2nd Gen Non-Engine/Transmission Speedometer accuracy

[jacklapointe]

I noticed my truck seemed to be traveling a little slower than the speedometer indicated. If I did the speed limit, other folks were blowing my doors off as they went around me. Got out the GPS and discovered a 4 MPH gap all the way from 5 to 70 MPH. In other words, when my speedometer says 54 MPH, I'm really doing 50 MPH, 64 is really 60, and so on. Ive seen this before with low-profile tires so I did some checking. I didn't buy the tirers on the truck, but they are 245/75R16E's - just what they should be. I also found a web site, www.dodgeram.org, that has a tire/gear/MPH calculator and the values there sync up nicely with my speedometer readings. So we've got a 4 MPH mystery here. The specs say the speedometer is right, but my GPS system is a better measure of true speed. Wonder what's going on? Anyone seen this before? What causes it, and more importantly, how do I correct it? I'm aware that the dealer can adjust the speedometer for tire size by updating the PCM, but it looks like the tire size is correct for my truck. Any thoughts?

 

[vssman]

The '96 has a vehicle speed sensor on the side of the transmission. If you remove the sensor, you can change gear sizes to correct the speedometer. The gears are dealer items and are around $8. 00 Rickson wheels resells them too.

 

[MCummings]

Originally posted by jacklapointe

I didn't buy the tirers on the truck, but they are 245/75R16E's - just what they should be.

Sounds like you may have bought this truck used.



My Theory:



Maybe the previous owner had bigger tires, and had the speedometer compensated for, and before he sold it, he only put the correct size tires on it, but did not re-correct the speedometer.



BTW, the "Fast" speedo is putting more miles on your truck than you are really driving.

 

[jacklapointe]

Thanks for the input.

Your assumption is correct. I bought the truck about 3 mo. ago with relatively new tires. I wondered about the tire size issue, but ruled it out when I found that tire/gear/MPH calculator. (www.dodgeram.org/tech/specs/axle/47auto.htm) The speeds described on that page are just what I'm seeing on my speedometer, eventhough my GPS says otherwise.

I guess if I can't come up with some other, perhaps unexpected, explanation, I'll try the speed sensor gear swap. Sounds easy enough, although I thought you had to have a dealer reflash the PCM. Seems like a larger gear would turn slower thus lowering the reported speed, right? I've got to lose 4 MPH somehow.

Again, thanks for your suggestions.

 

[Marv Windecker]

Speaking of tire sizes and speedos, a few years ago I wrote a little program for fun to calculate MPH, RPM, Tire size, OD ratio,

and axle ratio. If you give it any of the 4 you know, it gives you

the missing one.



It's totally free for the download so I hope this isn't considered

advertising - if so please remove this post.



It ain't fancy but works well.



It can be found on Marv Klotz's site. Other interesting stuff

there too - especially if you are a machinist.

http://www.geocities.com/CapeCanaveral/Hall/4425/gearing.zip



Marv Windecker

 

[Dennis Garrett]

If you have 96, pull your dash trim, remove the clear plastic bezel. Drive it down the highway by the GPS. When you are traveling at a constant known speed, pick the needle off the speedo, and reposition it on the little post where it indicates the correct speed!. Worked for me, takes a little pressure to pick it off to start with might want to take it off before you start driving, then put it back at the correct speed. Before you do that, change the gear going into the transmission tail shaft and get the odometer as close as you can, then "calibrate" the speedo as above.

 

[vssman]

Jack,



I used to make the vss sensors before the company that made them moved to Mexico. Correcting the speedometer on 12v trucks is accomplished by changing the gear. 24v trucks use a sensor off of the ring gear that is non-adjustable. DCX did this when they combined speed sensing with ABS functions. If you pull the unit straight out of the transmission without twisting it, you'll probably be able to reuse the o-ring. The unit can be installed two ways depending on the number of teeth. The gear pulls off of the adapter housing. It's very easy to do.

