C2 speedometer revisited

[John P.]

Hello All,

As always it is great to have access to a wide array of experienced tinkers here.

My speedometer on my 1965 329/350 convertible appears to be reading 8-10 mph too high. It is my impression that this is a fairly new situation. I havent changed tire size, though I am running modern radials.

I have removed and cleaned the cable-it is an AC branded metal cable. I cleaned lubricant from the cable housing attachment at the rear of the speedometer in the cluster. No change. At this point my next step was is to be to look at the driven gear at the transmission.

Would broken teeth on the driven gear yield a speedo reading too high? The speedo needle is not bouncing.

I have looked at discussions in the archives about a speedo restoration paper, and find that there are questions about it.



What would be my best source of further info on the speedo issues, and accepted repair principles? Thoughts on the high reading?

[Duke W.]

Re: C2 speedometer revisited

8-10 MPH at what speed? There are two kinds of speedo error. Gain error is a fairly constant percent error at all speeds, so you need to take readings at say, 20-80 MPH in 10 MPH increments with GPS or a modern car with known speedometer accuracy, and compute the PERCENT error at each speed. An incorrect speedo driven gear for the installed axle ratio and tire size typically causes gain error, which can usually be corrected by installing a correct driven gear.

Offset error is a fairly constant speed error at all speeds. This usually requires removal of the speedo and removing/replacing the needle to zero of the offset.

What is the installed axle ratio?

What make, model, size tires are installed? Look them up at Tire Rack or the manufacturers Web site and tell us the revs per mile figure.

Duke

[John P.]

Re: C2 speedometer revisited

My front are Bridgestone 20570R15 with revs per mile of 791. The rear are Firestone Affinity-not finding any specs for that particular size.

I am not seeing any differential numbers, but maybe not looking where I need to. Not sure about the ratio.

The error seems to be consistently 8-10 at 30 40 50 60-but that is just a "clinical impression" as we say. I'll have to see if I can cajole my wife to waste some gasoline to compare a vehicle with accurate modern speedometer to mine.

I am curious how one use GPS to calculate real time speed..

[Gary B.]

[Duke W.]

Re: C2 speedometer revisited

It's been years since I used a portable GPS (pre-smart phone), but they within 2 percent or less... don't know about the phone aps.

Get an accurate reading of actual speed at 60 MPH, cell phone GPS ap or modern chase car.

Axle ratio = RPM @ 60/760

The speedo gearing was basically set up for the pre-'65 760 revs/mile 6.70-15 tires. The new '65 7.75-15 tire size is about 775 revs per mile so 791 revs/mile tire would read about 4 percent high at all speeds.

Duke

[Justin S.]

Re: C2 speedometer revisited

Don't mile markers exist on highways to prove speedo accuracy or am I misremembering their purpose?

[Donald O.]

Re: C2 speedometer revisited

Don't mile markers exist on highways to prove speedo accuracy or am I misremembering their purpose?

Yes, the US interstate highways have mile markers that are 1 mile apart. I checked the speed in all of my cars using the phone speedometer AND the stopwatch on my Swiss made chronometer. All agreed to within +/- 0.5 mph. Close enough to me.

[John P.]

Re: C2 speedometer revisited

After taking the car out, it looks like I am off about 4 mph now at all speeds. 39 is really 35, 45 is really 41, 50 is about 46 etc.

So perhaps the tire change is the only real discrepancy. There is the matter of the speedometer noise however. It is still present to a degree.

[Donald O.]

Re: C2 speedometer revisited

After taking the car out, it looks like I am off about 4 mph now at all speeds. 39 is really 35, 45 is really 41, 50 is about 46 etc.

So perhaps the tire change is the only real discrepancy. There is the matter of the speedometer noise however. It is still present to a degree.

send it to our own Joe Ray along with your tach for any repairs or calibrations.