GlockTalk.com
Home Forums Classifieds Blogs Today's Posts Search Social Groups



  
SIGN-UP
Notices

Glock Talk
Welcome To The Glock Talk Forums.
Reply
 
Thread Tools Display Modes
Old 02-22-2013, 06:21   #1
Kevin108
HADOKEN!
 
Kevin108's Avatar
 
Join Date: Mar 2005
Location: Portsmouth, VA
Posts: 6,943


Speedometer Correction, Can you explain why this formula works?

Two formulas for speedometer correction are presented "High-Performance Jeep Cherokee Builder's Guide."

Formula 1:

Old Tire Diameter / New Tire Diameter x New Gear Ration / Old Gear Ratio x Old Tooth Count = New Tooth Count


Formula 2:

(63360 / (MSOD x 3.14159) x Axle Ratio) / 74.5 = # of Teeth in Speedo Gear


My algebra days are long behind me, so can I get someone to double-check my numbers here and explain to me how these formulas work?

Since I was too lazy to pull my existing gear to start with, I used Formula 2 first after getting actual measurements on my tires. I have GPS verified the accuracy of my speedo with my old tires with a diameter of 28.25".

(63360 / (28.25 x 3.14159) x 3.55) / 74.5 = # of Teeth in Speedo Gear
28.25 x 3.14159 = 88.7499175
63360 / 88.7499175 = 713.91615659811740106687986498692
713.91615659811740106687986498692 x 3.55 = 2534.4023559233167737874235207036
2534.4023559233167737874235207036 / 74.5 = 34.018823569440493607884879472531

34 teeth and lots of decimal places!

Now, back to Formula 1...
The Okie Corral


I didn't change my gear ratio, meaning old / new would be 1, so I'll leave that part out.

28.25 / 30.625 x 34 = New Tooth Count
28.25 / 30.625 x 34 = 31.363265306122448979591836734694

So I need to go all hillbilly and lose 3 teeth?

The Okie Corral


But seriously, what is represented or determined by the numbers 63360 and 74.5?
__________________
Kevin108.com | OpenCarry.org | VCDL.org

Those who would give up Essential Liberty to purchase a
little Temporary Safety, deserve neither Liberty nor Safety.
_____- Ben Franklin
Kevin108 is offline   Reply With Quote
Old 02-22-2013, 06:25   #2
Steve0853
Senior Member
 
Join Date: Aug 2003
Location: North Carolina
Posts: 5,087
uhhhhhhhhhhhhh............12?
Steve0853 is offline   Reply With Quote
Old 02-22-2013, 06:25   #3
wprebeck
Die, bird, die!
 
wprebeck's Avatar
 
Join Date: Oct 2002
Location: In a swamp, if I'm lucky
Posts: 8,444
Don't know about the math, but I would advise you to not rely upon GPS as accurate. Its usually accurate to within a few meters - while great for figuring out where you are in a city, its not exactly mathematically accurate for determining speed.
wprebeck is offline   Reply With Quote
Old 02-22-2013, 06:31   #4
Two_Clicks
Senior Member
 
Two_Clicks's Avatar
 
Join Date: Mar 2010
Location: Southern Ohio
Posts: 709
You need the gear ratio for the speed the tire turns and the diameter for the distance the tire travels each turn, that is as simple as I can put it.
Two_Clicks is offline   Reply With Quote
Old 02-22-2013, 07:10   #5
syntaxerrorsix
CLM Number 301
Anti-Federalist
 
syntaxerrorsix's Avatar
 
Join Date: Oct 2000
Location: Lakeland, FL.
Posts: 9,515
Quote:
Originally Posted by wprebeck View Post
Don't know about the math, but I would advise you to not rely upon GPS as accurate. Its usually accurate to within a few meters - while great for figuring out where you are in a city, its not exactly mathematically accurate for determining speed.
Garmin's specifications quote 0.1mph accuracy but due to signal degredation it could be as much as 0.5mph.

Seeing as this is an average based on how many times the device pings the sat and reassess your position the only time GPS wouldn't be this accurate is if you were only driving a few feet (within it's error of location) per minute. At normal driving speeds the GPS is quite accurate.
__________________
Sappers Forward
841st Eng (Cbt/Hvy) 81ARCOM, 84th Eng (Cbt/Hvy) 2ACR, 40th Eng (Mech) 1AD, 588th Eng (Mech) 4ID


Last edited by syntaxerrorsix; 02-22-2013 at 07:14..
syntaxerrorsix is offline   Reply With Quote
Old 02-22-2013, 07:20   #6
devildog2067
Senior Member
 
devildog2067's Avatar
 
Join Date: Apr 2005
Location: Near Chicago, IL
Posts: 15,766
Quote:
Originally Posted by Kevin108 View Post
Formula 1:

Old Tire Diameter / New Tire Diameter x New Gear Ration / Old Gear Ratio x Old Tooth Count = New Tooth Count
You need the velocity of the speedo gear, in # of teeth per second, to remain the same with the old tires/gears and new tires/gears. I wouldn't even really call it a "formula" it's just 3 ratios that you want to equal 1, then the bottom term on the right is cross-multiplied.

