Speedo problem

MorganMcCann · Mar 13, 2015

My speedo on my 99 Ranger (3.0L 4x4 off road package) is always 3 mph off. Is this a sign of a bad VSS? or what else could it be?

Thanks!

adsm08 · Mar 13, 2015

Tire size or calibration are the most likely culprits. I think 3 MPH is within the margin of error though.

RonD · Mar 13, 2015

Yes, ^^^
I am sure your '99 doesn't have its original tires, they would have worn out long ago and were replaced.
Look on the Drivers door sticker, it should list the factory tire size it came with.
And check what you have on there now.

I think these sizes were all available on a '99 Ranger 4x4
235/75R15 = 28.9" diameter and 698.7 revolutions per mile
265/75R15 = 30.6", 658.4
245/75R16 = 30.5", 662.3

Your(all) speedometers are calibrated for the original tire diameter and its revolutions per mile(distance traveled).

But as adsm08 said Factory speedometer can have a +/- 4% error which is 2.4MPH at 60mph

MorganMcCann · Mar 14, 2015

The factory size is 245/75R16, currently I have 265/75R16 duratracs on it. However when I was running the factory size the speedo was almost 9 mph off when doing 70, but it varied when doing other speeds.

RonD · Mar 14, 2015

Maybe a bad day at the factory when programing the modules

Anyway the modules(ABS and GEM) can be reprogrammed to read within 1 or 2 MPH of actual speed with corrected tire size.

Good read here on how that is done: http://www.explorerforum.com/forums/showthread.php?t=146122

Most DIYers are not going to have a Ford scan tool, so this is something a Dealer would have to do.
However if you are only 3MPH off at higher speeds then I doubt the expense would be worth it for maybe 1MPH difference.

MorganMcCann · Mar 14, 2015

I tried several dealers before I got new tires, and they all told me they could not re-program it.

adsm08 · Mar 14, 2015

The dealer can in fact recalibrate the speedometer, within certain pre-set limits. They can't calibrate it to make it read at a certain spot, like to readjust yours to get that 3 MPH error out. They can change the tire size and gear ratios being used to calculate the speed, but they can only be reset to things that were factory options. So you can change from 215/75/15 to 235/80/15, but not to 35". Or you can change from 3.73 to 4.10, but not not 4.88 or 5.13.

RonD · Mar 14, 2015

As a last resort or "best resort" depending on your point of view, lol, you can use a Dakota Digital speedometer interface, the SGI-5

Here is the manual for it: http://www.dakotadigital.com/pdf/sgi-5c.pdf

With this it won't matter what tires or gears you use, you can change the calibration went you want.

This is the catalog page: http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/

MorganMcCann · Mar 16, 2015

RonD said:
As a last resort or "best resort" depending on your point of view, lol, you can use a Dakota Digital speedometer interface, the SGI-5

Here is the manual for it: http://www.dakotadigital.com/pdf/sgi-5c.pdf

With this it won't matter what tires or gears you use, you can change the calibration went you want.

This is the catalog page: http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/

That looks pretty cool, do any of you have experience with it?

Speedo problem

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