Help for speedometer correction after gear change without tune
10/14/22, 02:19 PM
#61
Legacy TMS Member
Update,
This is all strange... I compared my old video and found out my rpm at 80mph is the same with the stock gear and new gear! All at 2000 at 80mph.
The guy couldn't lied about the gear install... So I'm guessing maybe the part I ordered is somehow wrong? I double-checked and what I ordered is 3.73 from ford performance.
This is strange...
This is all strange... I compared my old video and found out my rpm at 80mph is the same with the stock gear and new gear! All at 2000 at 80mph.
The guy couldn't lied about the gear install... So I'm guessing maybe the part I ordered is somehow wrong? I double-checked and what I ordered is 3.73 from ford performance.
This is strange...
10/14/22, 02:58 PM
#62
Member
Thread Starter
Is that 80 MPH the speedometer reading, or actual correct speed per some other source, such as GPS?
If it is the speedometer reading, it didn't change because it has not been corrected. The speedometer reading is a calculated value, not the actual vehicle speed. I think it is calculated from drive shaft RPM, rear gear ratio, and rear tire outer diameter (tire revolutions per mile). The amount of speedometer error depends on which gears you had before the change, and whether the speedometer was correct before the change or not.
(driveshaft RPM is the product of engine RPM and transmission gear ratio, but I think the system actually measures the driveshaft RPM rather than calculating it from engine RPM and transmission gear ratio)
If you went from 3.31 to 3.73 gears, and the speedometer was correct before that change, and you didn't change anything else (such as rear tire outer/overall diameter) and you didn't "recalibrate" the speedometer, then the speedometer is reading high by about 12.7%; meaning when the speedometer says 80 MPH, the actual real correct speed is 80 - (12.7% x 80) = about 70 MPH
If it is the speedometer reading, it didn't change because it has not been corrected. The speedometer reading is a calculated value, not the actual vehicle speed. I think it is calculated from drive shaft RPM, rear gear ratio, and rear tire outer diameter (tire revolutions per mile). The amount of speedometer error depends on which gears you had before the change, and whether the speedometer was correct before the change or not.
(driveshaft RPM is the product of engine RPM and transmission gear ratio, but I think the system actually measures the driveshaft RPM rather than calculating it from engine RPM and transmission gear ratio)
If you went from 3.31 to 3.73 gears, and the speedometer was correct before that change, and you didn't change anything else (such as rear tire outer/overall diameter) and you didn't "recalibrate" the speedometer, then the speedometer is reading high by about 12.7%; meaning when the speedometer says 80 MPH, the actual real correct speed is 80 - (12.7% x 80) = about 70 MPH
My speedometer is accurate and there is no change in RPMs when cruising at 80 mph. The guy told me to do a gear oil change and count the gear ratio to make sure...
10/14/22, 06:13 PM
#63
THE RED FLASH ------ Master-Moderator
Did you ever find out from the shop/person who did the gear change swap which gear ratio you have? Until you find out for certain, you're just going to continue spinning your wheels, second guessing yourself not knowing for certain if your speedo is calibrated accurately or not 
10/14/22, 06:19 PM
#64
Member
Thread Starter
Did you ever find out from the shop/person who did the gear change swap which gear ratio you have? Until you find out for certain, you're just going to continue spinning your wheels, second guessing yourself not knowing for certain if your speedo is calibrated accurately or not
In the future, I must double-check the parts before handing them over to the shop... If I took a photo of the gear set, I wouldn't have this trouble now...
The following users liked this post:
m05fastbackGT (10/14/22)
10/14/22, 06:47 PM
#65
THE RED FLASH ------ Master-Moderator
Agreed! Until counting the gear teeth after uninstalling the differential cover, I don't see any other way of determining which gear ratio you currently have.. Perhaps if the model/serial is stamped somewhere inside the casing? those numbers could also determine what the gear ratio is as well.. In the meantime, just keep in mind that we all make mistakes, as nobody is perfect... Just as you mentioned, for future reference just make certain to double check all the parts/numbers before handing them over to the shop.
10/15/22, 01:42 PM
#66
Member
Thread Starter
Agreed! Until counting the gear teeth after uninstalling the differential cover, I don't see any other way of determining which gear ratio you currently have.. Perhaps if the model/serial is stamped somewhere inside the casing? those numbers could also determine what the gear ratio is as well.. In the meantime, just keep in mind that we all make mistakes, as nobody is perfect... Just as you mentioned, for future reference just make certain to double check all the parts/numbers before handing them over to the shop.