The "tire diameter" and "gear ratio" terms give you the change in the angular velocity of the speedo cable ("how fast it's spinning" in terms of RPM). If the speedo cable spins 20% faster for a given speed, well than you want a speedo gear with 20% fewer teeth so that the speedo will count an unchanged number of teeth going by.

Now lets talk about more important things: is this the old BAR Lucky Strike Honda car?

The Okie Corral
devildog2067 is offline   Reply With Quote
Old 02-22-2013, 07:33   #7
kiole
Senior Member
 
Join Date: Feb 2008
Posts: 3,124
Quote:
Originally Posted by wprebeck View Post
Don't know about the math, but I would advise you to not rely upon GPS as accurate. Its usually accurate to within a few meters - while great for figuring out where you are in a city, its not exactly mathematically accurate for determining speed.
GPS speed accuracy is usually very good. It's more accurate then your cars speedometer.
kiole is offline   Reply With Quote
Old 02-22-2013, 07:42   #8
wprebeck
Die, bird, die!
 
wprebeck's Avatar
 
Join Date: Oct 2002
Location: In a swamp, if I'm lucky
Posts: 8,444
Quote:
Originally Posted by syntaxerrorsix View Post
Garmin's specifications quote 0.1mph accuracy but due to signal degredation it could be as much as 0.5mph.

Seeing as this is an average based on how many times the device pings the sat and reassess your position the only time GPS wouldn't be this accurate is if you were only driving a few feet (within it's error of location) per minute. At normal driving speeds the GPS is quite accurate.
Well, for court purposes, I'm going to need to see the last time the GPS was calibrated and the user's training records on the device.

Further, where did the OP state he used a Garmin device? What specific device is he using? Again, where are the calibration records for the device?

I don't trust my speedometer at all - it reads at least 3mph over the actual speed I'm traveling, based on calibrated radar hits.
wprebeck is offline   Reply With Quote
Old 02-22-2013, 09:20   #9
SC Tiger
Shaved Ape
 
SC Tiger's Avatar
 
Join Date: Aug 2011
Location: South Carolina
Posts: 6,997
Quote:
Originally Posted by Kevin108 View Post
Two formulas for speedometer correction are presented "High-Performance Jeep Cherokee Builder's Guide."

Formula 1:

Old Tire Diameter / New Tire Diameter x New Gear Ration / Old Gear Ratio x Old Tooth Count = New Tooth Count


Formula 2:

(63360 / (MSOD x 3.14159) x Axle Ratio) / 74.5 = # of Teeth in Speedo Gear


My algebra days are long behind me, so can I get someone to double-check my numbers here and explain to me how these formulas work?

Since I was too lazy to pull my existing gear to start with, I used Formula 2 first after getting actual measurements on my tires. I have GPS verified the accuracy of my speedo with my old tires with a diameter of 28.25".

(63360 / (28.25 x 3.14159) x 3.55) / 74.5 = # of Teeth in Speedo Gear
28.25 x 3.14159 = 88.7499175
63360 / 88.7499175 = 713.91615659811740106687986498692
713.91615659811740106687986498692 x 3.55 = 2534.4023559233167737874235207036
2534.4023559233167737874235207036 / 74.5 = 34.018823569440493607884879472531

34 teeth and lots of decimal places!

Now, back to Formula 1...
The Okie Corral


I didn't change my gear ratio, meaning old / new would be 1, so I'll leave that part out.

28.25 / 30.625 x 34 = New Tooth Count
28.25 / 30.625 x 34 = 31.363265306122448979591836734694

So I need to go all hillbilly and lose 3 teeth?

The Okie Corral


But seriously, what is represented or determined by the numbers 63360 and 74.5?
Here is how you do it:

Drive at what your speedo calls 100 mph past a cop. When he pulls you over, ask him what speed he clocked you at. Now you know how far off your speedo is.

