The speedometer in my 68 seems to be out of calibration. Compared it to a GPS and it appears to read slower than actual speed. (GPS is accurate with my other cars.) Not sure if it is out by the same amount at all speeds..but for the most part under 55 it appears slow by 7 mph or so.

Did some researh and apparently there are different speedometer "gears" available for different transmissions. Mine is a PowerGlide.

Couple of questions:

1. Where are these gears located? Are they in the speedometer head or in the transmission?

2. Each gear has a number (looks like a ratio) and a "teeth" count. Does this ratio have anything to do with the rear end gearing ratio (or am I out to lunch)?

3. What is required to replace these gears?

4. What is the correct speedometer gear for my car? 327 (4 bbl) with a PowerGlide.


Any info would help. Thanks!!!
Vic

 

I only have ford knowledge on this subject, but I can answer a couple questions for you, The speedo gear is super easy to replace, literally pull it out and stick the new one back in, unfortunately I cannot tell you where it's located on your application. Also, there should be a graph or something of the like, that shows you how many tooth speedo gear you need to compensate for the rear end gear. If your rear end gear is stock, changing the speedo gear most likely won't do anything to make your speedometer more accurate.

 

Thanks for the quick response, Cobra.
Sounds like the first step is to identified what the rear end gear ratio is.
Thanks!!
Vic

 

At the tail of the trans, remove the speedo cable, the lock screw and bracket that holds the speedo gear housing down, and slide it out. The gears go by color/tooth count for easy recognition. And yes, knowing your rear gear ratio would be helpful.

 

Thanks Camaro69.

I will check out the rear axle....and try to figure out what is installed. I will also check out the existing gear.
One more thought that just crossed my mind....I suspect that another factor in this is the actual wheel size, etc. I am running 14" tires on stock rally rims. I will post exact tire numer shortly.

Thanks again,
Vic

 

It's not your wheel size that's a factor, it's the overall diameter (height) of the tire.

 

Do you know the process of checking the gear ratio without tearing into anything? If not i'll go ahead and explain.
Step 1. Place a mark on the driveshaft that will be noticeabe and easy to see.
Step 2. Do the same with the tire.
Step 3. Get a buddy or whoever to watch the driveshaft or tire, have the buddy spin the tire one rotation slowly. If the mark on the drive shaft passes you 3.7 times, you have a 3:73 gear, 4.1 times, 4.10, 3.5 times, 3:55, and so on.

 

Step 3 above. Is assuming you have a posi, both tires need to be off the ground and you make one revolution of the tires.
Step 4. If you have an open differential, you need to have one rear tire on the ground and give the tire that's off the ground two revolutions.

 

Thanks guys...how about I just give you the rear axle code: BP 0228 G2

(Being a 1968) I interpret this as 2.73 ratio, 10 bolt non -positraction. My tire are P205/70R14...not sure if this tells us the height of the tire...???

Vic