Speedometer not accurate...

[bandit1]

'66 Mustang (V6,Auto):  The speedometer is not accurate.  It hasn't been for a while.  My guess is that it's about 5-10 MPH below the true speed.  I'd like to fix it, but don't know how.  Please advise.  Thanks, Tom.

[racedraper]

if you cant the tire size any at all will make your speedo incorrect. all you have to do is take it to a speedo chop and have the change the gears. it should be about 150 or more

[daver]

make sure the gear in your transmission correspond to your tire and rearend gear ratio combination. a larger tire or a lower gear ratio can have huge effects of the accuracy of your speedometer.

[bandit1]

1966 Mustang (Engine: 200 CID Straight-6, C-4 Automatic Transmission, Carburetor: Autolite 1100, Rear Axle: 2.83:1, Tires: P205/75R14, Style: Convertible, Color: Candyapple Red).
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Where do I locate and how do I remove the driven gear?  I looked, but can't find it.  I saw online that the 66 Mustang original tire size is 185/75-14.  Is this correct? If so, I don't know if the speedometer was previously calibrated for that size or the current size (205/75/14). Thanks.

 

[66GTKFB]

The driven gear is at the end of your speedometer cable. What you need to do first is see if the odometer is accurate. If your odometer is accurate, it's most likely the speedometer arm getting hung up.  Most states have a reasonably accurate milage marker along side major roads. Find a long stretch of road with markers, get up to 60 mph by your speedometer,  check the time elapsed between one mile markers with the second hand of your watch and the distance travelled on your odometer (remember the position of the tenths). At 60 mph, you should be doing one mile a minute and the odometer should read one mile.
Jim 

[bandit1]

The odometer is accurate. It has been tested.

[Soaring]

The original tire size of a 66 was a 6.95/14 which translates to a modern tire size of P185/75/R14.  I have 195/70/14's on mine and my odometer is correct.  The 205's could have an effect.  Take out the gear right where the cable plugs into the transmission and get two or three different colors to experiment with.  Here is a calculator which uses your tire size etc. to get you pretty close to the number of teeth and the color of the gear. http://www.mindspring.com/~steveflyer/speedo_gear_select.htm . I find the best way to check the accuracy, is to have someone ahead of you in a new/good  car travel at 60 mph for a few miles with you on their tail.  If you are maintaining 58 mph, then your gear is 2 mph off.  I would also pull the cable out of its' shield and use a good supply of graphite to make sure the cable isn't dragging inside the shield. 

[bandit1]

Speedometer is about 13-15 mph below what it should be.

[Soaring]

OK, that's fine.  Now use the calculator I gave you to find out which new gear you need to buy.  Once you figure that out, get one color on each side of the one you selected so you can get it down to a science. 

[66GTKFB]

No, No, No. A gear change will affect the odometer readings as well as the speedometer. Think, Glen, think. The odometer is correct, therefore it's the speedometer dial. There is a way a speedometer shop can fix it but don't try it yourself.
Jim

[Soaring]

I've never heard of that Jim, that the odometer and the speedometer run off of a different gear.  There is only one cable running from the transmission that is spinning both the odometer and the speedometer.  So, you are saying that the tech needs to change the speedometer gear inside the gauge pod somehow?

[66GTKFB]

There is no second gear. The speedometer is deflected by a magnetic rod (or bar, there are different systems) that is turned by the cable. The faster you go, the more deflection and the dial reads more. Break one open and look. There are two pivots (aka, friction points) and the magnet.
Jim

[bandit1]

I removed the driven gear. It's 16R and is light yellow. I believe 16 is the correct teeth based on the formula. I've heard that the drive gear is 7 and is specific to the C4 transmission. Is it possible that is not actually the one being used?  If so, could that be the reason for the issue, can it be removed and where is it located? What are other possible sources of the speedometer problem?

Tires= P205/75R14
Differential Gear Ratio= 2.83
Rear Tire Diameter (Inches)= 26.11
Speedometer Revolutions/mile = 773
Drive Gear Teeth Number = 7   

[Soaring]

Get you another gauge pod off of Ebay.  Obviously your odometer is working perfectly, but your speedometer is not correct.

[bandit1]

Ok. I drove the car and used a GPS device to show the actual speed I was driving.
At 30 mph (GPS), the speedometer showed 20mph. At 40 mph> 30mph. At 60 mph> 45mph.

[66GTKFB]

Ok. I drove the car and used a GPS device to show the actual speed I was driving.
At 30 mph (GPS), the speedometer showed 20mph. At 40 mph> 30mph. At 60 mph> 45mph.

What was the distance? The speed and distance are interconnected.
Jim

[bandit1]

I didn't record the distance.

[66GTKFB]

The speedometer drive gears in the transmission control the distance traveled, the odometer, directly. The speedometer is a secondary function of the rotating motion of the cable in the speedometer housing. Changing the gear to 'correct' the speedometer will affect the odometer aversely. Now, knowing that your odometer will be screwed up to 'fix' your speedometer by changing the gear, don't you think it would be wise to verify that the distance travelled is or is not correct? And I use my GPS receiver to verify all six of my car's odometer/speedometer readings.
Jim

[bandit1]

The distance traveled according to the odometer is correct.  I reset the trip on the GPS and compared it to the odometer. I thought you were asking for the total distance of the drive.

[Soaring]

Your driven gear is fine, your odometer is fine, but your speedometer is screwed up.  Either take it to a technician to try to fix the speedometer, or get one off Ebay and replace the whole pod.  That would be the cheapest way, but you would lose your old odometer reading.  If that is important, then find a technician.  Do not try to do this yourself.  Ask me how I know.