Formula 1 makes more sense to me because I don't know where the constants in formula 2 come from, and I don't know what MSOD stands for either. Either way the correction factor should be pretty simple to calculate.
__________________
Hug your barking irons as the Devil hugs a witch

Quote:
Originally Posted by Keoking View Post
If you're gonna be stupid, don't pull up short. Saddle up and ride it all the way in.
SC Tiger is online now   Reply With Quote
Old 02-22-2013, 09:56   #10
W
Book 'em, Spock
 
Join Date: Aug 2000
Posts: 2,341
They don't even give those girls a chair to sit on?

I'll offer them each a half a lap.
W is offline   Reply With Quote
Old 02-22-2013, 11:34   #11
syntaxerrorsix
CLM Number 301
Anti-Federalist
 
syntaxerrorsix's Avatar
 
Join Date: Oct 2000
Location: Lakeland, FL.
Posts: 9,515
Quote:
Originally Posted by wprebeck View Post
Well, for court purposes, I'm going to need to see the last time the GPS was calibrated and the user's training records on the device.

Further, where did the OP state he used a Garmin device? What specific device is he using? Again, where are the calibration records for the device?

I don't trust my speedometer at all - it reads at least 3mph over the actual speed I'm traveling, based on calibrated radar hits.
Not everything's about being a cop. Your statement is patently untrue across the spectrum of devices. As the unit itself doesn't measure your speed it won't need calibrating. I suppose you'll have to subpoena the satellite for that
__________________
Sappers Forward
841st Eng (Cbt/Hvy) 81ARCOM, 84th Eng (Cbt/Hvy) 2ACR, 40th Eng (Mech) 1AD, 588th Eng (Mech) 4ID

syntaxerrorsix is offline   Reply With Quote
Old 02-22-2013, 15:02   #12
Never Nervous
Senior Member
 
Never Nervous's Avatar
 
Join Date: Aug 2009
Location: Metro ATL
Posts: 2,233
Quote:
Originally Posted by syntaxerrorsix View Post
Garmin's specifications quote 0.1mph accuracy but due to signal degredation it could be as much as 0.5mph.

Seeing as this is an average based on how many times the device pings the sat and reassess your position the only time GPS wouldn't be this accurate is if you were only driving a few feet (within it's error of location) per minute. At normal driving speeds the GPS is quite accurate.
I agree with this. My GPS is spot on with my speedometer.

NN
__________________
____________________________________________
"Religion is for people who are afraid to go to hell. Spiritually is for people who have already been there."
Never Nervous is offline   Reply With Quote
Old 02-22-2013, 15:27   #13
DanaT
Pharaoh
 
DanaT's Avatar
 
Join Date: Feb 2005
Location: CO & Baden –Württemberg
Posts: 15,828
Blog Entries: 1
Quote:
Originally Posted by wprebeck View Post
Don't know about the math, but I would advise you to not rely upon GPS as accurate. Its usually accurate to within a few meters - while great for figuring out where you are in a city, its not exactly mathematically accurate for determining speed.
GPS speeds I have seen are pretty damn accurate. Much more accurate than modern speedometers (electronic ones have a step function that are off more the fast you go) I have seen speedos read around 10-15% high at 125mph. They are showing 140ish and you are really going 125.
__________________
Quote:
Twice a week? 14 times a month?
Quote:
2x4=8, not 14.
Many of the truths that we cling to depend on our point of view.
DanaT is offline   Reply With Quote
Old 02-22-2013, 15:31   #14
Kevin108
HADOKEN!
 
Kevin108's Avatar
 
Join Date: Mar 2005
Location: Portsmouth, VA
Posts: 6,943


I used the GPS on my Droid X sitting next to my wife's Garmin Nuvi with cruise control on 55 mph as set by my gauge. All 3 devices displayed the same number.
__________________
Kevin108.com | OpenCarry.org | VCDL.org

Those who would give up Essential Liberty to purchase a
little Temporary Safety, deserve neither Liberty nor Safety.
_____- Ben Franklin
Kevin108 is offline   Reply With Quote
Old 02-22-2013, 15:35   #15
DanaT
Pharaoh
 
DanaT's Avatar
 
Join Date: Feb 2005
Location: CO & Baden –Württemberg
Posts: 15,828
Blog Entries: 1
Quote:
Originally Posted by wprebeck View Post
I don't trust my speedometer at all - it reads at least 3mph over the actual speed I'm traveling, based on calibrated radar hits.
Cough cough cough bs cough cough cough.


5.1.2. In the case of vehicles manufactured for sale in any country where imperial units are used, the speedometer shall also be marked in miles per hour (mph); the graduations shall be of 1, 2, 5 or 10 mph. The values of the speed shall be indicated on the dial at intervals not exceeding 20 mph and commencing at 10 or 20 mph. The indicated speed value intervals need not be uniform.


5.3 The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified
in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1)
and the true speed (V2).

