speedo / PCM calibration

[addkev]

In response to my other tranny thread, I found a dealer about 10-15 miles from here that will reprogram my computer so I can run my 31's again. Quoted me $70.

 

[Apten]

[tech nonsense mode on]
Dealers that use an NGS or CarDaq for EEC scanning and reflashing can go into the "VID" block and change "tire revolutions per mile". The VID block contains a very small number of things the dealer can actually program, including the vehicle's vin. In order for a dealer to be able to calibrate the speedo, they have to understand how to calculate tire revolutions per mile.

225x55x16 size tire would be:

((0.55 * 225) * 2) + ((16) * 25.4) = 653.9 mm in diameter
247.5 + 406.4 = 653.9/ 25.4 = 25.74 inches diameter
25.74 * 3.1415926 = 80.86 inches in Circumference
5280 feet per mile = 63360 Inches per mile
63360 / 80.86 Inch Tire size = 783.57 revs per mile

And that's how a dealer would calcualte "tire revolutions per mile". That does exculde some factors from the mix, but it will get you close.

Also if you drastically change gears it will throw off the automatic transmission shift pattern.
[/tech nonsense mode off]

I belive that started in 98 with explores, as it did in 99 with mustangs. There is no longer a changable gear for the speedometer. You can buy an aftermarket device to compensate, or you can just get a custom chip. The computer has a static value for tire size and axle ratio, so that's what needs to get changed with a chip or reflash.



Brian

 

[sonkakaro]

You guys think i should get my speedo recalibrated after i put on my new wheels? I am running stock 255/70/16 wheels right now but i bought a set of P245/65R17 wheels which will be put on in the next few months. I checked the miata tire calculator site which is very good at http://www.miata.net/garage/tirecalc.html
It says the speedo reading w/ the new size will be 1.7% to fast so when it reads 60mph i'm only going 59mph. That doesnt' seem to bad to me.

 

[addkev]

I wouldn't bother, honestly how accurate do you think your speedo is straight from the factory?

 

[Okrazie1]

Originally posted by addkev
I wouldn't bother, honestly how accurate do you think your speedo is straight from the factory?

From the '99 Ford Service manual:

"Odometer Accuracy Test

To test the odometer accuracy, drive the vehicle over a measured distance of at least 16 consecutive kilometers (10 mile). Check measured distance against odometer measured distance. Acceptable odometer measured distance is 15.5-16.7 km (9.6-10.4 mile)."

Correlating this to the speedometer reading, a reading of 57.6 to 62.4 MPH at an actual 60 MPH is acceptable.

Well, that's how accurate Ford thinks your speedometer is from the factory.

 

[sonkakaro]

well!

 

[Robert]

And besides that a new speedometer gear will change your speed in approximately 5% increments. The only time a new gear is needed is when you change your tire size by 3% or more. Any less than that and the speedometer can be off by that much from the factory.

 

[Apten]

forgot to mention, for dramatic gear changes it will cause the transmission shifts on 96-up automatics to act differently. Fixable via chip, but otherwise noticable. For example, going from 3.08's to 4.27's

 

[beachn974door]

I am a tech at a dealership in town and have never had a problem recalibrating a speedometer for anyone. I had and Excursion on 39" Mickeys and was able to get it within 5 mph of being dead on. Just my experience

 

[addkev]

I am a tech at a dealership in town and have never had a problem recalibrating a speedometer for anyone.

Well, trust me you are a rare commodity. It took me months, and I mean months, to find a dealer that would reprogram my '99. Ask Alec, he is still looking I think.