Home » Public Forums » GMCnet » [GMCnet] Speedometer Correction Gear Adapter
| [GMCnet] Speedometer Correction Gear Adapter [message #147293] |
Fri, 21 October 2011 12:52  |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Senior Member |
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Good Afternoon
I finally got around to checking out my speedometer gear reduction calculations
and I was very pleased, my calculations were dead nuts.
When I started playing with this, I quickly realized that using "MILES PER HOUR"
was not going to work, so I did all of my calculations using "REVOLUTIONS PER
MILE.
As an example, most GMC's setup OEM from the factory came with 8.75 x 16.5 tires
and a 3.07 final drive.
8.75 x 16.5 tires (depending on brand) rotate approx. 708 Revolutions Per Mile.
This means that your final drive ring gear is turning at the same speed, 708
Rev's Per Mile.
With the ring gear turning at 708 Rev's Per Mile, multiply that times the final
drive ratio of 3.07 and the pinion gear AND final drive input shaft are turning
at 2,174 Revolutions Per Mile.
THAT is the magic number, 2,174 Rev's Per Mile on your final drive input shaft
!! That number is what is needed to make your speedometer read correctly.
Now, if you are running the OEM tire size, you can simply divide the 3.07 final
drive ratio by the new final drive ration and get the number of the gear reducer
you need.
Example: 3.07/3.70 = .8297, meaning you only want to use 82.97% of the final
drive output shaft speed, or a 17.03% REDUCTION.
What kills the above formula is when you are running different tire sizes. The
Revolutions Per Mile change significantly with different tire sizes.
As an example, on my coach I run 245/75R16 tires, the specific brand I have turn
at 662.5 Rev's Per Mile.
So let's do the same calculations
A. 3.07/3.70 = .8297
B. Now take 662.5 tire Rev's Per Mile, that equals 662.5 ring gear Rev's Per
Mile x 3.70 = 2,451 pinion gear and final drive output shaft Rev's Per Mile.
Divide the magic number above 2,174/2,451 and the TRUE gear reducer I need
is a .8869......NOT the .8297 calculated above in A.
When I put in my new final drive and my speedo was reading 5-6% slow, the
problem was that I had a gear reducer that was determined by the calculation in
A and not per the formula shown in B.
The difference between 82.97% and 88.69% is 5.72%.....my gear reducer was
slowing down my final drive input speed 5.72% too much.
So what I've done is put together a spreadsheet with most of the combinations of
tires and final drives on it so no one has to screw around with the
calculations.
I just loaded it to the picture site here: http://tinyurl.com/3bwgrrr
If anyone has comments or finds an error, please let me know.
Thanks
Ray
Ray & Lisa Erspamer
78 Royale "Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] Speedometer Correction Gear Adapter [message #147295 is a reply to message #147293] |
Fri, 21 October 2011 13:25  |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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Are you doing your calculations with new tires?
The revolutions per mile will, of course, change as the tires wear.
Emery Stora
On Oct 21, 2011, at 11:52 AM, Ray Erspamer wrote:
> Good Afternoon
>
> I finally got around to checking out my speedometer gear reduction calculations
> and I was very pleased, my calculations were dead nuts.
>
> When I started playing with this, I quickly realized that using "MILES PER HOUR"
> was not going to work, so I did all of my calculations using "REVOLUTIONS PER
> MILE.
>
> As an example, most GMC's setup OEM from the factory came with 8.75 x 16.5 tires
> and a 3.07 final drive.
>
> 8.75 x 16.5 tires (depending on brand) rotate approx. 708 Revolutions Per Mile.
> This means that your final drive ring gear is turning at the same speed, 708
> Rev's Per Mile.
>
> With the ring gear turning at 708 Rev's Per Mile, multiply that times the final
> drive ratio of 3.07 and the pinion gear AND final drive input shaft are turning
> at 2,174 Revolutions Per Mile.
>
> THAT is the magic number, 2,174 Rev's Per Mile on your final drive input shaft
> !! That number is what is needed to make your speedometer read correctly.
>
>
>
> Now, if you are running the OEM tire size, you can simply divide the 3.07 final
> drive ratio by the new final drive ration and get the number of the gear reducer
> you need.
> Example: 3.07/3.70 = .8297, meaning you only want to use 82.97% of the final
> drive output shaft speed, or a 17.03% REDUCTION.
>
> What kills the above formula is when you are running different tire sizes. The
> Revolutions Per Mile change significantly with different tire sizes.
> As an example, on my coach I run 245/75R16 tires, the specific brand I have turn
> at 662.5 Rev's Per Mile.
