[C2] There is a slight difference in the odometer vs actual mileage
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
There is a slight difference in the odometer vs actual mileage
When I bought my car it came with radial tires, and since owning it I've reinstalled bias ply tires from Kelsey Tires. Recently I was doing some reading in my PV manual and if I remember correctly one of the things they make you do in the PV certification is to drive a KNOWN distance of something like 10 miles after resetting the trip odometer to zero.
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The other day in my 67 I drove a known distance of exactly 10 miles, previously measured with my wife's Honda Accord, and while I haven't double-checked the same route with my truck I am presuming her Honda is fairly accurate largely because it matched the mile sections out in the country. After resetting my trip odometer and driving the same route, my 67 Corvette clocked only 9.65 miles. This is probly a reason for the examiner to FAIL my PV certification attempt.
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Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
.
The other day in my 67 I drove a known distance of exactly 10 miles, previously measured with my wife's Honda Accord, and while I haven't double-checked the same route with my truck I am presuming her Honda is fairly accurate largely because it matched the mile sections out in the country. After resetting my trip odometer and driving the same route, my 67 Corvette clocked only 9.65 miles. This is probly a reason for the examiner to FAIL my PV certification attempt.
.
Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
Last edited by CADbrian; May 29, 2026 at 05:47 PM.
Team Owner
Joined: Sep 2002
Posts: 38,357
Likes: 2,079
From: California
When I bought my car it came with radial tires, and since owning it I've reinstalled bias ply tires from Kelsey Tires. Recently I was doing some reading in my PV manual and if I remember correctly one of the things they make you do in the PV certification is to drive a KNOWN distance of something like 10 miles after resetting the trip odometer to zero.
.
The other day in my 67 I drove a known distance of exactly 10 miles, previously measured with my wife's Honda Accord, and while I haven't double-checked the same route with my truck I am presuming her Honda is fairly accurate largely because it matched the mile sections out in the country. After resetting my trip odometer and driving the same route, my 67 Corvette clocked only 9.65 miles. This is probly a reason for the examiner to FAIL my PV certification attempt.
.
Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
.
The other day in my 67 I drove a known distance of exactly 10 miles, previously measured with my wife's Honda Accord, and while I haven't double-checked the same route with my truck I am presuming her Honda is fairly accurate largely because it matched the mile sections out in the country. After resetting my trip odometer and driving the same route, my 67 Corvette clocked only 9.65 miles. This is probly a reason for the examiner to FAIL my PV certification attempt.
.
Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
Team Owner
Joined: Jul 2006
Posts: 22,188
Likes: 4,178
From: Kannapolis NC
When I bought my car it came with radial tires, and since owning it I've reinstalled bias ply tires from Kelsey Tires. Recently I was doing some reading in my PV manual and if I remember correctly one of the things they make you do in the PV certification is to drive a KNOWN distance of something like 10 miles after resetting the trip odometer to zero.
.
The other day in my 67 I drove a known distance of exactly 10 miles, previously measured with my wife's Honda Accord, and while I haven't double-checked the same route with my truck I am presuming her Honda is fairly accurate largely because it matched the mile sections out in the country. After resetting my trip odometer and driving the same route, my 67 Corvette clocked only 9.65 miles. This is probly a reason for the examiner to FAIL my PV certification attempt.
.
Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
.
The other day in my 67 I drove a known distance of exactly 10 miles, previously measured with my wife's Honda Accord, and while I haven't double-checked the same route with my truck I am presuming her Honda is fairly accurate largely because it matched the mile sections out in the country. After resetting my trip odometer and driving the same route, my 67 Corvette clocked only 9.65 miles. This is probly a reason for the examiner to FAIL my PV certification attempt.
.
Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
What size tires did you put on it?
Racer
Joined: Dec 2025
Posts: 287
Likes: 98
From: Pachuta, MS
The answer is maybe although 4% is reasonably good. Pull the speedometer driven gear (or at least pull the cable and note the color of the gear) and count the teeth. You want to choose a gear with probably 1 fewer teeth. For example if your current gear has 22 teeth then a a 21 tooth gear puts you within about 1%. Just compute the ratio of the new gear to the original and compare that to your error to get the needed gear. The color of the gear translates to the number of teeth. The rub is that if you change the gear it also changes the speedometer which will need to be recalibrated.
Last edited by acstephenson; May 29, 2026 at 08:59 PM.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
For both distance and speed monitoring, I use a free app for my iphone; it's just called Speedometer. I believe it's at least as accurate as the sign posts on the highway. I'm sure there are similar apps for Android.
What is the diameter of your tires compared to '67 tire dimensions? Is your speedometer accurate?
If the speedo is accurate, then it's probably not a tire diameter or transmission speedometer gear issue.
