| [GMCnet] Speedometer Correction Gear Adapter [message #147293] |
Fri, 21 October 2011 12:52  |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma:
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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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