Reprogramming For Tire Size and Gear Ratios

[Mort]

I've got that little fuel computer thing, which is annoying, but it would be more annoying if it were off. Does this reprogramming fix that as well?

 

[mynameisaric]

my guess would be yes.

 

[wm_sorg]

What is the procedure for an 03. Can the dealer reprogram the PCM for different tire size?

 

[ExplorerDMB]

Yes, an '03 is similar

 

[tktrain]

The computer doesn't care what gear ratio you have when it comes to your speedometer, by reprogramming, it knows that one revolution of the axle (read by abs toner ring) = (x) inches and it calculates speed from there. It can then use that information to calculate shift points.

Problem with the measuring of the circumfrance of the tire is that with road deflection of the tire, the measurement can be incorrect, if you mark and roll the wheel under the load of the truck you get a more accurate reading. I am sure it would get you very close however.

Josh

O.K., I have a question about all of this. Am I reading correctly that on a tone ring Explorer (1998 up) that changing gears will have no effect on the vehicle in terms of the speedometer and shift points if the tire diameter remains the same as factory original? I am looking at putting 4.10's in my X and I want to have everything mapped out and ready to go in advance.

 

[mynameisaric]

will not affect your speedo.... the shift point thing i duno about.

 

[tktrain]

will not affect your speedo.... the shift point thing i duno about.

I guess 1998 was a transition year for Ford. If you look at SCT's web site, it says that the SCT2 adjusts the gear ratio and tire size on 1999 and up vehichles. There is a post in their forums that says that 1998 Mustangs used a drive gear off the transmission for the speedometer and transmission output speed feedback. 1998 X's used a tone ring via the GEM/ABS system to control the speedometer. So my question is, do I need to tell the PCM that the differential gear ratio has been changed or does the GEM module tell the PCM whats going on?

 

[tktrain]

Bump.

 

[mynameisaric]

it goes by revolutions of the wheel. so lets say you're at 3.55 gear ration with 100 revs per mile on 33 inch tires(made up number), and you bump your gear ratio to 4.10 on 33 inch tires... your revs per mile is still 100 per mile. your engine turns more, but the tire rotations stay the same. since the gizmo in the rear end reads revs per mile, you can do whatever you want to the gear ratio, and it will still read accurately as long as it is setup properly for your tire size.

hope that helps.

 

[nofortunatesonII]

Doug904...

Another tire size related website...
http://www.miata.net/garage/tirecalc.html

 

[rwenzing]

For the 98~2001 4-door Explorers that make the speedo correction in the 4WABS Module: The factory shop manual gives the correct tire revs per mile value for factory Explorer tire sizes. Revs/mile for other tires can often be found on tire manufacturers' sites.

If you're stuck with a tire and no specs, you can calculate the revs per mile youself. A method using a chalk mark to determine rolling circumference was given earlier in this thread. Here is an alternate method from the Explorer factory shop manual:

• Inflate the tires to the correct cold inflation value.
• Park on a level surface with the weight of the truck on the tire.
• Carefully measure from the level ground to the top of the tread.
• Divide 20,168 by the tire height measurement (in inches) to get the tire revs/mile.
• Use the dealer configuration tool to enter the revs/mile value into the 4WABS Module.

Here are a few generic factory values for reference:
(revs/mile)

225/70R15 - 00761
235/75R15 - 00721
255/70R16 - 00691
31x10.50R15 - 00677

 

[stsmoker]

Allright, so I just read through this thread and if I read it right, correcting the Speedo wont correct the shift, so could i Dealership correct both the speedo and rpm's and shift points? And I'm still in High School, and im in auto and we have a Scan tool, not sure what kind but its a nice little thing. You think the scan tool may be able to correct the speed and shifts?

 

[uefear]

What can you use to reprogram a 2002 Sport?

 

[rwenzing]

What can you use to reprogram a 2002 Sport?

A 2002 Sport does the speedometer correction in the PCM. A dealer can adjust for a tire diameter and/or gear ratio change using an NGS or equivalent tool. It could also be done via an aftermarket flash programmer like those from SCT or Sniper.