Sponsored

Speedometer not accurate

K4fxd

Well-Known Member
Joined
Apr 2, 2020
Threads
102
Messages
10,230
Reaction score
8,288
Location
NKY
First Name
Dan
Vehicle(s)
2017 gt, 2002 FXDWG, 2008 C6,
Mine is accurate according to GPS and road side radar signs.
Sponsored

 

Linkster1666

Well-Known Member
Joined
Jun 28, 2019
Threads
5
Messages
734
Reaction score
611
Location
AZ
First Name
Link
Vehicle(s)
2019 Mustang GT Premium Orange Fury PP1 A10 401A 3:55
Vehicle Showcase
1
Why are you comparing GPS speed with an electric/mechanical sensor?

You understand how GPS works? You have to have at least Four sats for any speed reading, let alone an accurate one. Back in the olden days, you set a GPS waypoint, you could get back to it within 150 feet.

And having used GPS for about 30 years, you are at the mercy of many, many things. First and foremost is the software and the hardware that is deciphering the satellite data. The settings of the sats, they need updated constantly, due to a variety of factors. Thirty years ago, WHOSE Birds where you using. Space weather, a minor flare can and will mess with the birds, your hardware AND your Data. Line of sight, not as big a deal now or is it?

So you may want to do a LOT of research on Who made your hardware and what the limitations are. Also, Bob the App Writer may not know shit. (Sorry Bob)

Just an opinion.
 
OP
OP
joe603

joe603

Well-Known Member
Joined
Feb 2, 2021
Threads
48
Messages
567
Reaction score
376
Location
ATL
First Name
Joe
Vehicle(s)
2021 GT Premium PP
It's not a quirk, nor specific to a Mustang. It's completely normal with every vehicle.

No vehicle has a perfectly accurate speedometer.

What do you think happens when tires wear?
Both my 2014 Durango R/T and my 2016 Charger SCAT didn't have this issue...Again, just wanted to see if something was wrong with my car, or if it was a known issue. Thanks.
 

Zooks527

Well-Known Member
Joined
Aug 24, 2018
Threads
65
Messages
1,650
Reaction score
1,298
Location
02048
Vehicle(s)
2019 KB GT, 401A, 6MT, PP1, S&S, MR, AE, B&O / 2005 Toyota Tacoma
Speedo is calibrated for the stock rim/tire combo. As long as you can get the revs per mile right (and something that can reprogram it), you can get the speedo dead nuts on. I got the revs per mile for my setup (285/35/19 indy 500) from tirerack, programmed it to the car, and my speedo perfectly matches my GPS.
And then your tires start to wear down from full tread diameter, the circumference decreases, and the speedometer starts reporting a higher speed than actual once again.
 

Bob Lob Law

Well-Known Member
Joined
Apr 18, 2023
Threads
0
Messages
132
Reaction score
213
Location
417
First Name
Jeff
Vehicle(s)
'22 GT Premium
Why are you comparing GPS speed with an electric/mechanical sensor?

You understand how GPS works? You have to have at least Four sats for any speed reading, let alone an accurate one. Back in the olden days, you set a GPS waypoint, you could get back to it within 150 feet.

And having used GPS for about 30 years, you are at the mercy of many, many things. First and foremost is the software and the hardware that is deciphering the satellite data. The settings of the sats, they need updated constantly, due to a variety of factors. Thirty years ago, WHOSE Birds where you using. Space weather, a minor flare can and will mess with the birds, your hardware AND your Data. Line of sight, not as big a deal now or is it?

So you may want to do a LOT of research on Who made your hardware and what the limitations are. Also, Bob the App Writer may not know shit. (Sorry Bob)

Just an opinion.
My GPS app used 13 satellites to calculate my speed and it was spot on with the radar signs(2 of them) that I intentionally drove past to check. My car was dead on, until I changed wheels and tires.
 

Sponsored

Linkster1666

Well-Known Member
Joined
Jun 28, 2019
Threads
5
Messages
734
Reaction score
611
Location
AZ
First Name
Link
Vehicle(s)
2019 Mustang GT Premium Orange Fury PP1 A10 401A 3:55
Vehicle Showcase
1
My GPS app used 13 satellites to calculate my speed and it was spot on with the radar signs(2 of them) that I intentionally drove past to check. My car was dead on, until I changed wheels and tires.
I consistently find the Radar Signs 2 MPH off anyway so now we are at a 4 mph margin?
 

WildHorse

N/A or GO HOME
Joined
Jun 28, 2017
Threads
216
Messages
8,355
Reaction score
6,412
Location
Home World: CLASSIFIED
First Name
ⓇⒾⒸⓀⓎ ⓈⓅⒶⓃⒾⓈⒽ
Vehicle(s)
'17 S550
Vehicle Showcase
1
Nope, OEM rims/tires. 2mph is the difference between getting a warning and a ticket...
Move. In my neck of the old backwoods its 11 mph over b4 you get a ticket.
 