I would Advise the Old School Test with Pace Car to get all your Ducks in a Row as Well! On a Side Note Gears are Normally Engraved With Tooth Count. I can't Imagine any Tech of Note Not Checking These Before Install!
Well Here you Go!
80Mph = 2600 Rpm????
https://youtu.be/XQXWHfUQoYI
KC
Well Here you Go!
80Mph = 2600 Rpm????
https://youtu.be/XQXWHfUQoYI
KC
So the guy actually took a video when he installed the gear and I can count the teethes in the video. If I'm seeing this correctly, I have 11 teeth on the pinion and 41 teethes in the ring gear which 41/11 is 3.727(3.73).
If I'm correct, then I can confirm my speedometer and my RPMs did not change at all after changing from stock gear to 3.73 gear....
Am I correct?
10/15/22, 01:59 PM
#67
Administrator, Shop Manual PDF Poster, Parts Locator & Spam Bot Eliminator!!
Join Date: November 27, 2004
Location: Visalia Ca.
Posts: 10,268
Received 2,621 Likes
on
1,939 Posts
I See 3.73 Stamped on Pinion gear!
Lets Get a Pace Car & Get Out on the Road & Put this Mystery to Rest! Hell With the High Tech!
KC
Lets Get a Pace Car & Get Out on the Road & Put this Mystery to Rest! Hell With the High Tech!
KC
10/15/22, 06:00 PM
#68
THE RED FLASH ------ Master-Moderator
Charlie! Now that you know your current gear ratio is 3:73.1.. If your speedo and RPM's did not change after swapping from stock to 3:73, then your installer electronically flashed/uploaded a tune into your vehicle's ECM to make a gear change adjustment for the 3:73 gears, otherwise, if the stock gear ratio were still uploaded into your vehicle's ECM, your speedo and RPM's would most definitely be way off...
Therefore, you cannot change the laws of physics... That being said, I'm 99.9% certain your installer will confirm that he did in fact upload a gear ratio adjustment change electronically into your vehicle's ECM with a hand-held tuning device...
Therefore, you cannot change the laws of physics... That being said, I'm 99.9% certain your installer will confirm that he did in fact upload a gear ratio adjustment change electronically into your vehicle's ECM with a hand-held tuning device...
10/15/22, 09:24 PM
#69
Legacy TMS Member
Just imagine that whomever had the car before had already put 3.73 gears in and the speedo was already adjusted for the gears 
Just a thought since the speedo is still reading the same thing. By chance is there anyway you can look at the old gears?
As Rocky said the speedo cannot not be correct after a gear change but as I do know the Speedo VS Tach cannot remain the same either
Just a thought since the speedo is still reading the same thing. By chance is there anyway you can look at the old gears?
As Rocky said the speedo cannot not be correct after a gear change but as I do know the Speedo VS Tach cannot remain the same either
The following 2 users liked this post by Siber Express:
Bert (10/17/22),
m05fastbackGT (10/16/22)
10/16/22, 01:23 PM
#70
THE RED FLASH ------ Master-Moderator
Just imagine that whomever had the car before had already put 3.73 gears in and the speedo was already adjusted for the gears
Just a thought since the speedo is still reading the same thing. By chance is there anyway you can look at the old gears?
As Rocky said the speedo cannot not be correct after a gear change but as I do know the Speedo VS Tach cannot remain the same either
Just a thought since the speedo is still reading the same thing. By chance is there anyway you can look at the old gears?
As Rocky said the speedo cannot not be correct after a gear change but as I do know the Speedo VS Tach cannot remain the same either
Meanwhile, great call
Last edited by m05fastbackGT; 10/16/22 at 01:27 PM.
The following 2 users liked this post by m05fastbackGT:
Bert (10/17/22),
Siber Express (10/16/22)
10/17/22, 10:48 AM
#71
Legacy TMS Member
Join Date: January 25, 2010
Location: Massachusetts
Posts: 3,782
Received 1,509 Likes
on
1,031 Posts
yeah, the only explanations I can think of are:
1) it already had 3.73 gears and the speedo was already calibrated for that
2) the speedometer was wrong before, and by some miracle the change corrected it, so now it is right
3) there were some other changes done at the same time that offset the gear change (like much larger outer diameter rear tires?) but that is a stretchf
PS -- it is definitely impossible to change the rear gear ratio without changing the speed of the car, at the same engine RPM and transmission gear ratio, if everything else (mostly rear tire revolutions per mile) stays the same ... because that's how gear ratios work
1) it already had 3.73 gears and the speedo was already calibrated for that
2) the speedometer was wrong before, and by some miracle the change corrected it, so now it is right
3) there were some other changes done at the same time that offset the gear change (like much larger outer diameter rear tires?) but that is a stretchf
PS -- it is definitely impossible to change the rear gear ratio without changing the speed of the car, at the same engine RPM and transmission gear ratio, if everything else (mostly rear tire revolutions per mile) stays the same ... because that's how gear ratios work
Last edited by Bert; 10/17/22 at 11:01 AM.