0≤(V1– V2)≤0,1 V2+ 4 km/h

http://eur-lex.europa.eu/LexUriServ/...40:0048:EN:PDF

Or maybe you have a super special car that was built in a special plant that makes cars that dont share common parts with other cars that have to meet worldwide requirements.
__________________
Quote:
Twice a week? 14 times a month?
Quote:
2x4=8, not 14.
Many of the truths that we cling to depend on our point of view.
DanaT is offline   Reply With Quote
Old 02-22-2013, 16:25   #16
omega48038
Senior Member
 
omega48038's Avatar
 
Join Date: Aug 2007
Location: Detroit, Michigan USA
Posts: 840
Quote:
Originally Posted by Kevin108 View Post
Two formulas for speedometer correction are presented "High-Performance Jeep Cherokee Builder's Guide."

Formula 1:

Old Tire Diameter / New Tire Diameter x New Gear Ration / Old Gear Ratio x Old Tooth Count = New Tooth Count


Formula 2:

(63360 / (MSOD x 3.14159) x Axle Ratio) / 74.5 = # of Teeth in Speedo Gear


My algebra days are long behind me, so can I get someone to double-check my numbers here and explain to me how these formulas work?

Since I was too lazy to pull my existing gear to start with, I used Formula 2 first after getting actual measurements on my tires. I have GPS verified the accuracy of my speedo with my old tires with a diameter of 28.25".

(63360 / (28.25 x 3.14159) x 3.55) / 74.5 = # of Teeth in Speedo Gear
28.25 x 3.14159 = 88.7499175
63360 / 88.7499175 = 713.91615659811740106687986498692
713.91615659811740106687986498692 x 3.55 = 2534.4023559233167737874235207036
2534.4023559233167737874235207036 / 74.5 = 34.018823569440493607884879472531

34 teeth and lots of decimal places!

Now, back to Formula 1...
The Okie Corral


I didn't change my gear ratio, meaning old / new would be 1, so I'll leave that part out.

28.25 / 30.625 x 34 = New Tooth Count
28.25 / 30.625 x 34 = 31.363265306122448979591836734694

So I need to go all hillbilly and lose 3 teeth?

The Okie Corral


But seriously, what is represented or determined by the numbers 63360 and 74.5?
Yeah, that's a lot of decimal places, but all meaningless. Look up the term ''significant digits''.
omega48038 is offline   Reply With Quote
Old 02-22-2013, 18:21   #17
pesticidal
CLM Number 181
Eh?
 
pesticidal's Avatar
 
Join Date: Jun 2002
Location: North Dakota
Posts: 38,109
Send a message via Yahoo to pesticidal


Quote:
Originally Posted by wprebeck View Post
Well, for court purposes, I'm going to need to see the last time the GPS was calibrated and the user's training records on the device.

Further, where did the OP state he used a Garmin device? What specific device is he using? Again, where are the calibration records for the device?

I don't trust my speedometer at all - it reads at least 3mph over the actual speed I'm traveling, based on calibrated radar hits.
Their is no GPS calibration for speed. That's a fine-tuner for location.
__________________
Never pass up the opportunity to pet your dog, talk to an old friend, or play catch with your kid.
pesticidal is offline   Reply With Quote
Old 02-22-2013, 18:29   #18
Trapped_in_Kali
Senior Member
 
Join Date: Aug 2012
Location: Los Angeles, Northern Mexico
Posts: 1,040
Quote:
Originally Posted by DanaT View Post
...

Or maybe you have a super special car that was built in a special plant that makes cars that dont share common parts with other cars that have to meet worldwide requirements.

All speedos will be off as tires wear. If you put new tires on that are at all different in their circumference that will change the speedo accuracy.
Trapped_in_Kali is offline   Reply With Quote
Old 02-22-2013, 18:41   #19
boozer
Senior Member
 
Join Date: Jul 2007
Location: Hilltop, USA
Posts: 1,784
That's a lot of decimal places, here in Ohio they don't give you a ticket until you are four mph over the limit.
boozer is offline   Reply With Quote
Old 02-22-2013, 19:29   #20
hockeyrcks9901
Senior Member
 
hockeyrcks9901's Avatar
 
Join Date: Mar 2009
Location: St. Petersburg, FL
Posts: 1,714
Quote:
Originally Posted by DanaT View Post
Cough cough cough bs cough cough cough.


5.1.2. In the case of vehicles manufactured for sale in any country where imperial units are used, the speedometer shall also be marked in miles per hour (mph); the graduations shall be of 1, 2, 5 or 10 mph. The values of the speed shall be indicated on the dial at intervals not exceeding 20 mph and commencing at 10 or 20 mph. The indicated speed value intervals need not be uniform.


