Digital dash speedometer accuracy?

W.O.T. Stang

Well-Known Member
Joined
Dec 21, 2019
Threads
16
Messages
320
Reaction score
343
Location
Texas
First Name
Jim
Vehicle(s)
2019 Mustang GT Premium PP1, 2021 Kawasaki Z900
Has anyone else noticed an inaccuracy in your speed? My Performance pack 1 (19" wheels) with digital dash seem to be consistently 2 mph low (reads 37mph when im going 35mph, or 67 mph when im going 65mph). It was confirmed by multiple GPS apps and radar stations that were setup along the side of the road.

I'm guessing maybe this was planned to keep you from getting speeding tickets - because it is so easy to speed, but it's hard to say. I am wondering if anyone else notices this, and i also wonder if it's possible to calibrate it somehow.

 

NoVaGT

Banned
Banned
Banned
Joined
Sep 29, 2016
Threads
115
Messages
5,682
Reaction score
4,398
Location
Northern Virginia
Vehicle(s)
2019 PP1 GT Kona
2MPH doesn't cause or stop you from getting a ticket.

2MPH could just be tire pressures. And with 18K on the tires, that could also cause a 2MPH difference.
 
Last edited:

FreePenguin

Well-Known Member
Joined
Sep 29, 2019
Threads
80
Messages
5,227
Reaction score
3,456
Location
Ohio
First Name
Donald
Vehicle(s)
17 mustang
Vehicle Showcase
1
can you change tire sizes using forescan? Change it to another number make closer +2mph lol Idk
 

FreePenguin

Well-Known Member
Joined
Sep 29, 2019
Threads
80
Messages
5,227
Reaction score
3,456
Location
Ohio
First Name
Donald
Vehicle(s)
17 mustang
Vehicle Showcase
1


OP
OP
W.O.T. Stang

W.O.T. Stang

Well-Known Member
Joined
Dec 21, 2019
Threads
16
Messages
320
Reaction score
343
Location
Texas
First Name
Jim
Vehicle(s)
2019 Mustang GT Premium PP1, 2021 Kawasaki Z900
I was just looking at that thread.. Good info there! I suppose the only way to know for sure is to have an actual radar reading taken... and i'm not that worried about it. Also GPS is not always entirely accurate. I'm sure its within a margin of error and not a big deal. Just curious if anyone also noticed it.
 

Zooks527

Well-Known Member
Joined
Aug 24, 2018
Threads
53
Messages
1,498
Reaction score
1,064
Location
02048
Vehicle(s)
2019 KB GT, 401A, 6MT, PP1, S&S, MR, AE, B&O / 2005 Toyota Tacoma
Varies with temperature, tire type, tire wear, etc.

On my stock tires when new, my speedometer was either dead-on or 1 mph high, varying with the temperature of the day. With 6500 miles or so on them, the stock tires were running 1 to 2 mph high when I switched to my winter set in November.

On my winter tires when new, my speedometer was 1 to 2 mph high, varying with the temperature of the day, even though the rears were the same nominal diameter as stock. With 4000 miles or so on them, the winter tires are now running 2 to 3 mph high at the moment.

You can play with it in Forscan if you want, but unless you're doing something like changing the rear axle ratio or the gross rear tire diameter, you'll find yourself chasing your tail.
 
OP
OP
W.O.T. Stang

W.O.T. Stang

Well-Known Member
Joined
Dec 21, 2019
Threads
16
Messages
320
Reaction score
343
Location
Texas
First Name
Jim
Vehicle(s)
2019 Mustang GT Premium PP1, 2021 Kawasaki Z900
More interesting tech info i found regarding the speedometer.

Speedometer

The PCM receives the wheel speed data from the GWM over the HS-CAN1 . The GWM receives the wheel speed data from the ABS module over the HS-CAN2 . The PCM uses tire size stored in the vehicle configuration file along with wheel speed inputs to generate a vehicle speed signal.

The IPC receives the vehicle speed message from the GWM over the HS-CAN3 . The GWM receives the vehicle speed message from the PCM over the HS-CAN1 .

The IPC provides a tolerance that allows the speed indication to display between 3% below and 7% above the actual vehicle speed. This means that with an actual vehicle speed of 97 km/h (60 mph), the speedometer can indicate between 94-103 km/h (58-64 mph). Incorrect tire size or tire size configuration could potentially affect the speedometer accuracy.
 

NoVaGT

Banned
Banned
Banned
Joined
Sep 29, 2016
Threads
115
Messages
5,682
Reaction score
4,398
Location
Northern Virginia
Vehicle(s)
2019 PP1 GT Kona

Fly2High

Well-Known Member
Joined
Sep 9, 2019
Threads
69
Messages
1,161
Reaction score
582
Location
Long Island
First Name
Frank
Vehicle(s)
2019 Mustang GT PP2
A smaller diameter tire will have a shorter circumference. This will cause the speedo to read higher. You will get more revolutions per duration which equates to a higher estimated speed.

Same rationale for running lower pressure will read higher. Higher pressure will make it read lower since it bulges the tire just a bit.