The following users liked this post:
m05fastbackGT (10/17/22)
10/17/22, 04:07 PM
#72
Member
Thread Starter
yeah, the only explanations I can think of are:
1) it already had 3.73 gears and the speedo was already calibrated for that
2) the speedometer was wrong before, and by some miracle the change corrected it, so now it is right
3) there were some other changes done at the same time that offset the gear change (like much larger outer diameter rear tires?) but that is a stretchf
PS -- it is definitely impossible to change the rear gear ratio without changing the speed of the car, at the same engine RPM and transmission gear ratio, if everything else (mostly rear tire revolutions per mile) stays the same ... because that's how gear ratios work
1) it already had 3.73 gears and the speedo was already calibrated for that
2) the speedometer was wrong before, and by some miracle the change corrected it, so now it is right
3) there were some other changes done at the same time that offset the gear change (like much larger outer diameter rear tires?) but that is a stretchf
PS -- it is definitely impossible to change the rear gear ratio without changing the speed of the car, at the same engine RPM and transmission gear ratio, if everything else (mostly rear tire revolutions per mile) stays the same ... because that's how gear ratios work
10/17/22, 04:39 PM
#73
THE RED FLASH ------ Master-Moderator
Yes! You should most definitely ask the guy who installed the 3:73 gears... Either he programmed your vehicle with his devices to adjust for the speedo or as we also posted, the previous owner of your car had already swapped out the stock gears for the 3:73's and also did the programming for the speedo adjustment as well.. As we also mentioned from the previous post, if your still have your old ring and pinion gears laying around? Take a look at them, as this will also determine what your old gear ratio was...
Last edited by m05fastbackGT; 10/17/22 at 04:41 PM.
The following users liked this post:
m05fastbackGT (10/18/22)
10/17/22, 07:47 PM
#75
Administrator, Shop Manual PDF Poster, Parts Locator & Spam Bot Eliminator!!
Join Date: November 27, 2004
Location: Visalia Ca.
Posts: 10,268
Received 2,621 Likes
on
1,939 Posts
I would Advise the Old School Test with Pace Car to get all your Ducks in a Row as Well! On a Side Note Gears are Normally Engraved With Tooth Count. I can't Imagine any Tech of Note Not Checking These Before Install!
Well Here you Go!
80Mph = 2600 Rpm????
https://youtu.be/XQXWHfUQoYI
KC
Well Here you Go!
80Mph = 2600 Rpm????
https://youtu.be/XQXWHfUQoYI
KC
Put in 80 Mph, 28 Inch Tire, 3.73 Gear Ratio and .69 Trans Ratio You Get 2470 + Add 200 Rpm for Loss.
Try it with 2.73's and you get 1800 + Add 200 Rpm for Loss and There you Are!
I Can't Believe 80 Mph at 2000 Rpm with a 3.73. The Video and This Chart I Checked Puts you 400-600 Rpm Higher. I was not 100% on the tire Diameter Stock is 28" But 6th Overdrive Ratio on the 6R Trans is .69 The Drivetrain & Will Lose Some Power so Chart is a Little Lower Than Vid.
UNLESS, Sure That Vid was Your Car? Something Is Fishy Here! Just Gotta Dig!
The transmission has a relatively low 1st gear and two overdrive gears, the highest of which is 0.69:1. This provides exceptional towing performance when needed, while maximizing fuel economy by offering low engine speeds while cruising. The transmission has a relatively low 1st gear and two overdrive gears, the highest of which is 0.69:1. This provides exceptional towing performance when needed, while maximizing fuel economy by offering low engine speeds while cruising.
The 6R80 can be found behind the 3.7L V6 all the way up to the 6.2L V8.
Watch After All this He Has a Bum Tach! Or Something we have not Thought Of.
Anyhoo, Here is My Serve!
KC
Last edited by 05stangkc; 10/17/22 at 08:19 PM.
10/18/22, 09:21 AM
#77
Administrator, Shop Manual PDF Poster, Parts Locator & Spam Bot Eliminator!!
Join Date: November 27, 2004
Location: Visalia Ca.
