Speedometer reads too high

Problem: Speedometer and odometer read 25% too high, even though with oversize tires they should read low. Scanner attached to OBD reads about the same – ie too high. (Using GPS to read actual speed).

Otherwise the speedometer seems normal except maybe a bit slow to drop as the truck decelerates.

I am getting no error codes on the scan.

Questions:
• What is causing this?
• Since vehicle velocity is one of the data items sent to the fuel injection, is this affecting fuel consumption and overall performance?

Other info from my research:

I think the speedometer is cable-driven. In addition, I think, there is an electronic connection between the speedometer and the control module - maybe a speed sensor

It looks like there is a vehicle speed sensor on the transmission that is also connected to the control module.


So, which sensor is the control module reading: the speedometer or the speed sensor?

I have seen an YouTube video that seems to suggest that a sticky speedometer can cause it to read high, although I don’t understand why. Is it possible for the speed sensor in the transmission to give an erroneous reading?

Nothing in the Haines manual talks about any of this though the wiring diagram confirms that it is connected as I suggest above.

 

Your a 96 so your speedo is mechanical. So its possible that it need regrease behind the dash or a full new cable is needed.

There is no electric connection. Just the cable. kinda a paint to reconnect too..

did you buy the truck with aftermarket stuff on it? Swapping a small gear out on the transmission end is popular to make the speedo accurate with bigger tires. Do you also know if the truck was regeared in the diffs possibly?

 

Thanks.
I bought the truck earlier this year so don't know if anything has been re-geared, but it's possible.

A YouTube video about the speedometer shows an electrical connection to the speedometer, and so does the wiring diagram. The diagram suggests it is a speed sensor. I had concluded, and am glad you confirmed this, that it was a mechanical speedometer and that this, rather the the vehicle speed sensor in the transmission, is the cause of the error. However, the fact that the scanner agrees with the speedometer suggests that it is reading a speed sensor in the speedometer.

My main concern is whether this is sending incorrect information to the fuel injection system and affecting performance.

 

I just went through this yesterday on mine. the friction mechanism needs some very LIGHT lubrication and it should settle down again.

Pull the cluster out, it's a fairly quick job.
remove the clear plastic cover from the gauges.
Pull 7 screws from the back of the unit, 4 are for the speedo, 3 are for the tach. The display will fall out at this point, take care of the reset pin and the tach needle stop pin, mine fell out and had to search for it finally gluing the thing back in.

On the back of the speedo there is a round brass colored gizmo, don't know the proper name that spins causing slight friction with the needle and this is what positions the needle according to speed.

Use a very light lubricant here. I tried Tri-Flow first, don't do this as I was going 75 mph in second gear when I tested it. Way too thick.

I pulled it apart second time and used some electrical contact cleaner because it was what I had on the shelf to dribble out the Tri Flow. The contact cleaner has a residue that prevents corrosion so I thought it would work fine. And it did. Noticeable improvement right away and the needle settled back to normal after about 50 miles of driving. Hope this helps.

A very light lubricant, just slightly higher viscosity then water is what you want.

Here is the link that helped me with the cluster removal.
http://www.customtacos.com/tech.old/index.php?article=77

 

The speedometer has a hall sensor on the back of it and this is one of the speed sensors the computer is reading. Two of the afore mentioned screws are the contacts for the speed sensor on the cluster gauge.

What leads me to believe you need some lubrication is that you stated the needle will drop slowly when you decelerate.

While you're in there, put some white grease on the gears and squirt some oil down the speedo cable.

 

One more post.

You'll need to pull the center console, fairly quick, to access the center vent duct. Remove the screw that holds the duct in place, you'll see it, then push that thing to the side so you can get your arm and hand behind the gauge.

Feel for the speedo cable, it's a plastic connector and just clips on. Feel for a small tab near the cable end of the of the connector, press and pull, mine came right off. at this point there is enough wire to fully remove the cluster and unplug the connectors, there are three and they only go back one way so no worries there.