5.3 The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified
in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1)
and the true speed (V2).

0≤(V1– V2)≤0,1 V2+ 4 km/h

http://eur-lex.europa.eu/LexUriServ/...40:0048:EN:PDF

Or maybe you have a super special car that was built in a special plant that makes cars that dont share common parts with other cars that have to meet worldwide requirements.
My car is 16 years old and is off by up to 7 mph. It's more accurate at lower speeds but when it indicates 80mph I'm actually doing 73.

Sensors can move out of calibration, that's why industrial facilities perform routine calibrations on them. My work performs quarterly calibrations on our nuclear density gauges.
hockeyrcks9901 is offline   Reply With Quote
Old 02-22-2013, 19:39   #21
syntaxerrorsix
CLM Number 301
Anti-Federalist
 
syntaxerrorsix's Avatar
 
Join Date: Oct 2000
Location: Lakeland, FL.
Posts: 9,515
Quote:
Originally Posted by hockeyrcks9901 View Post
My car is 16 years old and is off by up to 7 mph. It's more accurate at lower speeds but when it indicates 80mph I'm actually doing 73.

Sensors can move out of calibration, that's why industrial facilities perform routine calibrations on them. My work performs quarterly calibrations on our nuclear density gauges.
My Mini is off by 3 miles an hour on the speedo but I'm not running stock tires anymore either.

Standard speedo deviations fluctuate as much as 10% empirically.
__________________
Sappers Forward
841st Eng (Cbt/Hvy) 81ARCOM, 84th Eng (Cbt/Hvy) 2ACR, 40th Eng (Mech) 1AD, 588th Eng (Mech) 4ID

syntaxerrorsix is offline   Reply With Quote
Old 02-23-2013, 06:12   #22
Kevin108
HADOKEN!
 
Kevin108's Avatar
 
Join Date: Mar 2005
Location: Portsmouth, VA
Posts: 6,943


Quote:
Originally Posted by wprebeck View Post
I don't trust my speedometer at all - it reads at least 3mph over the actual speed I'm traveling, based on calibrated radar hits.
With stock-sized tires 225/75R15s, my speedo was too fast. It was accurate up to the speed limit with 235/75R15s. All this math is to select the proper speedo gear to correct for the 31x10.50-15s I'm running now.
__________________
Kevin108.com | OpenCarry.org | VCDL.org

Those who would give up Essential Liberty to purchase a
little Temporary Safety, deserve neither Liberty nor Safety.
_____- Ben Franklin
Kevin108 is offline   Reply With Quote
Old 02-23-2013, 09:39   #23
droidfire
Senior Member
 
Join Date: Apr 2012
Location: All over, United States.
Posts: 1,979
Quote:
Originally Posted by Kevin108 View Post
With stock-sized tires 225/75R15s, my speedo was too fast. It was accurate up to the speed limit with 235/75R15s. All this math is to select the proper speedo gear to correct for the 31x10.50-15s I'm running now.
When I went from 31's to 33's, I just didn't sweat it. I do a lot of math otherwise, that kinda sucked the fun out of the change for me.
droidfire is offline   Reply With Quote
Old 02-23-2013, 09:52   #24
wprebeck
Die, bird, die!
 
wprebeck's Avatar
 
Join Date: Oct 2002
Location: In a swamp, if I'm lucky
Posts: 8,444
And Dana fancies himself a scientist at times.

Hmm - can you think of any variables that might make my truck slower?
wprebeck is offline   Reply With Quote
Old 02-23-2013, 10:06   #25
DanaT
Pharaoh
 
DanaT's Avatar
 
Join Date: Feb 2005
Location: CO & Baden –Württemberg
Posts: 15,828
Blog Entries: 1
Quote:
Originally Posted by wprebeck View Post
And Dana fancies himself a scientist at times.

Hmm - can you think of any variables that might make my truck slower?
You wouldn't be admitting to breaking the law (defective vehicle) in a public forum by altering something required by law by DOT / nhtsa would you?

http://www.nhtsa.gov/people/injury/s...rocedures.html
__________________
Quote:
Twice a week? 14 times a month?
Quote:
2x4=8, not 14.
Many of the truths that we cling to depend on our point of view.
DanaT is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump




All times are GMT -6. The time now is 00:01.



Homepage
FAQ
Forums
Calendar
Advertise
Gallery
GT Wiki
GT Blogs
Social Groups
Classifieds


Users Currently Online: 1,042
254 Members
788 Guests

Most users ever online: 2,244
Nov 11, 2013 at 11:42