.5)x 100. This all because calculator made for mm section width and HW division in percentage.

 

[ThumperF150]

In 10th gear at 1700/1800 rpm the truck thinks is is going 60 mph and that is the signal it sendto dash.

With 4.11 gears, the axle is spinning 10% slower than it would be with the 3.73 gears and if I had stock tires on it, the actual speed would be 10% slower.

Of course I assumed the 10% larger tires would account for this but they do not.

I admit these numbers are not exact. Had the actual speed been writhing 2% of indicated speed I would have been fine.

But for me 8% error is too much.

The most important thing for me is that the truck is running, driving and shifting as it did from the factory.

The Hypertech module will automatically adjust the signal coming from the truck before it gets to the dash. It can be configured to look at old tire size, new tire size (actual not sidewall) old gears and new gears.

Additionally, it has a mode to add the differences of the indicated speed bs actual speed so I’m fairly confident it will correct my speedometer error without needing to get into and muck with the trucks computer.

 

[HammaMan]

In 10th gear at 1700/1800 rpm the truck thinks is is going 60 mph and that is the signal it sendto dash.

With 4.11 gears, the axle is spinning 10% slower than it would be with the 3.73 gears and if I had stock tires on it, the actual speed would be 10% slower.

Of course I assumed the 10% larger tires would account for this but they do not.

I admit these numbers are not exact. Had the actual speed been writhing 2% of indicated speed I would have been fine.

But for me 8% error is too much.

The most important thing for me is that the truck is running, driving and shifting as it did from the factory.

The Hypertech module will automatically adjust the signal coming from the truck before it gets to the dash. It can be configured to look at old tire size, new tire size (actual not sidewall) old gears and new gears.

Additionally, it has a mode to add the differences of the indicated speed bs actual speed so I’m fairly confident it will correct my speedometer error without needing to get into and muck with the trucks computer.

Have you verified the trans is what sets the speed and not the ABS? Never seen it confirmed. What you're describing sounds like the ABS is what's reading the speed. ABS reads wheel speed, which has not changed. If trans was responsible for setting the speed it'd wash.

I did tell you how to properly inform the truck of both gearing and tire size. It's common to change tire size and recal the truck's speed. Otherwise your mileage is off (and of course, speed). The raptor comes w/ 4.10s so it's an easy value. Personally I would have stuck w/ 3.73s due to the 10 speed and mass of your truck. My tires turned my effective gearing from 3.73s into ~3.55 and I had no mileage hit and even at 6400lbs will spin all 4 tires.

 

[ThumperF150]

Have you verified the trans is what sets the speed and not the ABS? Never seen it confirmed. What you're describing sounds like the ABS is what's reading the speed. ABS reads wheel speed, which has not changed. If trans was responsible for setting the speed it'd wash.

I did tell you how to properly inform the truck of both gearing and tire size. It's common to change tire size and recal the truck's speed. Otherwise your mileage is off (and of course, speed). The raptor comes w/ 4.10s so it's an easy value. Personally I would have stuck w/ 3.73s due to the 10 speed and mass of your truck. My tires turned my effective gearing from 3.73s into ~3.55 and I had no mileage hit and even at 6400lbs will spin all 4 tires.

No have not verified if the dash gets signal from trans sensor or wheel sensors. If it was from the wheel sensors I think the indicated speed and RPMs would be different than stock. Since mine seams to be the same as stock, I think the signal comes from trans. But in the end it doesn’t matter where the signal comes from as the Hypertech will correct it.

I’m glad I went with 4.11s. Also thrilled with my 5.13s on my jeep but I know many that are happy with 4.88s.

 

[HammaMan]

You've got more than twice the power on hand than a jeep and a 10spd trans. Lack of torque on the ground won't be an issue. Surprised you haven't gotten a 'raptor bash plate' and trimmed the front end to fit it. Just need the front mounting brackets. Same as the tremor skid plate, but the front facia pieces that come w/ the tremor plate kit won't fit the 24.

https://www.f150gen14.com/forum/threads/tremor-bash-skid-plate-install-on-2024-f-150-stx.28195/

 

[ThumperF150]

You've got more than twice the power on hand than a jeep and a 10spd trans. Lack of torque on the ground won't be an issue. Surprised you haven't gotten a 'raptor bash plate' and trimmed the front end to fit it. Just need the front mounting brackets. Same as the tremor skid plate, but the front facia pieces that come w/ the tremor plate kit won't fit the 24.

https://www.f150gen14.com/forum/threads/tremor-bash-skid-plate-install-on-2024-f-150-stx.28195/

FTR, my jeep has 300hp and an eight speed. So not all that different in my opinion.

Feel free to build your truck the way you want.

I’m VERY happy with the mods I have made to my truck.

 

[ThumperF150]

OK so I just finished with the final install of the Hypertech in-line Speedometer Calibration Module.

If you have a 23 or earlier, the HT instructions are golden. If you have a 24 or newer F150 with the 12" infotainment screen you can toss the HT instructions in the trash.

I spent quite a bit of time tearing apart my instrument cluster as the instructions said, only to learn the plug ends are not the same and not compatible. I called their tech support and after some miscommunication, I learned that I should have torn apart my infotainment screen

HT tech swears up and down that it says in the instructions to remove the infotainment screen for 24s and up. IT DOES NOT. I read it four times. I asked them to send me exactly what they were looking at. They did and it was exactly what I was looking at on their site and THERE AINT NOTHING IN IT ABOUT REMOVING THE STEREO/NAV SCREEN.

Anyway, once I figured that out, I pulled the center dash apart like I had done for my stereo upgrade. Once you remove the screen, there is a sliver aluminum box behind it held on with four bolts from the front and two from the top which can be a bit tricky to get to but totally doable.

Once you get to that, the HT harness truly is plug and play.

As for the box itself, you need to first plug it into a PC and download their app and then tell the box what changes you made. I changed tire size and gears and entered that information. I changed both tires and gears because I wanted to retain stock overall gearing as closely as I could and I incorrectly assumed my speedo would be accurate. So when the HT box did IT's calculations it came up with a correction factor of less than one percent.

So of course, the test drive showed I was still a solid 8% off - indicated 60 was actually 65 per gps.

So I went back in and reprogramed the HT module with JUST my old and new tire sizes and told it I was using the stock gears. This time it gave a correction factor of about 9%.

This test drive proved the speedo to be dead on accurate, 60 mph indicated is 60 mph actual and my RPMs in 10th are about 1,600 - I think stock would be about 1,700/1,750 ish but I never thought to check.

So at the end of the day, I achieved my goal of bigger tires with an accurate speedometer and extremely close highway RPMs with factory shift points.

Thank you for everyone's input.

 

[HammaMan]

Hrm, so it sounds like speed is coming from the ABS then. Gearing wouldn't have any impact on ABS derived speed. Curious what values the device modifies

 

[ThumperF150]

Hrm, so it sounds like speed is coming from the ABS then. Gearing wouldn't have any impact on ABS derived speed. Curious what values the device modifies

I’ve been going back and forth on this. I was firmly with you on the wheel speed sensor sending the signal to the dash. But…..

My axles are turning 10% slower due to gears while my larger tires result in a 10% faster speed. So I’m thinking if the signal was coming from the wheel, the difference would be a 20% error.

If the signal to the dash is coming from the transmission sensor then it had not changed but my actual speed was higher by 10% due to my taller tires.

In the end, I told HT module the old tire size and the new tire size and it calculated to increase the signal by a factor of 1.099x. So right at 10% increase and it appears to be right on the money.

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