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 02-22-2013, 06:21 #1 Kevin108 HADOKEN!     Join Date: Mar 2005 Location: Virginia Beach, VA Posts: 6,116 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... 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? 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
 02-22-2013, 06:25 #2 Steve0853 Senior Member   Join Date: Aug 2003 Location: North Carolina Posts: 4,923 uhhhhhhhhhhhhh............12?
 02-22-2013, 06:25 #3 wprebeck Need this gun..     Join Date: Oct 2002 Location: Louisville KY Posts: 8,074 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.
 02-22-2013, 06:31 #4 Two_Clicks Senior Member     Join Date: Mar 2010 Location: Southern Ohio Posts: 705 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.
02-22-2013, 07:10   #5
syntaxerrorsix
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Quote:
 Originally Posted by wprebeck 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.
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Last edited by syntaxerrorsix; 02-22-2013 at 07:14..

02-22-2013, 07:20   #6
devildog2067
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Quote:
 Originally Posted by Kevin108 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?

02-22-2013, 07:33   #7
kylenewman
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Quote:
 Originally Posted by wprebeck 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.

02-22-2013, 07:42   #8
wprebeck
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Quote:
 Originally Posted by syntaxerrorsix 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.

02-22-2013, 09:20   #9
SC Tiger
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Quote:
 Originally Posted by Kevin108 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... 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? 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.
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 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.
02-22-2013, 11:34   #11
syntaxerrorsix
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Quote:
 Originally Posted by wprebeck 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
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02-22-2013, 15:02   #12
Never Nervous
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Quote:
 Originally Posted by syntaxerrorsix 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
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02-22-2013, 15:27   #13
DanaT
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Quote:
 Originally Posted by wprebeck 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.
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 02-22-2013, 15:31 #14 Kevin108 HADOKEN!     Join Date: Mar 2005 Location: Virginia Beach, VA Posts: 6,116 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
02-22-2013, 15:35   #15
DanaT
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Quote:
 Originally Posted by wprebeck 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.
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02-22-2013, 16:25   #16
omega48038
Senior Member

Join Date: Aug 2007
Location: Detroit, Michigan USA
Posts: 837
Quote:
 Originally Posted by Kevin108 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... 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? 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''.

02-22-2013, 18:21   #17
pesticidal
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Eh?

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Quote:
 Originally Posted by wprebeck 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.
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02-22-2013, 18:29   #18
Trapped_in_Kali
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Quote:
 Originally Posted by DanaT ... 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.

 02-22-2013, 18:41 #19 boozer Senior Member   Join Date: Jul 2007 Location: Hilltop, USA Posts: 1,657 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.
02-22-2013, 19:29   #20
hockeyrcks9901
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Join Date: Mar 2009
Location: St. Petersburg, FL
Posts: 1,676
Quote:
 Originally Posted by DanaT 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.

02-22-2013, 19:39   #21
syntaxerrorsix
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Quote:
 Originally Posted by hockeyrcks9901 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.
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02-23-2013, 06:12   #22
Kevin108

Join Date: Mar 2005
Location: Virginia Beach, VA
Posts: 6,116

Quote:
 Originally Posted by wprebeck 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.
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02-23-2013, 09:39   #23
droidfire
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Quote:
 Originally Posted by Kevin108 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.

 02-23-2013, 09:52 #24 wprebeck Need this gun..     Join Date: Oct 2002 Location: Louisville KY Posts: 8,074 And Dana fancies himself a scientist at times. Hmm - can you think of any variables that might make my truck slower?
02-23-2013, 10:06   #25
DanaT
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Quote:
 Originally Posted by wprebeck 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
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