Speedo Off

My 348 has a problem with the speedometer not being correct. I guess this is pretty common but it is really annoying. I checked it with my GPS and it varies from being off a few mph at 30 mph to being about 12 mph off at 70. Since it isn't linear it makes me think it may be the tires. They were on it when I bought it so I don't know if they are the right size.

Here are the tires I have:
BF Goodrich G Force Super Sports
Front- 215/ 50RZ17 91W
Rear- 255/ 45RZ17 98W

Are these the correct size and what can I do to fix it?

 

Should I change the sender? I read a bunch of the previous posts and they all seemed to discuss that they weren't working at all. Mine works but it is just off.

 

Thanks Bruce. I just read some old posts on changing the ratios and the math works out perfectly. I assumed they changed 5th from the standard 4.34 to 3.53 which is a ratio of .813364 and works out almost perfectly to what I am seeing on the speedo. Now the real question. It drives me crazy trying to drive around when it is so far off. Does anyone make anything to correct this? I work in factory automation and I can do it with a PLC to correct but I don't want to add a big box in the engine compartment to do such a simple conversion. I just wish they used the old gear type speedos that I could just change them out.

 

I'll have to check that but it does seem to be closer at lower speed which would be a different gear.

 

What does the sender pick up off of? I assume that it is just a prox switch and sends a pulse at whatever frequency it sees from the gear it is close to. Is this correct, does anyone know if it send a frequency or voltage signal back to the speedometer? I know this is a common problem and there has to be some way to correct it even if I need to come up with some scaling device in between the pick up and whatever it sends it to?

 

That means that it is missing pulses since it is running slow. If it has 5 voids and is missing one it would be .8 x the speed and that is what I am seeing. I looked at previous posts and I saw where it is located but haven't tried to play with it. I'll do it this weekend. Is there any height adjustment on it? I would think if is missing pulses that it might just need to move in a bit?

 

That is what I was referring to with the .8 x speed or 80% of actual. If you see 4 out of 5 voids you should see 80% of what the actual speed is. It should be a percentage of the speed or

(x pulses per rev of the shaft/ sec) x (the circumference of the rear wheels/rev of the tire))= mph after scaling. With a hall effect sensor, if it can be adjusted closer it might not miss as many pulses. It either has to be that or that the frequency counter in the electronics is off and it doesn't count correctly or am I missing something?

 

Rob, I just checked out the SpeedoHealer and this looks perfect. I am going to run a few tests to see if my car stays a consistent percentage off at different speeds and if it does I will order one and let everyone know how it works out. Here is the site if anyone else is interested http://www.healtech-electronics.com/

 

Just got off the phone with the company that makes SpeedoDRD. They sound like a better fit and are anxious to help. The other guys are out of Hungary so any support would be minimal. The guys at SpeedoDRD have already recommended a product and sent me the wiring diagram so it may work. I'll keep you informed.

 

You mean all this time I haven't been going 0-60 in 2.8 seconds?

 

Does anyone know if the speedometer receives a variable frequency or is it a variable voltage from the ECU? The device I found adjusts the frequency but not the voltage so I need to clarify this one point before I order. If anyone has the info it will save me from tearing apart the dash to verify it.

 

I have the unit on order that I think will correct the speed issue. I'll let everyone know if I have success and try to do a write up of how to do it for everyone with pictures.

I need help with a wiring schematic of the speedometer if anyone has one. I need to splice the wire that has the feedback to the speedometer. If anyone knows which wire it is or where I can get the drawing it will make this much easier. I'm sure I can figure it out but it would be helpful know which one it is going in.

One other thing. Is there a link to removing the instrument cluster? I did a search but couldn't find anything.