Does your odometer differ from your trip odometer reading over the same distance?
If so it's likely just error within the two sets of tumblers.
What is the diameter of your tires compared to '67 tire dimensions? Is your speedometer accurate?
If the speedo is accurate, then it's probably not a tire diameter or transmission speedometer gear issue.
Does your odometer differ from your trip odometer reading over the same distance?
If so it's likely just error within the two sets of tumblers.
Race Director
Joined: Jan 2003
Posts: 17,684
Likes: 3,680
From: Central Arkansas
For both distance and speed monitoring, I use a free app for my iphone; it's just called Speedometer. I believe it's at least as accurate as the sign posts on the highway. I'm sure there are similar apps for Android.
What is the diameter of your tires compared to '67 tire dimensions? Is your speedometer accurate?
If the speedo is accurate, then it's probably not a tire diameter or transmission speedometer gear issue.
Does your odometer differ from your trip odometer reading over the same distance?
If so it's likely just error within the two sets of tumblers.
What is the diameter of your tires compared to '67 tire dimensions? Is your speedometer accurate?
If the speedo is accurate, then it's probably not a tire diameter or transmission speedometer gear issue.
Does your odometer differ from your trip odometer reading over the same distance?
If so it's likely just error within the two sets of tumblers.
Check your speedometer against a GPS phone app. I would get the speedometer corrected to the app as close as possible by changing the driven speedometer gear at the transmission. That’s all you can do - the accuracy of the odometer reading is dependent on the speedometer accuracy.
Drifting
Joined: Nov 2004
Posts: 1,506
Likes: 678
From: Folsom Ca
... Recently I was doing some reading in my PV manual and if I remember correctly one of the things they make you do in the PV certification is to drive a KNOWN distance of something like 10 miles after resetting the trip odometer to zero.
.
…This is probly a reason for the examiner to FAIL my PV certification attempt.
.
Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
.
…This is probly a reason for the examiner to FAIL my PV certification attempt.
.
Is there a gear in the transmission and/or speedometer cable that I can change out to get more accurate mileage recorded on my odometer?
how about tires? If you are getting 9.65 mile reading on a known 10-mile route, then a shorter set of tires would help.
different brands, as well as modern metric sizing can easily affect the calibration.
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Racer
Joined: Dec 2025
Posts: 287
Likes: 98
From: Pachuta, MS
I find the Interstate mile markers to be very accurate as long as you don't start or stop where it would have been impossible to locate one (e.g. at an exit or junction) which in that case they're located as close as practical. The other gotcha's are the first and last marker going in or out of a state but otherwise using 10 of those mile markers will be very close and the speed calibration is done after you get the distance right (if you care at this point) but you have to tell the calibration shop how much to change because otherwise they will do a standard calibration.
P.S. Be glad you aren't looking for a 6 cylinder Powerglide driven gear because there the choice of gear sizes is exactly 1 and they are very hard to find though fortunately any of the 50-54 Powerglide gears will work and they are metal gears instead of that plastic stuff.
P.S. Be glad you aren't looking for a 6 cylinder Powerglide driven gear because there the choice of gear sizes is exactly 1 and they are very hard to find though fortunately any of the 50-54 Powerglide gears will work and they are metal gears instead of that plastic stuff.
Last edited by acstephenson; May 30, 2026 at 04:36 PM.
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
The car has 32,xxxx miles on it now. I think it had 31,xxx when I bought it, and spoke to all surviving prior owners who confirmed low miles during their ownership. I've had the car for 5.5 years and shortly after I got it I removed the radial tires and I stalled the bias ply tires from kelsey tires. I remember Brian Tilles telling me that he's never seen a n instrument cluster as clean as mine, so I doubt it has any significantly different wear on it than what the odometer shows.
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I'll talk with the folks at Kelsey tires next week and see if they know how their tires differs from original tires. Lots to learn about with this topic.
Racer
Joined: Jun 2018
Posts: 290
Likes: 117
From: ashburnham, MA
I'd say leave it alone. 3 1/2 % is pretty close. When the tires wear down, that could easily take off 2%.
Also, the original bias ply tires were supposed to be inflated to 24 psi. Over inflation will work against you.
I don't see how anyone can expect better than what you have.
Ray
Also, the original bias ply tires were supposed to be inflated to 24 psi. Over inflation will work against you.
I don't see how anyone can expect better than what you have.
Ray
Team Owner
Joined: Aug 2008
Posts: 33,033
Likes: 6,546
From: Rochester NY
St. Jude Donor '09 thru '26
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
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I've never checked it but I will now (soon).
