Odometer (in)accuracy
Thread Starter
Racer
Joined: Mar 2019
Posts: 276
Likes: 87
From: Melville, NY
Odometer (in)accuracy
I took a nice 85 mile drive today. At least that's what the odometer said until I google mapped the drive, and it was actually 93 miles.
Just curious whether that kind of discrepancy (> 8%) is pretty much par for the course?
My car is a 1970 454 with Firestone Wide Oval Redlines GR70-15 (pretty close in size to 225/70R 15).
Just curious whether that kind of discrepancy (> 8%) is pretty much par for the course?
My car is a 1970 454 with Firestone Wide Oval Redlines GR70-15 (pretty close in size to 225/70R 15).
Safety Car
Joined: Sep 1999
Posts: 4,895
Likes: 696
From: grand rapids, lowell mi. area
yes my 72 is about 9 mph off ,when it shows im going 70 ,im really going 61mph ,some day im gonna take the gear off of the end of the cable and get a different one ,someday!
Race Director
Joined: Oct 2008
Posts: 19,147
Likes: 1,667
From: Long Island
The difference is about one gear ratio difference. Like a 3:08 and a 3:36.
All my Speedos are right on, about 1 MPH off at 70.
Get the SPEED app for your phone and compare it to the actual reading.
Using the available speed vs RPM charts see which rear you have and get the correct gear for the speedometer drive gear - about $12. Use the RPM and Phone speed to calculate the rear.
All my Speedos are right on, about 1 MPH off at 70.
Get the SPEED app for your phone and compare it to the actual reading.
Using the available speed vs RPM charts see which rear you have and get the correct gear for the speedometer drive gear - about $12. Use the RPM and Phone speed to calculate the rear.
Last edited by BLUE1972; Aug 15, 2019 at 09:33 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
The odometer and speedometer should have the same 'offset'. If speed is low by 10%, the odometer will be off by 10%. Have you checked speed accuracy with a GPS device...or timed a one-mile distance when set at 60mpy? As mentioned above, accuracy can be 'adjusted' by altering number of teeth on the [removable] transmission drive gear. If speedo is low, reduce the number of teeth; if speedo is high, add teeth. Roughly, one tooth change will be about 4-5 mph at 60mph with 3.36 rear gear. (That rate will vary with different rear axle ratios.)
Tech Contributor
Joined: Aug 1999
Posts: 15,135
Likes: 3,939
From: Connecticut, USA
I agree, I suspect a possible gear change at some point without regard to the speedo gear. My original 72 with 225 tires was dead on until I installed 336's over the 308's and knew the speedo would be off. It now reads about 4-5 mph slower but I don't have lift to get to the trans gear and no longer like do creeper work so I live with it. So far I drove the car about 250 miles this year so it doesn't matter in the grand scheme of things.
Thread Starter
Racer
Joined: Mar 2019
Posts: 276
Likes: 87
From: Melville, NY
The odometer and speedometer should have the same 'offset'. If speed is low by 10%, the odometer will be off by 10%. Have you checked speed accuracy with a GPS device...or timed a one-mile distance when set at 60mpy? As mentioned above, accuracy can be 'adjusted' by altering number of teeth on the [removable] transmission drive gear. If speedo is low, reduce the number of teeth; if speedo is high, add teeth. Roughly, one tooth change will be about 4-5 mph at 60mph with 3.36 rear gear. (That rate will vary with different rear axle ratios.)
Just checked the speedometer compared with a GPS speedometer app. Assuming the app is accurate, your post is dead on! I compared speeds at 10 MPH intervals from 20 MPH up to 80 MPH and found the speedo in the car to be low throughout. As you stated, as the speed is off (low - 55MPH) by 5 MPH at an actual road speed of 60 MPH, the car likely has a 3.36 rear gear and I need to replace the transmission drive gear with one that has one less tooth.
BTW, proportionately the MPH discrepancy is almost exactly the same as the odometer discrepancy mentioned in my initial post. When I drove 93 miles, the odometer registered only 85 miles, so the speedo and odometer have the same offset as you indicated.
Thanks again for the help.
Race Director
Joined: Oct 2008
Posts: 19,147
Likes: 1,667
From: Long Island
If you jack up the driver side of the car you can replace the gear. Be careful they are a tight fit on the hole to pull them out and they do get brittle.
I just did my friends 72 and used a black gear for his 3.36 rear. It's right on.