Also, a tire, as it wears, will cause the speedo to read higher.

That is why cops tend not to pull people over for anything less than 5 mph over the limit.

GPS can be off due to government limitations on accuracy and I believe radar guns can read off due to needing calibration and angle.

Find a straight road with mile markers and a stopwatch and you might just get a better estimate.
 

Zooks527

Well-Known Member
Joined
Aug 24, 2018
Threads
53
Messages
1,498
Reaction score
1,064
Location
02048
Vehicle(s)
2019 KB GT, 401A, 6MT, PP1, S&S, MR, AE, B&O / 2005 Toyota Tacoma
...... This will cause the speedo to read higher. Y

..... Same rationale for running lower pressure will read higher.

..... Also, a tire, as it wears, will cause the speedo to read higher.

That is why cops tend not to pull people over for anything less than 5 mph over the limit.
Actually, if all those conditions result in your speedometer showing a higher speed than you are going, that gives the cop a reason to pull you over at 1 mph above the limit, as your speedometer was reading even higher than that.

The real reason they don't pull you over is because everyone is going much more than 5 mph over. As an example, coming to work this morning on Rt.495 in Massachusetts, the middle lane was doing 82 mph in a 65 mph zone, and the left lane cars were either matching or passing the cars in the middle.
 

FreePenguin

Well-Known Member
Joined
Sep 29, 2019
Threads
80
Messages
5,227
Reaction score
3,456
Location
Ohio
First Name
Donald
Vehicle(s)
17 mustang
Vehicle Showcase
1
It’s actually stupid how fast traffics going since covid... look, I drive the speed limit, I go 69-70 in 65, but everyone I mean everyone except me is going 85-95 now. In 55mph people going 70-80, even on 35-40 areas people are going 60!

its pissing me off really, because I don’t speed, I’ve never had a ticket and people are literally on my ass, and I’m in furthest right lane, they are almost causing wrecks getting around me and Ive even gotten annoyed people who fly around me then stomp brakes because I’m going speed limit.

(I’ve been with government 13 years and many of those as police officer).
The people who go the even faster are those with the crappiest beat up cars or soccer moms with 9 kids in the vans texting eating and doing make up.

it’s literally like no one has a single fear of being pulled over and it makes me mad. We are becoming a society with zero respect for law. I just don’t see why people have to do 80-100mph in a 65, if I go 71 I worry about getting a ticket, but these guys doing reckless all day everyday don’t give a shit.

Im boring, I fly from 0-70 onto highways etc. but I never exceed the speed limit + 5.
 
Last edited:

PoCoBob

Well-Known Member
Joined
Jan 10, 2021
Threads
7
Messages
192
Reaction score
149
Location
Port Coquitlam B.C.
First Name
Bob
Vehicle(s)
2019 Mustang GT Premium
Virtually every car the speedometer reads slightly high. That's because the lawyers get involved with designing things. Imagine if it was even slightly the other way round, reading low. How many people would be claiming the manufacture is responsible for their tickets and fines. Years ago one of the big Japanese car companies got sued because their speedometers were so off peoples warranties were expiring 7-10% sooner then they should have, the odometer would read 60K miles when in fact the car had only traveled 54K miles. But based on the odometer reading the warranty was over.
 

NoVaGT

Banned
Banned
Banned
Joined
Sep 29, 2016
Threads
115
Messages
5,682
Reaction score
4,398
Location
Northern Virginia
Vehicle(s)
2019 PP1 GT Kona
It’s actually stupid how fast traffics going since covid... look, I drive the speed limit, I go 69-70 in 65, but everyone I mean everyone except me is going 85-95 now. In 55mph people going 70-80, even on 35-40 areas people are going 60!

its pissing me off really, because I don’t speed, I’ve never had a ticket and people are literally on my ass, and I’m in furthest right lane, they are almost causing wrecks getting around me and Ive even gotten annoyed people who fly around me then stomp brakes because I’m going speed limit.

(I’ve been with government 13 years and many of those as police officer).
The people who go the even faster are those with the crappiest beat up cars or soccer moms with 9 kids in the vans texting eating and doing make up.

it’s literally like no one has a single fear of being pulled over and it makes me mad. We are becoming a society with zero respect for law. I just don’t see why people have to do 80-100mph in a 65, if I go 71 I worry about getting a ticket, but these guys doing reckless all day everyday don’t give a shit.

Im boring, I fly from 0-70 onto highways etc. but I never exceed the speed limit + 5.
7BSP.gif
 

Doc2020GT

Well-Known Member
Joined
Jul 9, 2020
Threads
0
Messages
123
Reaction score
155
Location
Hampton Roads, VA (formerly NOVA)
First Name
Don
Vehicle(s)
2020 GT Premium PP 2018 Explorer. 2012 Toyota Tacoma
I’ve ridden motorcycles for the last 50 years, bikes speedos always show higher than what they’re actually doing. I read an article in one of the bike mags years ago and they explained why, I’ll be damned if I can remember why now.
 

 
89 - Solo Performance - 1
Top