SpeedO Correction Options 37s+? Tires and Gears

[5GENIDN]

Guys keep in mind when you run 38s your computer thinks you have 34.7in tires so me changing the gears in the computer actually puts my computer in line with what real world is. So I would think it would shift better.

I would suggest not necessarily.... Torque requirements will change with tire size.

Sponsored

 

[Ducati1098]

Had to play with some numbers to even have a clue as to what you were referring to.... I am guessing the 2800mm is referring to the circumference of the tire. I thought (and please correct me here) that the FP input for the tire was in revolutions per mile?

Yes, forscan has you enter circumference in mm. But in reality the numbers don’t match up very well for some reason. 37’s should be close to 2950mm, but in reality that was far off of actual GPS speed.
I’ve never adjust size with the FP tuner, but I believe your correct that it’s revolutions per mile.

 

[5GENIDN]

Yes, forscan has you enter circumference in mm. But in reality the numbers don’t match up very well for some reason. 37’s should be close to 2950mm, but in reality that was far off of actual GPS speed.
I’ve never adjust size with the FP tuner, but I believe your correct that it’s revolutions per mile.

Thank you for that... I am only guessing based on what I have read... I will be facing this dilemma here very shortly... So I have been reading everything I can about it. But I have no practical experience as of yet. and that does make me feel better because I came up with 2950 and assumed there must be some reason for the difference.

 

[Bradley Thornton]

Thank you for that... I am only guessing based on what I have read... I will be facing this dilemma here very shortly... So I have been reading everything I can about it. But I have no practical experience as of yet. and that does make me feel better because I came up with 2950 and assumed there must be some reason for the difference.

To be clear you can't put in 2950 the highest you can go and the computer will take will be ((((2890))))

 

[One more upgrade]

I will give what I have and will update as others chime in.

I'm looking to update my computer to give me the correct readout for my 38s.

Tire Calibration
Lets start with ForScan for Tire calibration the highest number you can go is ((((2890)))) my understanding is this get you to a 37in tire or very close.

This can also be done with a tuner.

Gear adjustment Options?
The only option I can find is Ford Performance tune. It will let you change gears up to a 5.38.
Any other tuners? or Other Options?
Forscan? (Does anyone know if or how to do it)

Tool needed
https://www.tremec.com/calculadora.php

How to use and figure out what you need (Gear options I know of 4.88, 5.13, 5.38)

As an example I have 35s and 4.7 gears factory. Put in (Magnum XL 2.97 .63) Then add 2500 RPMs, 4.7 gears and Factory Sasquatch tire size came in at 34.7in tire. Now hit Calculate Speed. This gives you 6th gear 87.16mph reading.

Now with this data start a new Tab and check you tire size at same RPM and change gears size as needed.

For me if I change it to a 38in tire and use 5.13 that gives me a 87.45mph reading so I would leave my tire calculation alone and just add a 5.13 gear to the computer and call it a day. Close enough


Personally I'm looking for a different opinion vs Ford performance tune? Any help on other option would be nice.

I looked high and low, and Ford Tuner had me within 1-2 MPH of my radar detector with GPS on the freeway running 70-80 MPH. I run 5.13 gears and 38's. I am now very fond of the FP Tuner; no gear hunting, and it shifts smoothly. San Tan Ford did the gear install and programming, and I was very happy with how factory everything came out. My vehicle is shockingly quick on the street with large tires and runs 75 MPH at 2200 RPM.

 

[Bradley Thornton]

I looked high and low, and Ford Tuner had me within 1-2 MPH of my radar detector with GPS on the freeway running 70-80 MPH. I run 5.13 gears and 38's. I am now very fond of the FP Tuner; no gear hunting, and it shifts smoothly. San Tan Ford did the gear install and programming, and I was very happy with how factory everything came out. My vehicle is shockingly quick on the street with large tires and runs 75 MPH at 2200 RPM.

we are talk just putting it in the computer not changing the gears.

 

[5GENIDN]

To be clear you can't put in 2950 the highest you can go and the computer will take will be ((((2890))))

I understand.... that is why the reference to the "90" more....

 

[One more upgrade]

we are talk just putting it in the computer not changing the gears.

I realize that. In my opinion, the 38's tires were too big for 4.7 gearing on and off-road. Just letting you know what I experienced. Good luck getting what you need and I am interested in hearing what you find.

