5969vette

Recently picked up a 69 a couple weeks ago. All gauges work well, but the speedo is off by about 30MPH. The motor is a 1981 350 and the tranny is a Muncie M20(2 piece speedo cable attached). Could this be a mismatch and throwing the accuracy off? Can this unit be calibrated, or a gear off?

Just looking for advice where to start - thanks

doorgunner

If the speedometer needle does not fluctuate rapidly you may only need a new plastic DRIVEN GEAR. Remove the gear from the transmission.....shine a light into the trans. to see the color of the internal gear.....then go to a vendor like Willcox and click on the speedometer gears for sale/also click on the Technical Info Section concerning speedometer calibration charts to see which NEW plastic gear that you need so it will match the internal gear that is in the trans.( the current tire diameter and the differential gear ratio affect the plastic gear size)

7T1vette

Unless the differential in that car has been swapped out with a very low ratio unit, it is unlikely that a speedo gear change will resolve your problem. It may be that the speedo head is defective.

69FASTFUN

What size tire are you running? That has an effect on the speedometer

doorgunner

He could buy a new plastic gear 2 sizes different from the driven gear that he now has.....

If that doesn't correct the 30 mph difference or it doesn't make the reading more accurate....

then it proves that you are right about the head being defective

I wonder if there is a way to repair and re-magnetize the metal in the speedometer head to make it accurate again

loup68

I think that in 1969 there were only one piece speedometer cables. That could be part of the problem. Lou.

69FASTFUN

My 69 was a one piece speedometer cable for a 4 speed.. My 72 which is an automatic is two piece.

7T1vette

To correct a 30 mph error due to wrong speedo gearing, you would have to change out the drive gear on the trans output shaft as well as the driven gear (which is accessible). Changing only the drive gear by 3 teeth would make about a 10 mph correction.

It is because of the magnitude of the error that suggests to me that the speedo head could be the problem.

doorgunner

In the next couple days I will try to correct the 20 mph "too fast reading" on my TH400 A/T with 3.36 stock diff. gears...I will use my GPS to compare the reading and post the results here.

7T1vette

doorgunner....

What has changed to cause it to read 20 mph too high?

Red 71

Adding to what 7T1 stated here, you can only use speedo gears with a certain number of teeth on certain tail shaft gears. For example, a speedo gear with let us say a arbitrary number of 10, 11, or 12 teeth may work on a certain tail shaft gear but any less or more teeth will no doubt chew up the teeth on the speedo gear in a relatively short time. If you needed to go to a different speedo gear you would need to change the tail shaft gear also. You need to determine exactly what ratio rear end you have first to be able to eliminate possible problem areas.

Here are a couple calculators you can play around with to simulate your speed at given RPM's to tell you what speedo gears it says you would have to have in order for the speed you are seeing to be correct. These may eliminate some possible problem areas.

http://www.crawlpedia.com/rpm_gear_calculator.htm
http://wahiduddin.net/calc/calc_speed_rpm.htm

ps: I am sure this is understood but when you verify the RPM versus speed to input in the calculators use fourth gear and thus a 1:1 transmission ratio. Do like a exactly 2000 or 2500 RPM setpoint. You can juggle the speed and rear end ratio numbers to see what the speedometer thinks it has. Having to tear into the dash is the last thing you want to do unless it is the last resort.

doorgunner

Thanks for the explanation Red71 and other members!

We'll solve the speedometer readings now!

5969vette

Ok - lots of info here First off I was told multiple things from the previous owner about the speedo and the speedo needle is not jumping around.

He told me the speedo was off due to tire size and possibly the low ratio unit?

Tire size is 235/50 ZR17
Tranny P0B26A (Muncie 4spd 1970 Feb 26th M20 Ratio
Axle ID cant tell: Top number is either 88171975 or the first 8 is a 9 or 3?
Side of case looks like A784 (7 is questionable but can't imagine it is anything else but maybe a 1) but then again cant find a translation for A784
Bottom of case is GM 10
2 piece speedo cable

Thanks!!!

7T1vette

To get a 40% jump in indicated speed (from actual speed), the gear ratio for a '69 car would have to be in the area of 4.56:1 and the tires would need to be as large as would fit under the car. If that were the case, your tachometer would be showing about 3000 rpm at 50 mph.

Is that the situation you have? If so, you will have to use one of the speedo gear calculators to determine which trans output shaft gear you need AND which speedo drive gear you need. The only other solution would be to purchase one of those speedo drive correction units and install it in the speedo cable system.

If none of this matches what you have in the car, your speedo head need recalibration.

5969vette

Thank you 7t1vette. I will hook up GPS to get precise speed and watch tach - but I think I am very close to that - need to confirm and will try when I get home tonight. Also I assume this is in 4th gear - correct?

Do you think the 2 piece speedo cable could also be an issue?
Also having a hard time confirming Axle ratio from numbers on box - just doesn't add up?

doorgunner

The car was a basket-case when I bought it in '13. The differential gears are original....but everything else has been altered----tire diameter is smaller----wrong driven gear in TH400 transmission---possibly wrong drive-gear also---damaged odometer drive gear---damaged cable....all these problems add up.

All the other problems have been corrected/I only need to replace the driven gear.....then I will know if the head has a magnetism problem if the speedometer is still way-off.

5969vette

Ok now I have the data and chart I added below:

-Tire size is 235/50 ZR17 (probably the largest that fits under with out modification)
-Tranny P0B26A (Muncie 4spd 1970 Feb 26th M20 Ratio
-Axle ID build date A184 (Jan 18, 1974): I know this is probably irrelevant since I am almost positive the car was re-geared very low 4:??.

The gear in the transmission I just removed is red - so I assume 21 teeth.

So I should I start with a 18 tooth gear to see what happens?

1969 Corvette - GPS actual vs Speedometer (4spd)
Numbers in green are the Deltas from Act vs the Speedo then Tach Reading and Gear

Act Speedo
Spd Read Delta Tach Gear
25 43 18 2000 3
35 57 22 1700 4
45 70 25 2000 4
50 78 28 2300 4
55 85 30 2500 4
60 94 34 2800 4
65 102 37 3000 4
70 108 38 3200 4
75 115 40 3500 4
80 126 46 3800 4

doorgunner

My new driven gear arrived USPS....I will try to install it this afternoon and post the info.

5969vette

Oops - meant to say start with a 24 tooth. Also I know the Muncie 4 spd only goes to 25 teeth max - it looks like the OD changes above the 21 tooth that I have in now. Goes from 1.84" to 1.76" is that ok?

doorgunner

I have brain lock----can't help you......or me! I hope I went the right direction with the tooth-count......will know for sure later today!