 

When you are saying that the speedometer is reading too high, could you be more specific? Are you saying that the speedometer is showing a speed greater than what your GPS is showing, or the exact opposite? As for the speed sensor, there is a sensor located in the cluster, where the speedometer gauge itself is located. This is the electronic sensor. The actual signal from the transmission is cable-driven, as you can tell from other posts in the thread. Good recommendations on how to clean and lubricate. The bad news is, without knowing if the gears or anything else has been changed, it will be difficult to tell what is throwing things off. GPS speed tests, while good, are also notoriously inaccurate unless you are going in a perfectly straight line with no obstructions to at least 3 satellites or more. The best way I recommend checking is either talk to a police officer you know well, or another that I have done was to drive at a consistent 60MPH on my speedometer for 10 miles on the interstate, using a stopwatch. Don't just do 1 or 2 miles, since there can be small inaccuracies in timing of you clicking the stop watch, and your maintaining of 60 MPH. 10 miles is a good standard.

 

Very helpful - thanks much! I'll plan to do this. Do you have any idea how the control module uses this and whether it is affecting engine performance?
Thanks again!

 

It does affect engine performance, not sure in what way. My speedo cable broke a few years back, i said screw it and used the gps for speed until the check engine light started to come on. Never ran funny, just had the light on until the cable was replaced, then it (the light) was happy again. It shouldn't cause any damage, most likely a fuel ratio change somewhere in the throttle curve, not sure exactly though.

 

When the speedometer reads 70, the GPS says it is going 55. I've checked it against another vehicle, so I think the GPS is approximately right. I have oversized tires, but they should make the speedometer read low, not high.

I'll try cleaning and relubing the speedometer as suggested, but I'm really curious whether the speed sensor in the cluster is actually used for the fuel ratio, or whether it is merely there to make sure you don't disconnect the speedometer.

 

Never thought of that, you could be right. lol
I noticed when it was apart, I couldn't reset the trip odometer with the push pin, but after re-installing, it worked fine. Not that I would roll my mileage back, I'm quite proud of my high mileage pickup

Light viscosity lube on the speedo mechanism, can't stress this enough or you'll peg the needle in second gear. I ended up using electrical contact cleaner and dribbled it in to prevent overspray staining. This morning's cold weather test went just fine, smooth needle and it's accurate again.

White grease on the plastic gears just for kicks since you're there, oil the cable for the same reason.

 

The local Toyota dealer seems to have no idea what the speed sensor on the speedometer is used for. My best guess so far, after looking at the shop manual, is that it is used for cruise control, automatic transmission or ABS brakes, none of which my Tacoma has.

 

Sounds like the previous owner regeared to compensate for the larger tires, and overcompensated.
If the speedo needle is not jumping around, and both the speedo and OBD port match, I would not suspect it to be a lubrication issue, since the electronic part of the system agrees with the mechanical.

Is the odometer off by the same amount? That is a direct-geared connection to the cable and not subject to the same type of distortion from lack of lube as the needle.

 

Page BE-32 of the shop manual indicates it is "speed control unit."

 

The speedometer does not jump, though it drops a bit slowly as the vehicle decelerates. The reading from the scan is a bit lower than the speedometer (e.g. When the speedometer reads 70, the scan reading is 65, and the GPS speed indicates 55 - the latter confirmed by an accompanying vehicle.

The odometer is off by the same amount - about 25%

 

The OP's speedo is off as if the truck has been re-geared.

When I re-geared from 4.10s to 4.88s, the speedo was off very similar to his. The exact error depends on what the original gear ratio was and what the new gear ratio is.

OP, how big are your tires? What is the gear code on the sticker on the inside of the drivers door panel? B03A?

 

Originals were 225/75R15
The ones on it now are 31/10.5 R15.

 

It makes sense that the odometer would be directly geared to the cable, Rich, so it seems right that this is a gearing problem rather than a speedometer lube problem. Thanks

 

That's about a 10% difference (slowing the speedometer).
If the PO switched the diff from 3.91 to 4.88, that would throw it 25% the other direction, leaving you with a net error of around 15%.

What are your RPMs at 65 (GPS), and what is the code for the diff ratio on the door sticker?

 

At 55 (GPS) the RPM is about 2330. There's no tach, so this is from my scanner.
I'm not sure how to read the ratio - here's some of what's there:
FRT 2650 with P225/75R15 TIRES
15 X 6JJ RIMS AT 26 PSI COLD
RR 2800 WITH P22R/75R15 TIRES
15 X 6JJ RMS AT 29 PSI COLD

RZN161L-TRMDKAB
C/TR
A/TM B02A/W59

Does any of that give the gear ratio?