 

[Brian_B]

Here’s FP

 

[Brian_B]

Here’s FP

Just tried it out - changed the software ratio to 3.37 and ran out a couple of miles.

No check lights lit.
GPS still tracked with speedometer - so it did not adjust the speedometer
4H/4L/Hill descent and goat modes all seemed ok.

There was one logged DTC when I plugged the dongle back in to revert

 

[Bradley Thornton]

Just tried it out - changed the software ratio to 3.37 and ran out a couple of miles.

No check lights lit.
GPS still tracked with speedometer - so it did not adjust the speedometer
4H/4L/Hill descent and goat modes all seemed ok.

There was one logged DTC when I plugged the dongle back in to revert

So my idea does work?

Now just need to know if it can be done with ForScan and another tunes?

Also did it shift the same/ride the same as before? In theory it should.

 

[5GENIDN]

Just tried it out - changed the software ratio to 3.37 and ran out a couple of miles.

No check lights lit.
GPS still tracked with speedometer - so it did not adjust the speedometer
4H/4L/Hill descent and goat modes all seemed ok.

There was one logged DTC when I plugged the dongle back in to revert

That invalid data code is interesting..... I would guess that rpms did not match wheel speed.... Maybe???

 

[Brian_B]

So my idea does work?

No, it doesn't. Gearing didn't affect the speedometer reading at all in my testing. At least up to 50mph, and I changed the ratio by 1.09, so that definitely should have shown a big change (25% give or take?).

It doesn't appear to break anything obvious, but it did throw a DTC code.

There are some F150 forums with that same DTC code saying it broke some things with cruise control (which I did not test in my limited testing)

https://www.f150forum.com/f118/u0422-00-2f-invalid-data-received-body-control-module-545616/

I have a manual so I don't know if it would also affect Shift points or anything.

 

[FordOwner]

FWIW - I have a 2.7L with 4.70 gears and went from factory 35" sasquatch tires to 37'' BFGoodrich All-Terrain T/A KO2 tires. I have the Ford performance calibration and adjusted the revs/mile to 569 when I put on the 37" tires because that is what I found online here. My speedometer seems about 1-2 MPH off based on smartphone installed GPS based apps.

Tonight I went to https://tiresize.com/calculator/ and put in 36.5/12.5R17 and got a result saying 553 revs/mile as the number I should use. I put 553 revs/mile into my Ford performance calibration and went for a drive. The 'service advancetrack system' light stayed lit up during my test drive, the speedo seemed even further off and cruise control would not work. If I tried to turn it on it immediately shut off. I reverted back to 569 revd/mile. The 'service advancetrack system' light disappeared and cruise control works again.

Oh and take a look at this post and the rest of this thread for Forscan change info: https://www.bronco6g.com/forum/thre...can-for-2021-bronco.23064/page-8#post-1336626

 

[‘21OBX]

I will give what I have and will update as others chime in.

I'm looking to update my computer to give me the correct readout for my 38s.

Tire Calibration
Lets start with ForScan for Tire calibration the highest number you can go is ((((2890)))) my understanding is this get you to a 37in tire or very close.

This can also be done with a tuner.

Gear adjustment Options? Not changing the gear just putting it in the computer.
The only option I can find is Ford Performance tune. It will let you change gears up to a 5.38.
Any other tuners? or Other Options?
Forscan? (Does anyone know if or how to do it)

Tool needed
https://www.tremec.com/calculadora.php

How to use and figure out what you need (Gear options I know of 4.88, 5.13, 5.38)

As an example I have 35s and 4.7 gears factory. Put in (Magnum XL 2.97 .63) Then add 2500 RPMs, 4.7 gears and Factory Sasquatch tire size came in at 34.7in tire. Now hit Calculate Speed. This gives you 6th gear 87.16mph reading.

Now with this data start a new Tab and check you tire size at same RPM and change gears size as needed.

For me if I change it to a 38in tire and use 5.13 that gives me a 87.45mph reading so I would leave my tire calculation alone and just add a 5.13 gear to the computer and call it a day. Close enough


Personally I'm looking for a different opinion vs Ford performance tune? Any help on other option would be nice.

I’m at 2780 in forscan for the nitto 37x12.50/17 for a dead on speedo.

Sponsored