 

[KBarr]

I believe Garrett is on to it. If the problem was with the speed sensor gear, you would have a PROPORTIONAL variance, say 10%. You seem to have a constant 4mph. I went to 185s and I have about a 10% difference. I'm actually moving 33 when my speedo says 30, and 66 at 60. You can also verify the odom/speedo as well as your GPS (which probably is most accurate only if you are travelling level ground) on your next interstate trip. Reset you trip meter to 0 when you pass a mileage marker. When you get to the one 10 miles down the road, see how much short (or long) you are. I wouldn't expect the markers to be placed close enough for a 1 mile test, but at 10 they should average out pretty close. If your odo says 9. 9 after 10 miles, you are 1% off. More or less.

Another way is to find a state trooper parked next to the intersate, and see how fast you can go. He'll probably jump at the chance to help you out...

 

[jacklapointe]

Again, thanks for all the suggestions guys. I am beginning to see the problem in a somewhat different light. For example, when I use Marv's gearing formula I get a speed more like the GPS numbers. In addition, I do see an issue with the constant 4 MPH differential - should, in fact, be proportional. I'm not so sure about recalibrating the speedometer pointer, however, the speedo is zeroing out nicely at this time. If the vss gear is as easy and inexpensive to change as you say, that should at least bring the hwy speed numbers in line. Picking a replacement gear will be the trick though. Once again I do appreciate the the help.

Jack

 

[Dennis Garrett]

The needle will still park at "0" when you stop, after you re-position it. You can try all the different gears, 31,32, or 33 teeth, but don't count on the odo/speedo to both be right at the same time. You might the odo right, but the speedo will be a few off & vice versa. About the only way I could get mine right, was regear until I got the odo as close as I could, then correct the speed by repositioning the needle.

 

[MCummings]

repositioning the needle. [/B]

Sounds like the best advice so far.



I agree, the speedometer should be proportional. (10%)



Sounds like repositioning the speedometer pointer would be the best bet.



How does your speedometer act when close to 0MPH, say idling in gear does the speedo "Jump" to 4MPH when you are in reality only doing 1-2 MPH?



Info like that might be able to tell you if the speedo is off, which would warrent only a repositioning of the "pointer" or if changing the transmission gear would be nessacary.



Maybe the previous owner had a different "face" which would require removal of the pointers to install a replacement face, maybe the dealer had to replace, (or previous owner) replaced the "aftermarket face" with the original one and didn't get the speedo pointer just right. (In that case other pointer could also be "off")

 

[fortunate1]

After the original 245/75/16 tires wore out, I changed to 235/85/16s. They are taller and roll farther than stock in one revolution of the tire. I have done some "tests" on my own with a GPS, highway mile markers, and my speedo & odo. Here's what I found out:

1) The highway markers that I used are right on the money; 100 mile markers was exactly 100. 1 miles on the GPS.

2) My speedo was very close to the GPS speed. At highway speeds, the difference between the two was only a tenth mph.

3) My odo was off by almost 3%. Travelling 100 true miles caused the odo to click off only 97. 1 miles. So I've actually driven a little bit more than the odo says. When the tires are worn and ready for replacement, the difference is more like 2. 7%.

4) I'm not going to bother with changing any gears or needles for this.



Phil

 

[KBarr]

The GPS is great, but remember that it usually only tells your speed in a single plane. If you are going up and down hills, your odo and speedo will show higher values than the GPS. It will be amazingly accurate with your boat on the lake, but very WRONG on mountainous roads. This may or may not be true with the more advanced units, it certainly is possible to mathematically calculate the groundspeed since the unit knows the altitude changes, but all the units I've used didn't do this.

But the argument is good enough for this purpose. The real point may be what is "accurate enough" from a speedo? 2-3% is ok with me. 5% could be a problem for those of us who push the speed limit to the very edge. And I'm sure those police radar units are 99. 9% accurate and calibrated before every shift. Right?