Hello from a newbie.

I am changing my '98 Expedition rims/tire and will increase the tire height from 30" to 31.46". By my calculations, my speedometer will read slow by about 5%. I've searched on the internet and it sounds like I need to replace the VSS gear--actually decrease it by one tooth number (though I'm not sure what number I have now).

From what I've read, this is a pretty simple 10 minute project. Of course, if you know how to do it, anything is simple I suppose. I am not mechanically inclined when it comes to automobiles although I am comfortable around the toolbox.

So I have a couple of issues. Where can I get a VSS gear with one less tooth than I have now. The local Ford dealer was not much help. From my online research, it seems like it should only cost ~$20 or so.

Once I have the gear, I've read that it's a very quick fix to put it in, but I can find no detailed advice on how to do that. What I DON'T want to do is pay for a minimum one hour labor at ~$70 for a job that's going to take 5-10 minutes.

Any and all advice is appreciated.

Thanks in advance.

BTW, my stock tires are 255/70-16 and I am switching to 285/60-18s. In perusing these forums I read about guys changing tires and then needing to change rear gears. Their tire heights were changing by several inches. Mine will change by 1.4." Is this something I need to be concerned about.

And I have a 4x4 with trailer tow pkg, 5.4L V8. What would my stock gear ratio be right now? (I think its 3.73).

Rob

 

The PSOM in the spedometer needs to be reprogrammed. The "tone ring" is not touched. Factory (not Chilton, Haynes, etc.) service manuals have the instructions, or just pay the dealer labor to reprogram it. This is MUCH simpler than disassembly of the rear axle and separating the ring gear.

Jim

 

Jim,

Thanks for the reply. If I understand correctly, since I have a '98, it's a VSS gear change vs a reprogramming. I have a mechanical odometer/speedometer, not a digital one.

Am I missing something?

If it's as involved as getting into the rear axle, I'll definitely take it to the dealer.

Rob

 

I'm not sure about your Expedition, but the VSS is not a direct gear/gear relationship like a mechanical speedo gear on a transmission. In the VSS an electronic sensor fixed in the rear axle housing reads information magnetically from the 'tone ring', which rotates and is attatched to the big ring gear in the axle. I've not heard of tone rings with varying tooth patterns for adjusting the speedo.

In any case, replacing the tone ring is by no means a 10 minute job. It involves pulling the axle shafts and removing the ring gear assembly. As JimandMandy stated, reprogramming the speedo is the way to do it on vehicles with the VSS, so far as I know.

Your tire size difference is not all that much, I went from 31.5 to 33 and didn't do the speedo reprogram and my speedo isn't off by all that much and I've calculated that if I drive it this way for about a 100K miles, my odomter might be off by 5K miles. No biggie to me. Since you likely have an electronic controlled transmission, your trans may shift a little later than it should since you will be going a little faster than the computer thinks you are.

Superlift, as well as a couple other companies, sells an electronic box that goes on the wire comming from the VSS and corrects the information for various tire sizes before it gets to the computer in the truck. These boxes are not cheap. Maybe there is some information about reprogramming your speedo in the 'tech articles' section of this website? There is an article about it for the '90's F-series trucks, not sure what other Ford vehicles it may apply to.

 

Thanks for the replies guys.

I took my Expedition to the dealer today. He plugged two different machines into the port and was unable to find any menu to adjust the speedometer.

He did say that in 1999, they went to the electronic speed sensor and he can reprogram those but not 1998 models. So he said changing the gear was probably the only option.

My Expedition has 255/70-16s with a diameter of 30.07." My new tires are 285-60-18s with a diameter of 31.46."

The option when I purchased this vehicle was for 265/70-17s with a diameter of 31.61" which is VERY close to what I'll end up with. So I asked the guy in parts to look up and see if the VSS gear is listed with two different part numbers for the factory 16" and 17" rims.

Sure enough it is. So I'm thinking that maybe it will be a matter of replacing the $8 gear after all.

Since I made my initial post, this seems to be the prevailing theory on various "truck forums." I guess it gets more complicate finding the gears when you move away from factory specs, but I'm hoping in my case, the gear change will do it.

I understand that otherwise my speed and odometer will only be off by about 4.5%, but if I can fix it cheaply, why not?

Thoughts?

 

Geez, now I'm replying to myself.

Now I am really confused. The dealer was unable to find a way to program the module to correct for my new wheels.

There are in fact two different speedometer gears available for a 98 Expedition--one for a 16" OEM wheel and one for an 18" OEM wheel. The respective part numbers are C0DD-17271-B and C3D2-17271-C.

I got curious this afternoon and removed the one bolt that holds on the VSS to the rear transfer case. Instead of finding any part with gears, the end of the cable is terminated with a cylindrical device about 1 - 1-1/4" long and maybe 3/4" in diameter. I assume this is the sensor that picks up the rotation of the axle. I stuck my finger in the hole and felt a gear inside pretty much under the hole where the VSS is attached.

So if the dealer can't program it, and there is no gear to switch out, what's left aside from a third-party box which I'm told doesn't work on pre-99 model years anyway.

Thanks for any insight.

 

Hmmm... maybe one of the Superchips or Hypertech or other brand of 'tuners' has that feature??

 

That may be my next choice, though at SOME price, I'd be willing to leave it alone and put up with the speed discrepancy.

I'm going to try another dealer tomorrow just for grins. If one of the factory options was for tires similar in size to the ones I want, I find it hard to believe that Ford would "lock it in" at the plant. But I've seen stranger things.

Hey, anyone in Cincinnati have a tuner they'd be willing to lend out for a few minutes??