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Goodyear 775/15 Power Cushion Tire – 5/8″ White Stripe
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These are the tires that I purchased from Kelsey Tires without even thinking that they might NOT be exactly the same size as original white stripe tires for a 67. But to be honest, I don't know what the precise dimensions were of original white stripe tires back in 67.
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With that said, here are the specs from Kelsey's website:
Tire Specifications*
Size:775/15
Rim Width:5.5 - 6"
Section Width7.70"
Overall Diameter:26.27"
Rolling Circumference:82.5"
Rev./Mile:767
Tread Width:5.27"
Max Load:1490 lbs. @ 32 psi
Tire Weight:18 lbs
I've never checked it but I will now (soon).
.
.
.
Goodyear 775/15 Power Cushion Tire – 5/8″ White Stripe
.
These are the tires that I purchased from Kelsey Tires without even thinking that they might NOT be exactly the same size as original white stripe tires for a 67. But to be honest, I don't know what the precise dimensions were of original white stripe tires back in 67.
.
With that said, here are the specs from Kelsey's website:
Tire Specifications*
Size:775/15
Rim Width:5.5 - 6"
Section Width7.70"
Overall Diameter:26.27"
Rolling Circumference:82.5"
Rev./Mile:767
Tread Width:5.27"
Max Load:1490 lbs. @ 32 psi
Tire Weight:18 lbs
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
I detached the speedometer cable and took this picture (below). Are we able to tell the color of the driven gear by this photo (below) or do I need to disassemble further?
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Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
I'm downloading it now... Thanks.
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As explained in another post (just a few minutes ago, I don't know what the precise dimensions were of original white stripe tires back in 67 but these are the tires that are on my car right now... I'll be on the phone with Kelsey this week to see what they can tell me about the differences between the tire I purchased from them vs the original tires back in 67.
.
Goodyear 775/15 Power Cushion Tire – 5/8″ White Stripe
.
These are the tires that I purchased from Kelsey Tires without even thinking that they might NOT be exactly the same size as original white stripe tires for a 67. But to be honest, I don't know what the precise dimensions were of original white stripe tires back in 67.
.
With that said, here are the specs from Kelsey's website:
Tire Specifications*
Size:775/15
Rim Width:5.5 - 6"
Section Width7.70"
Overall Diameter:26.27"
Rolling Circumference:82.5"
Rev./Mile:767
Tread Width:5.27"
Max Load:1490 lbs. @ 32 psi
Tire Weight:18 lbs
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I don't know but I can't wait to find out. My original fan clutch is currently out of the car and I'm getting ready to send it in to be rebuilt (Kirkconnell) and I might install an NOS service replacement fan clutch that I have in my parts inventory while my original is getting rebuilt. Once I get a fan clutch reinstalled I'll be able to drive my car and use that speedometer app to find out.
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I sent my instrument cluster into Brian Tilles to be rebuilt a couple of years ago and I don't remember if there was anything about the instrument cluster that I kept back. So if I sent the speedometer and the trip odometer both into Brian then I'm sure he would have restored everything as that was his policy - that he would restore/rebuild anything and everything that was sent to him.
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If the speedometer is accurate, and it's not a tire diameter or transmission speedometer gear issue, then what else could it be? Let's come back to this question AFTER I drive my car and find out how accurate my speedometer is.
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I'm not sure but that's something else I can check next time I can drive my car.
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
Check your speedometer against a GPS phone app. I would get the speedometer corrected to the app as close as possible by changing the driven speedometer gear at the transmission. That’s all you can do - the accuracy of the odometer reading is dependent on the speedometer accuracy.Will do as soon as possible.
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
Ok, I think I understand this. What you're saying is make sure the odometer is recording miles correctly with the right gear installed... <<< Is this what you're saying?
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
Wait a minute... Is the "driven gear" the little gear between the speedometer cable and the transmission OR is the "driven gear" a metal gear INSIDE the transmission??? Remember, I'm mechanically challenged.
Thread Starter
Melting Slicks
Joined: Dec 2017
Posts: 3,478
Likes: 548
From: 67401
I would say no, There may be external gear boxes to add that could alter the odometer calibration, but the only changeable gears are limited in tooth configuration. You may find a closer calibration, but it may involve taking the transmission apart to change the driven gear. how about tires? If you are getting 9.65 mile reading on a known 10-mile route, then a shorter set of tires would help. different brands, as well as modern metric sizing can easily affect the calibration.
I'll just throw up the "I'm in over my head" flag. After reading everyone's posts it's becoming clear to me that I don't understand what I need to understand to fully understand what's going on here. I was just thinking that I could choose a different little gear that goes between my speedo cable and my transmission but now we're talking about the possibility of taking apart the transmission. Yep, I'm in over my head now.
