Speedometer gear selection

[mgm1986]

Working on my 1968 300. I found a few threads on the topic but I did not have luck getting this resolved. Hoping to get additional insight before I buy another gear.

When I got the car the speedometer was around 6 MPH off; showed 50 on Speedo, GPS says 44. This was pretty consistent at all speeds but it jumped a bit, maybe because the 32 tooth gear had missing teeth.

After replacing the rear with 3.23 from Dr Diff and putting on new tires (235/70r15) I used a chart from Mancini to determine I needed a 31 tooth gear to correct the speedometer.

With this 31 tooth gear the speedometer is off by 10 mph, across all speeds. GPS says 40, speedometer says 50.

The engine is not original so I am not sure if the transmission is either. Engine is a 1972 or '73.

Any insights as to what the best approach is to get my speedometer working accurately? Did I incorrectly interpret what gear I needed or is there some other formula I can use to get the correct gear?

 

[volksworld]

one annoying thing i've found with my regular daily drivers is that their speedo's read higher than the maps app...was this designed in at the factory so we'd get less speeding tickets? not all tires of the same numeric size are the same diameter, this varies by brand...since you weren't accurate with your original setup and the needle bounced your speedo may be binding up a bit and might need some lube...if you're not into taking everything apart to do this you're back to playing with gears...I'd put your original 32 back in and see what that does and based on that you can guesstimate what one tooth more does and see which way to go

 

[CBODY67]

In checking calibrations on speedometers, look NOT at the speed, but the DISTANCE traveled instead. I know, this is opposite to common convention, it is the fact that the odometer is driven by the cable which attaches to the speedo gears in the transmission DIRECTLY, as the speed function is a result of that spinning cable, magnets, and a clockspring (which brings the needle back to "0". THOSE things can vary, but the number of times the cable turns/mile will NOT vary.

The odometer on my '67 Newport is dead accurate, but the speed indicated is about 10% too high (from my digital wristwatch timing between milemarkers on the Interstate, as 60mph = 60 seconds, before GPS was around).

The car came with 3.23 gears and had 8.55x14 tires from the factory. It now has P245/70Rx14 radials on it (which are the same diameter as the OEM 8.55x14 bias-ply tires).

IF you might change the speedo gears to get a more accurate speed reading, DO NOT complain about gas mileage suddenly decreasing with no vehicular changes. As the odometer will now be registering less miles than actually traveled.

CBODY67

 

[413]

More gear teeth slow down the speedo, so put the 32 back in and see.

The engine and trans swap should not affect this.

 

[Mopars & Missiles]

If 10% too fast, then use a gear with 10% more teeth. 5% fast, 5% more teeth.

If 10% too slow, then use a gear with 10% less teeth. You get the idea, I think.

 

[Big_John]

With this 31 tooth gear the speedometer is off by 10 mph, across all speeds. GPS says 40, speedometer says 50.

If the speedometer is off 10MPH at ALL speeds, it's definitely not the gear.

If the speedometer is off 20% at ALL speeds it could be the gear.

 

[mgm1986]

If the speedometer is off 10MPH at ALL speeds, it's definitely not the gear.

If the speedometer is off 20% at ALL speeds it could be the gear.

That's exactly what I was looking to confirm. I'll do more driving and confirm numbers before I do anything else.

 

[mgm1986]

It looks like the speedometer is off roughly 20% when compared to GPS at four different speeds. I am so curious though why the charts would suggest I needed a 31 tooth gear based on the rear end ratio and tire size. If I go up 20% more teeth I'm using a 37 tooth speedo gear.

I can buy the gear but it seems like I'm missing something.

 

[Big_John]

It looks like the speedometer is off roughly 20% when compared to GPS at four different speeds. I am so curious though why the charts would suggest I needed a 31 tooth gear based on the rear end ratio and tire size. If I go up 20% more teeth I'm using a 37 tooth speedo gear.

I can buy the gear but it seems like I'm missing something.

View attachment 724381

You've established that it could be a gear (and it probably is), but it could also be a speedometer problem.

As @CBODY67 mentioned, checking the odometer will tell you a lot. The odometer is mechanically connected, where a speedometer is isn't. The speedometer uses a spinning magnet in a spring loaded cup that's attached to the pointer.

So, if the odometer is off 20%, it would be a gear. If the odometer is right, or off a different (but significant) percentage, the problem is in the speedometer head.

 

[413]

Your speedo needs repair, not a 37 tooth gear

 

[mgm1986]

Considering this may keep my wife from getting us a speeding ticket, I'll put this one lower on the list and start researching what a speedometer repair involves.

Thanks as always for sharing your expertise, makes life easier learning from those who've been down this road before.

 

[volksworld]

fwiw...finally took mine on the highway since the gear swap...3.23's, 225/70 15s General Altimax (Tire Rack claims its a 27.4 inch tire), 32 tooth pinion ....when speedo reads 55 Maps says 58