Calibrating The Speedometer And Odometer
Mar. 4, 2007
 With a shop tachometer, tape recorder, notebook and calculator, Lynn sets off to calibrate the speedometer and odometer on Penny. |
Jump to:
- Correcting The Speedometer Reading, At What Cost?
- Taking It On The Road
- Penny Probably Has The Stock Differential Ratio
- Can Lynn Get An Overdrive For Penny?
Correcting The Speedometer Reading, At What Cost?
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From observing the speed registered on radar speed signs that have been installed in Livermore,
Lynn determines that the speedometer on Penny is reading 20% too high.
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When Lynn first acquired Penny, the speedometer was inaccurate, displaying a speed about 20% too high. He is fairly certain of the accuracy of this observation by comparing the speed displayed by Penny's speedometer compared with the radar speed signs that have recently appeared in Livermore. Tim Elliot, the previous owner of the car, told Lynn that a still earlier owner replaced the transmission with a newer model. Lynn suspects that the speedometer driven gear and the differential gear ratio were not properly matched.
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Lynn buys this Stewart Warner speedometer adapter (left) as a temporary fix for his
high reading speedometer.
Subsequently Lynn removes the speedometer driven gear (right) and finds that it
has 21 teeth, the proper number for a standard transmission L-38 Olds.
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As a temporary fix, Lynn buys an 0.80 speedometer reduction adapter on ebay. This has the desirable effect of reducing the speedometer reading to be bang on the reading from the radar speed signs. But is this fixing the symptoms and not the disease? What has this done to the odometer accuracy? Asking this question another way, what is the differential ration in Penny?
Lynn's suspicion now focuses on the differential gear ratio. He suspects that it is not the standard ratio for the car. Although he's taken the car onto the highway, it seems to him that the engine is spinning way too fast, so he thinks that the final drive ratio must be higher than standard for the car.
Taking It On The Road
On Sunday, Mar. 4, Lynn decides to make some measurements of the accuracy of the speedometer and odometer compared with mileage markers on Interstate 580. His plan is to do his best to maintain a steady 60 MPH according to the speedometer in Penny and time how long the car takes to pass mileage markers on the highway. He has also hooked up a shop tachometer to the car so that he can record the engine speed. Using the data he collects, Lynn expects that he can estimate the accuracy of the speedometer, odometer and final drive ratio of the car.
Lynn finds that the speedometer again appears to be fairly accurate. For example, after traveling seven miles down the highway trying his best to maintain 60 MPH, he measures 7 minutes, 2 seconds as he passes the seventh mile marker. OK, the speedometer is giving good readings.
However, he finds that the odometer isn't doing as well. The finest resolution on Penny's odometer in 1 mile, and the mechanism doesn't continuously move the mile digit, but relatively quickly flips it then holds it stationary for most of a mile. To get a handle on the accuracy of the odometer, Lynn times how long the odometer goes before advancing the number while driving at 60 MPH. If the odometer is accurate, it will advance every 60 seconds.
Instead, Lynn observes that it advances every 70 seconds. So the odometer is slow by 10/60 or about 16%. That's almost the exactly the amount of correction from his 0.80 speedometer reduction adapter, this adapter has the effect of slowing down the odometer by 20%.
So the odometer was more accurate without the speedometer adapter. It's clear now to Lynn that the speedometer is actually the instrument at fault. Probably it's return spring has gotten weaker with age.
Lynn feels that the speedometer is the more important instrument at the moment. Now that he knows that the odometer is reading low, he will simply multiply mileage readings and computed fuel consumption rates by 16% to get a more accurate answer.
What does this imply for the final drive ratio?
Penny Probably Has The Stock Differential
At 60 MPH, the engine in Penny is turning at almost exactly 3000 RPM. This surprises Lynn, somewhat. He would have guessed that it was a lot higher based on the sound. These older long stroke engines don't like to run at high speed, he rationalizes. What does this imply about the final drive ratio?
We can deduce the final drive ratio using the information we have. These data are related by the following formula.
- S2[mi/hr] = S1[rev/min] * C1[min/hr] * (1/R) * D[in] * PI * C2[ft/in] * C3[mi/ft]
where
- S2 is the car's speed in MPH,
- S1 is the speed of the engine in RPM,
- C1 = 60min/hr, converts between minutes and hours,
- R is the final drive ratio,
- D is the diameter of the tires in inches,
- PI = 3.14159...
- C2 = 1ft/12in, converts between inches and feet, and
- C3 = 1mi/5280ft, converts between feet and miles,
What is the diameter 'D' of the tires on Penny? She is wearing 7.10-15 tires, 7.10" wide tires on 15" diameter wheels. Assuming that they are "80" profile tires (height/width=0.8), the diameter of the tires are D = 15" + 2*0.8*7.1", or about 26.4". Lynn measures the diameter with a builders square and a tape measure. He gets a diameter of 27.5" to 28", somewhat bigger than the diameter computed from the tire size. This gives an aspect ratio more like 0.9 -- can that be right?
Solving for 'R', one gets the this expression.
- R = S1[rev/min] * C1[min/hr] * D[in] * PI * C2[ft/in] * C3[mi/ft] / S2[mi/hr]
or
- R = 3000RPM * 60min/hr * D[in] * 3.14159... * 1ft/12in * 1mi/5280ft / 60MPH
or
- R ~ 0.149(1/in) * D[in]
Using our measured diameter of 28", this gives a measured final drive ratio of about 4.2 (keeping only the number of digits that the accuracy of our input warrants). This is consistent with the stock standard transmission ratio of 4.37.
Our measured ratio does not seem to be as consistent with the stock automatic ratio of 3.55 or the stock "mountain" ratio of 4.62.
So it seems plausible that Penny has a stock standard ratio differential and the problem is simply that the speedometer (without adapter) is reading about 20% too high.
In the fullness of time, Lynn intents to take the cover off the differential and actually count the teeth on the gears. This would be the definitive test of determining the final drive ratio on Penny.
Can Lynn Get An Overdrive For Penny?
Lynn wonders what the possibility is of getting Penny equipped with an overdrive unit. He will make some calls in the future to vintage transmission shops and explore the possibility. Lynn would be a lot more happy cruising down the interstate with Penny if he could reduce the engine speed a bit.
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Last updated: May 24, 2009