Posts: 10,268
Received 2,621 Likes
on
1,939 Posts
I got 3.31's in The Shelby ,I should Check My Car against Chart & See What I Come up With! Been awhile the Battery May Not Be up!
Meanwhile I Add to Charlies Confusion!
Oh Well, We Shall See!
At this Point I would Just Like Mystery Solved!
KC
Last edited by 05stangkc; 10/18/22 at 10:06 AM.
10/18/22, 02:16 PM
#78
Administrator, Shop Manual PDF Poster, Parts Locator & Spam Bot Eliminator!!
Join Date: November 27, 2004
Location: Visalia Ca.
Posts: 10,268
Received 2,621 Likes
on
1,939 Posts
FORMULA TEST, Strictly to test Formula not Compare To the Mustang. But Rpm Range is Similar!
Tested on my 2002 Harley F-150 All Stock! Including Oem Replacement Tires. Realize The Faster You Go the More Aerodynamics Come into Play Etc. Also I Believe Lower Horsepower & Taller Gears Would Require More Rpm as Well!
3.73 Gears,29.7" Tires and .71 Overdrive. 340 Supercharged Hp!
50 Mph Dead On 1500 Rpm, 60 Mph Dead on 1800, 70 Mph Required 2200 Rpm 100 More Rpm Than Listed . I Would Guess 80 May Require 100-200 More Rpm. Chart Says 80 At 2400 Rpm. Also Speedo's & Tachs Can Be Off as Well. But I think the Formula is Pretty Valid.
Second Test 2007 Shelby Gt-500 500 Supercharged Hp!
3.31 Gear, 27" Tire & .63 Overdrive
50 Mph Within 50Rpm at 1300 Listed 60 Mph Within 50 Rpm at 1550 Rpm. 70 Mph Within 50 Rpm at 1800. 80 Listed at 2100 Untested.
KC
Tire Size Calculator for Good Measure!
https://tiresize.com/calculator/
Tested on my 2002 Harley F-150 All Stock! Including Oem Replacement Tires. Realize The Faster You Go the More Aerodynamics Come into Play Etc. Also I Believe Lower Horsepower & Taller Gears Would Require More Rpm as Well!
3.73 Gears,29.7" Tires and .71 Overdrive. 340 Supercharged Hp!
50 Mph Dead On 1500 Rpm, 60 Mph Dead on 1800, 70 Mph Required 2200 Rpm 100 More Rpm Than Listed . I Would Guess 80 May Require 100-200 More Rpm. Chart Says 80 At 2400 Rpm. Also Speedo's & Tachs Can Be Off as Well. But I think the Formula is Pretty Valid.
Second Test 2007 Shelby Gt-500 500 Supercharged Hp!
3.31 Gear, 27" Tire & .63 Overdrive
50 Mph Within 50Rpm at 1300 Listed 60 Mph Within 50 Rpm at 1550 Rpm. 70 Mph Within 50 Rpm at 1800. 80 Listed at 2100 Untested.
KC
Tire Size Calculator for Good Measure!
https://tiresize.com/calculator/
Last edited by 05stangkc; 6/7/23 at 02:08 PM.
10/18/22, 03:00 PM
#79
THE RED FLASH ------ Master-Moderator
At 65MPH in 5th gear with stock 3:55 gears...My speedo hovers between 2000-2200 RPM with 26.7" diameter 275/40/18" Firestone Firehawk Indy 500 tires on 2007-09 Shelby GT500 18x9.5" wheels on all 4 corners.
The following users liked this post:
05stangkc (10/18/22)
10/18/22, 03:09 PM
#80
Legacy TMS Member
Join Date: January 25, 2010
Location: Massachusetts
Posts: 3,782
Received 1,509 Likes
on
1,031 Posts
Gear ratios is one thing where the math always works correctly and the results should be very accurate -- because that's how gears work. The only reason the calculated speed would be wrong, is if one of the numbers in the math is wrong. Most likely culprit would be rear tire revolutions per mile; I assume there is no question about engine RPM, transmission gear ratio, or rear gear ratio. This is for manual transmission where there is no slip in the transmission, meaning there is no loss of RPM between the engine and the rear wheels (other than the changes through the gears).
If you don't have enough horsepower (to overcome drag), the engine won't be able to achieve that RPM, in that transmission gear.
Still wondering what is the explanation for this mystery!
If you don't have enough horsepower (to overcome drag), the engine won't be able to achieve that RPM, in that transmission gear.
Still wondering what is the explanation for this mystery!
The following users liked this post:
05stangkc (10/18/22)