Speedometer Question

olddog · #1 (**permalink**) 07-27-2008, 02:42 PM

If a speedometer is reading wrong because the gear ratio in the trany cable was not correct, I would expect the error to be a percentage. That is if it was reading 60 mph at a true 40 mph it is reading 150% of actual, therefore I would expect it to read 30 mph at a true 20 mph.

Here is what I got with a GPS.

10 spedo - 10 GPS
20 spedo - 23 GPS
30 spedo - 37 GPS
40 spedo - 47 GPS - local police radar 46
50 spedo - 57 GPS
60 spedo - 67 GPS
70 spedo - 77 GPS

So from 30 mph to 70 mph on my speedometer the actual speed is always 7 mph higher. The error is a zero shift problem not a span problem. The only way I am aware of correcting a speedometer is to change the gear in the trany at the cable connection. Is there a way to adjust a zero shift in a speedometer?

wtm442 · #2 (**permalink**) 07-27-2008, 04:10 PM

You can discount the small errors at low speeds. But the consistent 7 MPH offset is a zero error. Do not change the gear in the tranny.

I would suggest contacting the gauge manufacturer and ask them for their comments. There very well may be a zero adjustment screw, and if there is one, its probably accessible thru a small hole on the back of the speedo "canister".

I've got Smith gauges in my SPF, and amazingly it is only off by 2 MPH at 130 MPH.

Ibr8k4vetts · #3 (**permalink**) 07-27-2008, 09:36 PM

I believe you can change the speed by 6 MPH if you change the gear on the speedo cable at the trans.

Bob In Ct · #4 (**permalink**) 07-28-2008, 07:58 AM

Is the odometer accurate? The odometer should determine the gears in the tranny. BTW, almost all speedometers are designed so that the cable makes 1000 turns per mile of car travel.

Bob

trularin · #5 (**permalink**) 07-28-2008, 08:55 AM

Speedometers are generally calibrated to three points on the dial. In the case of after market, I think it would be best to set for 70 with a GPS or State trooper.

Mine is set right on by asking a trooper to ride next to me at 70 his car and my GPS. Actually I asked him to drive 70 on I75 and I got next to him. He was a very nice person, one of the guys who came out for the inspection.

olddog · #6 (**permalink**) 07-28-2008, 06:08 PM

Quote:

Originally Posted by trularin

Speedometers are generally calibrated to three points on the dial. In the case of after market, I think it would be best to set for 70 with a GPS or State trooper.

Mine is set right on by asking a trooper to ride next to me at 70 his car and my GPS. Actually I asked him to drive 70 on I75 and I got next to him. He was a very nice person, one of the guys who came out for the inspection.

How does one adjust these three calibration points? I have VDO gauges.

olddog · #7 (**permalink**) 07-28-2008, 06:10 PM

Quote:

Originally Posted by Bob In Ct

Is the odometer accurate? The odometer should determine the gears in the tranny. BTW, almost all speedometers are designed so that the cable makes 1000 turns per mile of car travel.

Bob

I have not checked to odometer. So you are saying the trany gear should be chosen to get the odometer correct?

olddog · #8 (**permalink**) 07-28-2008, 06:13 PM

I have been searching for VDO gauges, but all I find are venders who sell VDO gauges. I cannot seem to locate the manufacture, so I cannot find any type of manual.

Ibr8k4vetts · #9 (**permalink**) 07-28-2008, 09:16 PM

Quote:

Originally Posted by olddog

I have not checked to odometer. So you are saying the trany gear should be chosen to get the odometer correct?

I'm sorry for butting in... But yes, if you are 7 MPH off just change the gear on your cable. They make like five diffident gears and each one will change the speedometer by six MPH.

wtm442 · #10 (**permalink**) 07-29-2008, 02:33 AM

Quote:

Originally Posted by olddog

I have been searching for VDO gauges, but all I find are venders who sell VDO gauges. I cannot seem to locate the manufacture, so I cannot find any type of manual.

They are part of Continental.
www.vdo.com

I did a quick Google search. I only found instructions for an "electronic speedometer". I did not see an instruction for a "mechanical speedometer".

http://www.egauges.com/Inst_PDF.asp#Speedometer

There are instructions on how to calibrate the electronic versions. They use a common "sender" at the tranny.

I hate to ask, but do you have an "electronic speedo"?

Ron61 · #11 (**permalink**) 07-29-2008, 05:00 AM

I used the VDO Electronic Speedometer in my Cobra and loved it. They use a Hall Effect Sending unit and can easily be adjusted when you change tire sizes, gears, or anything else that will cause the reading to change. And mine was accurate to the exact MPH per the CHP radar at 50, 60, 70, and 80 MPH. The tach reading was also right on at those speeds. I could care less about the car looking original as nothing about it was even close. And I wanted something that was easy to use and worked more than a few weeks or months before failing. I had my car almost 7 years and never a gage problem.

Ron

olddog · #12 (**permalink**) 07-29-2008, 05:37 PM

Quote:

Originally Posted by Ibr8k4vetts

I'm sorry for butting in... But yes, if you are 7 MPH off just change the gear on your cable. They make like five diffident gears and each one will change the speedometer by six MPH.

Butt in all you like. That's why I posted.

My issue is that my spedo is off 7 mph at 60 and it is off 7 mph at 30 as well. So if I change the gear to make it read 66 at an actual 67, then it will read 33 at an actual 37. In other words it will still be off by 4 mph at 30 mph. I was hoping there is a better solution that the gear ratio option.

GlynMeek · #13 (**permalink**) 07-29-2008, 05:42 PM

I gotta go with Ron here. the VDO Electronic Speedo is the one I have and it works (and adjusts) PERFECTLY!!! If you' re not overly concerned about accurate appearance, this might be the way to go!

Glyn

olddog · #14 (**permalink**) 07-29-2008, 05:46 PM

Quote:

Originally Posted by wtm442

They are part of Continental.
www.vdo.com

I did a quick Google search. I only found instructions for an "electronic speedometer". I did not see an instruction for a "mechanical speedometer".

http://www.egauges.com/Inst_PDF.asp#Speedometer

There are instructions on how to calibrate the electronic versions. They use a common "sender" at the tranny.

I hate to ask, but do you have an "electronic speedo"?

Yes I have the mechanical gauge. I wish it was the electric, as it seems to be very adjustable.

After my last post yesterday, I was able to trace VDO back to Siemans, as the parent company. Siemans is a large european compay that sells drives, programable logic controller, and much more in industry, but I digress. Anyway I cannot find a gauge that looks like my speedometer, so I suspect it is no longer made. In reading their technical literature on current mechanical speedos it looks like the only calibration correction option is changing the gear.

olddog · #15 (**permalink**) 07-29-2008, 05:51 PM

Quote:

Originally Posted by GlynMeek

I gotta go with Ron here. the VDO Electronic Speedo is the one I have and it works (and adjusts) PERFECTLY!!! If you' re not overly concerned about accurate appearance, this might be the way to go!

Glyn

I agree it is the best option, but I would rather spend that money on a dyno tune. I guess I will put up with it being off for now and add it to my wish list.