Bluemustang

Well-Known Member
Joined
Nov 6, 2015
Threads
149
Messages
3,895
Reaction score
2,254
Location
Maryland
First Name
Ryan
Vehicle(s)
2015 Mustang Base GT
If you changed tire size, that's why.
 

KingKona

Well-Known Member
Joined
Jan 6, 2022
Threads
12
Messages
2,907
Reaction score
2,824
Location
Virginia
First Name
Shlomo
Vehicle(s)
2019 GT
Both my 2014 Durango R/T and my 2016 Charger SCAT didn't have this issue....
There is no issue.

Neither your Durango or Charger had 100% accurate speedometers. They were both off by a MPH or three or more.
 
Last edited:

ice445

Well-Known Member
Joined
Apr 13, 2020
Threads
33
Messages
6,009
Reaction score
7,116
Location
Salt Lake City, UT
First Name
Ryan
Vehicle(s)
2020 Mustang GT 6MT
And then your tires start to wear down from full tread diameter, the circumference decreases, and the speedometer starts reporting a higher speed than actual once again.
That's a pretty negligible change with summer tires that don't have that much tread depth to begin with lol
 

Sponsored

Zooks527

Well-Known Member
Joined
Aug 24, 2018
Threads
65
Messages
1,650
Reaction score
1,298
Location
02048
Vehicle(s)
2019 KB GT, 401A, 6MT, PP1, S&S, MR, AE, B&O / 2005 Toyota Tacoma
That's a pretty negligible change with summer tires that don't have that much tread depth to begin with lol
For the 275/40-19 Michelin Pilot Sport 4S rear tires that came on my car:
Initial diameter: 27.70"​
Initial tread depth: 0.266"​
Tread depth when worn: 0.0625"​
Diameter when worn: 27.29"​
% diameter change when worn: 1.5%​
Speedometer error when worn at 75 mph: reports 1.1 mph faster than actual​
If the tires were 225/45-17, the error would be 1.9%, or 1.4 mph faster than actual​
Which is in the middle of what the OP said he was seeing. If they were all-seasons instead of summer performance, the error number would be even greater.
 
Last edited:
OP
OP
joe603

joe603

Well-Known Member
Joined
Feb 2, 2021
Threads
48
Messages
567
Reaction score
376
Location
ATL
First Name
Joe
Vehicle(s)
2021 GT Premium PP
I just had new front tires put on yesterday...we'll see if it improves. Again, this isn't some earth-shattering issue, just something I noticed and wanted to see if it was a S550 thing or my car...like the BBQ tick.
 

Skye

Well-Known Member
Joined
Jan 18, 2022
Threads
11
Messages
1,642
Reaction score
2,309
Location
≈39N
Vehicle(s)
"Skye" Mach1 N2144
Conceptually, it's diffcult for me to see things being exact amongst one another.

Police: using sounding radar or laser

GPS: coded radio signals and stratum one timing

Car: derived from mechanical, electrical, possibly both means, maybe multiple input sources

Phone: probably GPS, via a programmed app or OS

+

Environment variations: tire wear, interfering weather, radio or physical conditions, location, etc.

Different systems and interfaces, each with their own measurement methods, and what I think could be most important: different 1) rates of measurement and 2) tolerances. Further still, what are any of the systems described above certified to? Level of accuracy? How long ago?

At the end of the day, IMO, it's what are the authorities measuring the car at. Anything over, you have to prove they're wrong. Have a different reading? Courts would expect the driver to take it up with whomever created and maintains the other system and then come back and prove the police/trooper was inaccurate.

Scenarios I describe aren't often feasible, if possible at all. Drivers should allow for a bit and give themselves something of a speed budget, so they are always under the speed of interest. If the car is consistently (key phrase) reporting a certain amount over the cars actual, verified speed, that amount, that gap, is the budget and they should not exceed it.

Example: car reports 65 MPH / 104 KMH but actual, authority-measured speed is 63 MPH / 101 KMH. The budget is 2. The driver needs to drive between 65-67 MPH, as reported by the car, preferably 65. The driver could drive at a car-reported 67, but they run the risk of all the variances ultimately showing otherwise to the police (higher, triggering ticket).
 
Last edited:

ORRadtech

Well-Known Member
Joined
Sep 12, 2019
Threads
21
Messages
2,448
Reaction score
2,148
Location
Atlanta, Georgia
First Name
Dave
Vehicle(s)
18 Mustang EcoBoost convertible, 14 Ford Fusion SE
Personally I'd rather the speedo read 2 over than 2 under.

Also, I know that it's jurisdiction/ LEO dependant but I've never been ticketed for anything under 10 over. Hope that doesn't jinx me...
 

K4fxd

Well-Known Member
Joined
Apr 2, 2020
Threads
102
Messages
10,230
Reaction score
8,288
Location
NKY
First Name
Dan
Vehicle(s)
2017 gt, 2002 FXDWG, 2008 C6,
Also, I know that it's jurisdiction/ LEO dependant but I've never been ticketed for anything under 10 over. Hope that doesn't jinx me...
Don't drive in Indiana, got popped for 5 over coming into some hick town.
Sponsored

 
 




Top