Explorer Speedo Calibration Limit

[umr_engnr]

So I took my 2000 Explorer to the Ford dealer to get the revs per mile changed because my Ex has 33" tires. The factory setting was 736 revs per mile and my tires by the manufacturer spec (32.6" overall diameter) should have been set to 619. The dealer did not do this a lot, and was concerned they couldn't do anythign, but once getting it to the tech they were able to get into the ABS module and change the setting. The problem is they said that the tool only lets them change it down to 679.

It isn't really a huge problem now, but if I end up going to 35" tires, I'm going to run into this again.

Can anyone out their confirm that the limit is 679 and are their any alternatives to get this corrected.

If it was just the speedometer I wouldn't be concerned, but I have heard that the revs/mile is used for shift points as well. Does anyone know exactly what the PCM uses the revs/mile field for?

 

[Blown]

Revs per mile is used to calibrate the speedo and MPH

Shifts are based upon TPS and MPH
WOT shifts are based on RPM and TPS

 

[umr_engnr]

Looks like an SCT x4 might be in my future. It appears to handle larger tire sizes. I've got an email to their support team to see what their range is.

Anybody here run an SCT x4 on a stock drivetrain? Any noticeable improvements.

At what point do you actually benefit from a custom tune from an outfit like LaSota Racing?

 

[fastpakr]

Based on the thread below, it looks like Forscan supports the change you need. Much cheaper than an SCT tuner.
Speedometer Calibration/Tire Size - FORScan forum

 

[Blown]

I do not see a way within programming for the 98 - 2000. The speedo is run off the rear ABS sensor and gem module. I am going to look into Forscan to change it.

On my stock 2000, 5.0 I added 3 degrees of timing at 5400 feet which may have gained 4 horses. The biggest gains I got was in tuning the tranny to shift later and shift harder 2-3 and 3-4 as well as downshifting a bit sooner when I stomp on it.

Looks like an SCT x4 might be in my future. It appears to handle larger tire sizes. I've got an email to their support team to see what their range is.

Anybody here run an SCT x4 on a stock drivetrain? Any noticeable improvements.

At what point do you actually benefit from a custom tune from an outfit like LaSota Racing?

 

[CDW6212R]

I'm interested to see how the Forscan works out. I'll likely end up with 29" tires on my 98, and the speedo is calibrated best for a 30" tire. There is a calibration device/box you might find that does work to adjust the signal. I have one on my 91 Mark VII that has 3.73 gears. It has four wires, I got mine used for about $50 in 1997. One power, one ground, and the other two run to the VSS wire(cut and spliced to them).

 

[Blown]

Bummer, I see that Forscan will only allow up too a 265/75 -15 658 revs/mile. That is likely Ford's max, unless Forscan only added so many options to choose from. It was a pick list, I was not able to enter the revs/mile, so chose the largest diameter tire.

I just put on new 265/75 16's, set the Speedo in the GEM module with Forscan to the max (265/75 -15), and my speedo is off by what you would expect, reading about 3% low. 3.3% to be precise.

There tuning considerations with larger tires on 98-2000 Explorers. Though that 3.3% is not a big difference in my rigs case. I can imagine tuning to shift later when it's reading lower..................best bet climbing with large tires.

 

[umr_engnr]

Thanks for following up. I can't believe there isn't easy way to do this.

 

[Blown]

A black box type thing like the old Tru-speed would be the only way to get the speedo to read right on these 98-2000 X's, but it wouldn't necessarily feed the correct signal to the EEC for shifting...........................................again that is were you would benefit from a tune. 33's are 12% larger than stock 245/70 16's, I'd add 12% to shift points to keep the engine in the power band for optimal performance. Like upshifting to 2nd at 44.8mph instead of 40 at WOT.

 

[SoHK_alumni]

Wonder if a Dakota Digital interface might be the answer.

Grab the speed signal output from the ABS unit (grey/black wire) and adjust the Dakora output to keep the PCM and speedo happy.

Not a ton of money.

 

[one chance]

I believe it will be the same as my 97. change the gear on the VSS located on the transfer case. you will want to increase the number of teeth on the replaceable gear to speed up your speedometer.

 

[MAS Tequila]

The simplest and most effective way is to swap gears to end up with the same rpms at speed.

With the 3.73 gears and a 33" tire my numbers come out to needing a 4.29 gear.

No one makes that gear for the front diff, so go 4.11 or do like me and go 4.56, I'm about 100 rpms higher than with the factory tires and gears at 75 mph.

Not only is the speedo close, but now it runs and shifts like it should.