>
> So let's do the same calculations
> A. 3.07/3.70 = .8297
> B. Now take 662.5 tire Rev's Per Mile, that equals 662.5 ring gear Rev's Per
> Mile x 3.70 = 2,451 pinion gear and final drive output shaft Rev's Per Mile.
> Divide the magic number above 2,174/2,451 and the TRUE gear reducer I need
> is a .8869......NOT the .8297 calculated above in A.
>
> When I put in my new final drive and my speedo was reading 5-6% slow, the
> problem was that I had a gear reducer that was determined by the calculation in
> A and not per the formula shown in B.
> The difference between 82.97% and 88.69% is 5.72%.....my gear reducer was
> slowing down my final drive input speed 5.72% too much.
>
> So what I've done is put together a spreadsheet with most of the combinations of
> tires and final drives on it so no one has to screw around with the
> calculations.
>
> I just loaded it to the picture site here: http://tinyurl.com/3bwgrrr
>
> If anyone has comments or finds an error, please let me know.
>
> Thanks
>
>
>
>
> Ray
>
>
> Ray & Lisa Erspamer
> 78 Royale "Great Lakes Eagle"
> Center Kitchen TZE368V101144
> Wauwatosa, Wisconsin 53226
> Email: 78GMC-Royale@att.net
> 414-745-3188
> Web Site: http://ray-lisa.page.tl/
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] Speedometer Correction Gear Adapter [message #147299 is a reply to message #147295] |
Fri, 21 October 2011 13:51 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Senior Member |
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Yes Emery, I researched the data for new tires. But a good point, tire wear
will affect the calc's, I just played with it and it could be as much 1.5% or
so.
But, the tires wear on the OEM coaches too, so I'm not sure exactly how they are
designed or setup, my goal was to get as close as I could. Tire Pressure will
also have a slight affect.
The good thing is that as your tires wear, the speedo will read faster than your
actual speed which is the better side to be on.
The gear reducer I have on hand and will use until I can get the exact one I
need, makes my speedo read slightly fast.....but minimal. It may save me a
ticket some day, who knows.
Thanks
Ray
Ray & Lisa Erspamer
78 Royale "Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
----- Original Message ----
From: Emery Stora <emerystora@mac.com>
To: gmclist@temp.gmcnet.org
Sent: Fri, October 21, 2011 1:25:35 PM
Subject: Re: [GMCnet] Speedometer Correction Gear Adapter
Are you doing your calculations with new tires?
The revolutions per mile will, of course, change as the tires wear.
Emery Stora
On Oct 21, 2011, at 11:52 AM, Ray Erspamer wrote:
> Good Afternoon
>
> I finally got around to checking out my speedometer gear reduction calculations
>
> and I was very pleased, my calculations were dead nuts.
>
> When I started playing with this, I quickly realized that using "MILES PER
>HOUR"
>
> was not going to work, so I did all of my calculations using "REVOLUTIONS PER
> MILE.
>
> As an example, most GMC's setup OEM from the factory came with 8.75 x 16.5
>tires
>
> and a 3.07 final drive.
>
> 8.75 x 16.5 tires (depending on brand) rotate approx. 708 Revolutions Per
>Mile.
>
> This means that your final drive ring gear is turning at the same speed, 708
> Rev's Per Mile.
>
> With the ring gear turning at 708 Rev's Per Mile, multiply that times the final
>
> drive ratio of 3.07 and the pinion gear AND final drive input shaft are turning
>
> at 2,174 Revolutions Per Mile.
>
> THAT is the magic number, 2,174 Rev's Per Mile on your final drive input shaft
> !! That number is what is needed to make your speedometer read correctly.
>
>
>
> Now, if you are running the OEM tire size, you can simply divide the 3.07 final
>
> drive ratio by the new final drive ration and get the number of the gear
>reducer
>
> you need.
> Example: 3.07/3.70 = .8297, meaning you only want to use 82.97% of the final
> drive output shaft speed, or a 17.03% REDUCTION.
>
> What kills the above formula is when you are running different tire sizes. The
>
> Revolutions Per Mile change significantly with different tire sizes.
> As an example, on my coach I run 245/75R16 tires, the specific brand I have
>turn
>
> at 662.5 Rev's Per Mile.
>
> So let's do the same calculations
> A. 3.07/3.70 = .8297
> B. Now take 662.5 tire Rev's Per Mile, that equals 662.5 ring gear Rev's Per
> Mile x 3.70 = 2,451 pinion gear and final drive output shaft Rev's Per Mile.
> Divide the magic number above 2,174/2,451 and the TRUE gear reducer I need
> is a .8869......NOT the .8297 calculated above in A.
