Be happy that the odometer works, many or most have failed in the last sixty years. That error is pretty normal for C2s.

 

That's a great point. Given that all of these cars are 60 years old or thereabouts, if the worst deficiency a C2 has is the odometer (and probably the speedometer as well) is off by <4%, I admire the standards of perfection sought but am doubting that this would be the most significant item needing attention, just given the car's age.

 

You should review the requirements in the NCRS "Corvette Operations Manual & Performance Verification Test Guide" for C2 Corvettes. It specifies the Odometer and Trip Odometer should be accurate within 5%. The Speedometer should be reasonably consistent with the estimated driving speed and minimal needle bounce at 50 MPH.

 

Hello,
If it helps my speedometer was rebuilt in 1988.
Im running radials 215/70/R15 on knock offs.
According to the speed signs I pass it shows my speed exactly 2mph off every time.
My speedometer is 2 miles slower than the speed sign.
I attributed this to the larger diameter tires.
My trip odometer starts turning over the odometer to the next mile at 7/10th on the trip odometer . It does reset to all zeros as I reset it to all zeros every time I fill up at the pumps.
It is not calibrated as I would expect the odometer below the speedometer should start rolling over when the trip odometer reads 9/10ths.
Hey it’s close enough for me.
If yours works as stated above your ahead of the game 👍

 

Here are the things we know:
1. the driven gear is made of natural (non colored) plastic and has 19 teeth.
2. my car has a 3.08 rear end (f that matters).
3. my car's speedometer measured 9.65 miles when it was probably closer to 10.0 miles (measured with another vehicle's odometer).
4. when my car's speedometer says I'm doing 64 mph, the speedometer app tells me I'm doing 70 mph.
.
Will a different driven gear get me closer to actual MPH and miles driven?
If yes, do I need a driven gear with 18 teeth or 20 teeth?

 

If you are worried about the measured mileage in a PV scenario, you need to increase by 5% (barely) or decrease the tooth count. The speed is slow by 10%ish. Did you reach out to the Team Leader to solicit his thoughts?

 

First, it is state of the art stupid to compare your distance measurement to another car's odometer unless you know the other car is accurate (or at least know how inaccurate it is and in which direction. You are much better off using distance markers (like at least 10 interstate or highway mile markers making sure they don't begin or end near an exit or intersection where it would be impractical/impossible to place them).

Having said that, let's assume your distance measurements are correct. In that case, you are underreporting distance so the cable needs to turn more times per mile. Fortunately, your speedometer is also wrong in the same direction so a gear with fewer teeth addresses both.

Distance correction switching from 19 to 18 tooth driven gear: (9.65 mi / 10.0 mi) * (19/18) ~ 1.02 -- You improve from -3.5% to +1.8% (both within the 5% margin of error)

Speed correction: 64 mi/hr * (19/18) = 67.6 mi/hr --- You improve from -8.6% to -3.5% with the change to within the 5% margin of error.

My advice would be to swap to the 18 tooth gear then have the speedometer calibrated to read 3.5% higher than it does after your gear change. That will make your speedometer essentially dead on and leave your odometer still at +1.8% and that should be close enough for anybody. The other problem is reducing the needle bounce and the best I have found for that is a product called Kable-Ease after cleaning the cable itself and housing with carburetor cleaner.

 

.
To have the speedometer recalibrated, isn't that going to require the removal of my instrument cluster or at the very least pulling it forward?

 

You may need to go down to a 17 tooth gear which I think is the smallest available. Kind of surprising though as a 3.08 was not the tallest gear available in some model lines available with a 4-speed. That might be an indicator that your speedometer calibration is off a little - no way of knowing from here. Yes, recalibration of the speedometer is normally a shop repair thing.

 

Yep. They essentially drive the speedometer with something like a drill press on its side (or a lathe) at a known speed (it's actually a dedicated device) and adjust a torsional spring until the correct reading is attained. In your case, since you know how much to correct you could do this yourself with nothing more than a variable speed drill to chuck your cable (as long as you are able to obtain repeatable speeds e.g. with your trigger stop), you simply adjust the speed up by about 3.5%. Once you see the spring and the adjustment lever, the method is obvious. The bad news is whether you send it out or do it yourself, to get to the adjustment lever, you have to open the speedometer up --- so it has to come out. You remove the two screws on the back and slip the cover off. That exposes the part of the speedometer with the spring and adjustment lever.

Your other option would be to simply install the gear and live with the speedometer inaccuracy. The point is, you can get your odometer very close and because the speedometer is semi-independent, you can get it very accurate but there ain't no such thing as a free lunch.

The 17 tooth gear corrects your speedometer to about +2% but overcorrects your odometer (again, assuming your distance is correct) by about 8%.