Speedo gear change - which one?

R

rjlester

Senior Member
In my 57 Monarch (same as US Mercury) with the push button automatic, the speedo is way off. On the GPS, it reads 62mph, but the speedo shows 70mph. 8mph too fast.

There is an 18 tooth gear right now, I know I have to put a larger one in there, but I have yet to confirm how many mph one tooth difference makes. Somewhere I read online that its 2mph per tooth, but at what speed? It's different at slower speeds for sure, my car is only off about 4 at 40mph.

It has the type of gear that is "left canted" or left rotation if that makes sense. From what I found, that is typically in a manual trans, not automatics. Strange.

I can find a 21 tooth on a couple of websites, I take it that will get me a lot closer that what it is right now.

Am I in the ballpark? Where is the best place online to order these gears? I also have to order a 20 or 21 tooth for my 54, hopefully a place has both.
 
Found the gears on the National Parts Depot website for both vehicles. Just need to figure out what size now.
 
Before you change any gears check the accuracy of the odometer. It's gear driven and more likely to be correct than the spring and spinning magnet controlled speedometer needle.

If the odometer is relatively accurate the error in the speedometer may be due to it needing cleaning, lubrication or re-calibration, not a gear change.

EDIT...
The odometer in my '55 Bird is right on the money compared to highway mile markers & GPS but, the speedometer is always 15 mph higher than it should be.
Conclusion: the drive gears on the cable are correct but the speedometer mechanism itself needs attention &/or adjustment.
.
 
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I will check the accuracy of the odo tonight, didn't think of that.

As for lubricating the speedo, I did take it out and lubed it. In fact, the speedo cable broke because the speedometer seized up! You couldn't even turn it with a pliers. A drop of oil in the right place(s) and I got it lubed perfectly. I've rebuilt these before so I know how to do it properly and have had great success with others.

That speedometer always had a bit of a quiver and the couple of hours of use before the cable broke, it was really jumping around. We were on a road trip and could not investigate the problem at the time.

The transmission was rebuilt by a previous owner and I suspect they put the wrong gear in there, have had that problem with rebuilt transmissions a few times.
 
70 is 113% of 62. 1.13 times 18 is 20.3 so a 21 tooth should have the speedometer reading a little slow. I think I'd opt for a 20 tooth.
 
Excellent, thanks. That formula makes sense. Might buy 21 and 20 just for the hell of it as they are cheaply priced.
 
I got my gears from ebay. Lots of choices there. I head that one tooth equals about 4 MPH. I don't remember when I did mine.


You have to go with a happy medium as far as reading. In my 54, the speedo reads low at lower speeds, right on at 60-65 MPH, which is what I wanted. I have a 4 speed in the 54.
 
Update, got the gears in the mail from National Parts depot. Tried the 21 tooth first, a little slow reading at 70mph. Put in the 20 tooth and it's perfect at 70! I didn't think I would get it that close. Glad I bought both gears as I know for sure which one works best.

Thanks guys for your tips and to GLAmes for the formula!
 
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