>
> When I put in my new final drive and my speedo was reading 5-6% slow, the
> problem was that I had a gear reducer that was determined by the calculation in
>
> A and not per the formula shown in B.
> The difference between 82.97% and 88.69% is 5.72%.....my gear reducer was
> slowing down my final drive input speed 5.72% too much.
>
> So what I've done is put together a spreadsheet with most of the combinations
>of
>
> tires and final drives on it so no one has to screw around with the
> calculations.
>
> I just loaded it to the picture site here: http://tinyurl.com/3bwgrrr
>
> If anyone has comments or finds an error, please let me know.
>
> Thanks
>
>
>
>
> Ray
>
>
> Ray & Lisa Erspamer
> 78 Royale "Great Lakes Eagle"
> Center Kitchen TZE368V101144
> Wauwatosa, Wisconsin 53226
> Email: 78GMC-Royale@att.net
> 414-745-3188
> Web Site: http://ray-lisa.page.tl/
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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|
|
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| Re: [GMCnet] Speedometer Correction Gear Adapter [message #147333 is a reply to message #147293] |
Fri, 21 October 2011 20:55 |
Andrew
Messages: 213
Registered: April 2011
Location: Connecticut
Karma: 0
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Senior Member |
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| Ray Erspamer wrote on Fri, 21 October 2011 13:52 |
So what I've done is put together a spreadsheet with most of the combinations of
tires and final drives on it so no one has to screw around with the
calculations.
I just loaded it to the picture site here: http://tinyurl.com/3bwgrrr
|
Ray, that's excellent.
The URL didn't work for me, but this one did:
http://www.gmcmhphotos.com/photos/showphoto.php?photo=41238&title=speedometer-gear-reducer-chart&cat=5837
My speedo is slow too, partially due to switching to 16" tires, but there's also something else going on (it varies non-linearly). When I get around to taking care of that issue, I'll also be looking for the proper reducing gear.
Can you order these things in arbitrary ratios, or are there a set of standard ones that we will have to choose from to match up to your calculations?
Great work!
1973 Sequoia 260 (since 2011)
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| Re: [GMCnet] Speedometer Correction Gear Adapter [message #264586 is a reply to message #147333] |
Mon, 20 October 2014 15:42 |
lw8000
Messages: 201
Registered: July 2012
Location: S.E. Michigan
Karma: 1
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Senior Member |
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Andrew wrote on Fri, 21 October 2011 20:55Ray Erspamer wrote on Fri, 21 October 2011 13:52
So what I've done is put together a spreadsheet with most of the combinations of
tires and final drives on it so no one has to screw around with the
calculations.
I just loaded it to the picture site here: http://tinyurl.com/3bwgrrr
Ray, that's excellent.
The URL didn't work for me, but this one did:
http://www.gmcmhphotos.com/photos/showphoto.php?photo=41238&title=speedometer-gear-reducer-chart&cat=5837
My speedo is slow too, partially due to switching to 16" tires, but there's also something else going on (it varies non-linearly). When I get around to taking care of that issue, I'll also be looking for the proper reducing gear.
Can you order these things in arbitrary ratios, or are there a set of standard ones that we will have to choose from to match up to your calculations?
Great work!
Does anybody have an updated location for this spreadsheet? Both links in this thread are no longer live, and I've tried searching around to no avail. We're currently looking to correct our speedo, too. Thanks in advance!
--
Chris
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -
S.E. Michigan
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| Re: [GMCnet] Speedometer Correction Gear Adapter [message #264588 is a reply to message #147293] |
Mon, 20 October 2014 16:37 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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Box stock, (Original wheels, 16.5 Firestone TransForces w/85% tread) mine was pessimistic about 4% on speed, I didn't calculate the distance cos a) I'm lazy; and B) I figure anything under 5% is akin to the picking of nits. But, for the purists, changing to Eagles and 225 75R 16 Discovery II Coopers, less than a percent change in indicated speed. (Actually, I ascribe the pessimism to wear and increased drag within the speedo itself).
The chart turns per mile are useful however. If the Heathern Chinee company I bought a batch of digital gages from ever do a speedo, it will be easy to calibrate. Thanks!
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Speedometer Correction Gear Adapter [message #264600 is a reply to message #264590] |
Mon, 20 October 2014 23:47 |
drdonaldg
Messages: 35
Registered: June 2013
Location: San Rafael, CA
Karma: 1
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Member |
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Another solution to speedometer calibration with different final drive ratios and different tire size and wear issues is to use a GPS speedometer. I have one from Custom Instrument Panels that's works well and provides two resettable trip readings and a compass.
1977 Palm Beach
TZE167V100274
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