He had a green gear that was broken, the tooth count was for a 3.08
Go on line and do the math, for RPM vs Speed and it will tell you the gear ratio. Just enter the numbers into the app. it will calculate the rear. Otherwise you are guessing the ratio.
https://www.summitracing.com/newsand...tio-calculator
I just did my friends 72 and used a black gear for his 3.36 rear. It's right on.
He had a green gear that was broken, the tooth count was for a 3.08
Go on line and do the math, for RPM vs Speed and it will tell you the gear ratio. Just enter the numbers into the app. it will calculate the rear. Otherwise you are guessing the ratio.
https://www.summitracing.com/newsand...tio-calculator
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Thread Starter
Racer
Joined: Mar 2019
Posts: 276
Likes: 87
From: Melville, NY
If you jack up the driver side of the car you can replace the gear. Be careful they are a tight fit on the hole to pull them out and they do get brittle.
I just did my friends 72 and used a black gear for his 3.36 rear. It's right on.
He had a green gear that was broken, the tooth count was for a 3.08
Go on line and do the math, for RPM vs Speed and it will tell you the gear ratio. Just enter the numbers into the app. it will calculate the rear. Otherwise you are guessing the ratio.
https://www.summitracing.com/newsand...tio-calculator
I just did my friends 72 and used a black gear for his 3.36 rear. It's right on.
He had a green gear that was broken, the tooth count was for a 3.08
Go on line and do the math, for RPM vs Speed and it will tell you the gear ratio. Just enter the numbers into the app. it will calculate the rear. Otherwise you are guessing the ratio.
https://www.summitracing.com/newsand...tio-calculator
Thanks Blue1972, for the info and for the link!
Thread Starter
Racer
Joined: Mar 2019
Posts: 276
Likes: 87
From: Melville, NY
Well, today's final test to confirm a gear change did NOT turn out as expected! Everything I checked until today led me to believe that a prior owner changed the gearing from the stock 3.08 (confirmed from the build sheet) to 3.36, but failed to change the transmission drive gear, thus explaining the inaccurate speedo and odometer. When I entered the numbers into the gear ratio calculator from Blue1972's post above, it came back that my car has the stock 3.08.
The numbers I plugged in from today's test drive were:
RPM: 2250
Tire Diameter: 27.7
MPH: 60 (measured with a speedometer app)
That returned a Gear Ratio of 3.08 (actually 3.09, but close enough.)
So, now what? I suppose the gearing could have been left stock and the speedo gear was replaced for some reason with the incorrect gear? Does that seem most likely, or could there be some other reason I haven't considered for the speedo/odometer inaccuracy?
The numbers I plugged in from today's test drive were:
RPM: 2250
Tire Diameter: 27.7
MPH: 60 (measured with a speedometer app)
That returned a Gear Ratio of 3.08 (actually 3.09, but close enough.)
So, now what? I suppose the gearing could have been left stock and the speedo gear was replaced for some reason with the incorrect gear? Does that seem most likely, or could there be some other reason I haven't considered for the speedo/odometer inaccuracy?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
No change in plan. The speedometer is off by 5 mph at 60 mph. Change the number of teeth on the speedo drive gear by ONE TOOTH (either higher or lower as you data suggest). Quit focusing on WHICH gear is installed. Just change it and the speedo will be more accurate.
Thread Starter
Racer
Joined: Mar 2019
Posts: 276
Likes: 87
From: Melville, NY
Thanks 7T1Vette! You're absolutely right. I think I just over thought the situation when I got a result I wasn't expecting. Appreciate your reply!
Race Director
Joined: Oct 2008
Posts: 19,147
Likes: 1,667
From: Long Island
1970 cars came with F70 -15 which were about 26.6", your GR70 - 15 are 27.7" . also which causes spedo problems... The larger tires cause you to go faster than the spedo reads.
Get a 3:08 gear and you should be good to go. Just check the speedo gear housing as they can only take certain # teeth. It's stamped on the housing.
Your spedo is reading correctly for factory tires and a 3.36 rear.
Get a 3:08 gear and you should be good to go. Just check the speedo gear housing as they can only take certain # teeth. It's stamped on the housing.
Your spedo is reading correctly for factory tires and a 3.36 rear.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
336 rear, stock tire diameter, I noticed my speedo off too, so did some GPS testing years ago, got it dead nutz on until 80 mph which was really 78.....100 mph indicated was really 94 mph....ran outta room, test over......
way before that testing, I went from a 4 speed to a 200 4r auto, for the o/drive.....
way before that testing, I went from a 4 speed to a 200 4r auto, for the o/drive.....