Terribly off speedometer

[RosieTheRedM3]

I'm sorry I'm advance as I'm sure this has been covered in the past but my car's speedometer becomes terrible off as my speed increases. I have quite a decent amount of modifications but here's the data I've gotten back from both an Escort Max 360 (which displays speed based off satellite, I think) and a speedometer app I downloaded. It occurs roughly after 30 mph and I have been told by several people that it's normal to be a few miles off l, about 5 mph.

35 mph: really 31 mph
45 mph: really 40 mph
65 mph: really 58-59 mph
85 mph: really 78 mph
110 mph: really 101 mph
125 mph: really 113-114 mph
140 mph: really 126-128 mph

I understand it's incredibly unsafe to be driving those speeds but at the same time, if it says I'm doing 140 mph, I could be doing 15 miles less than that. If anyone knows the answer to this or has the same issue, I'd love to hear an answer. I've heard about a thing you can have done by a mechanic for $75 that gets your car to show "true speed" on the speedometer but I'm not sure if this really is that true speed although it seems extremely off to show 4-14 mph off the speed shown on my dashboard. Thank you

[Shiza]

Do you have factory size wheels and tires on the car?

[fatalelement]

Speedometer usually will overshoot always on any car, but usually only like 5%ish. This seems a bit off, but my guess would be that you are fitting tires that are slightly nonstandard size.

[roastbeef]

It's common.

[Helmsman]

Wow thats a lot, wonder why. Mine show half, around 2.5mph/4kmph at 65mph. Anyways, you can code the digital speedo in the cluster to "true speed" in a minute with for instance Carly.

[SkiFly01]

I even run a 275/35/19 (26.6") in the rear which is +1.1% larger than the oem 265/35/19 (26.3") and it still reads higher than GPS speed. BMW does this with their motorcycles too, my R1200GS Adventure reads faster than GPS speed. You can play with tire sizing to get the speedo to be more accurate or even look into the yellow box. https://yellr.com/ something Engineering Explained used when swapping out a differential. As you said, your speed increases so does the speed increasingly get worse. It is exponential.

[RosieTheRedM3]

Originally Posted by Reed

Do you have factory size wheels and tires on the car?

No they are 275's in the rear and 245's in the front

[RosieTheRedM3]

Originally Posted by fatalelement

Speedometer usually will overshoot always on any car, but usually only like 5%ish. This seems a bit off, but my guess would be that you are fitting tires that are slightly nonstandard size.

Yeah I am 245 front, 275 rear

[RosieTheRedM3]

Originally Posted by Helmsman

Wow thats a lot, wonder why. Mine show half, around 2.5mph/4kmph at 65mph. Anyways, you can code the digital speedo in the cluster to "true speed" in a minute with for instance Carly.

What is the "Carly" thing?

[RosieTheRedM3]

Originally Posted by SkiFly01

I even run a 275/35/19 (26.6") in the rear which is +1.1% larger than the oem 265/35/19 (26.3") and it still reads higher than GPS speed. BMW does this with their motorcycles too, my R1200GS Adventure reads faster than GPS speed. You can play with tire sizing to get the speedo to be more accurate or even look into the yellow box. https://yellr.com/ something Engineering Explained used when swapping out a differential. As you said, your speed increases so does the speed increasingly get worse. It is exponential.

I'm running the same size in the rear

[ItsGary]

Originally Posted by RosieTheRedM3

What is the "Carly" thing?

https://www.mycarly.com/product/bmw-app/

[IanMan]

Out of all of the modern BMW's I've owned, they each seem to show a 3 mph bias towards a faster speed. This is common.

[thkfast]

I've also experienced my speedo reads about +3 mph. When cruising on the highway I just set it for 10 mph over the limit and call it good. I haven't checked the accuracy against my GPS at higher speeds because I'm terrible at multitasking.

[M3 Number 86]

Here's an old video where I put two points of data for speed.

In the speedo, it's literally what the speedo says.
On the right side, it shows GPS speed.

Different size wheels/tires are what's messing with it.

[M3 Number 86]

Originally Posted by RosieTheRedM3

I'm sorry I'm advance as I'm sure this has been covered in the past but my car's speedometer becomes terrible off as my speed increases. I have quite a decent amount of modifications but here's the data I've gotten back from both an Escort Max 360 (which displays speed based off satellite, I think) and a speedometer app I downloaded. It occurs roughly after 30 mph and I have been told by several people that it's normal to be a few miles off l, about 5 mph.

35 mph: really 31 mph
45 mph: really 40 mph
65 mph: really 58-59 mph
85 mph: really 78 mph
110 mph: really 101 mph
125 mph: really 113-114 mph
140 mph: really 126-128 mph

I understand it's incredibly unsafe to be driving those speeds but at the same time, if it says I'm doing 140 mph, I could be doing 15 miles less than that. If anyone knows the answer to this or has the same issue, I'd love to hear an answer. I've heard about a thing you can have done by a mechanic for $75 that gets your car to show "true speed" on the speedometer but I'm not sure if this really is that true speed although it seems extremely off to show 4-14 mph off the speed shown on my dashboard. Thank you

see my post above. You can't look at it in terms of nominal values. It will increase as speeds increase since it's off by a percentage value.

[Z K]

The values look the same as what I get from my speedometer. When I use my navigation apps on my phone, the speedometer in the car always reads about 5-10mph more under 100 mph. It's fine, I just compensate by driving faster than indicated. I'm running stock tire size.

[Transfer]

Speedometer reads off of the rear wheel/tire. I went from 275/35/19 which was accurate within about 1 mph to 285/30/19 which reads high by about 6-7% (based on GPS).

[1MOREMOD]

Pretty common, anyone who data logs at the track is well aware top speed on straight is well off of actual speed.

[M3 Number 86]

Originally Posted by 1MOREMOD

Pretty common, anyone who data logs at the track is well aware top speed on straight in speedo is well of of actual speed.

Hero mode.



On another though I just realized you (OP) made that IG post on Facebook group hahha

[Helmsman]

Originally Posted by Transfer

Speedometer reads off of the rear wheel/tire. I went from 275/35/19 which was accurate within about 1 mph to 285/30/19 which reads high by about 6-7% (based on GPS).

So you measured speedo to be on spot with OEM or even a tad larger rear wheels? Strange.
Think I remember Ive seen a (too complicated for me) way to code the friggin gas consumption to present accurate value (which it doesn't original). Cant remember details but sounds like included the speedo. Done any of that mate?

[Transfer]

Originally Posted by Helmsman

So you measured speedo to be on spot with OEM or even a tad larger rear wheels? Strange.
Think I remember Ive seen a (too complicated for me) way to code the friggin gas consumption to present accurate value (which it doesn't original). Cant remember details but sounds like included the speedo. Done any of that mate?

I didn't measure with 265/35 but was surprised how accurate 275/35 was. Then I dropped tire diameter by 3.4% and it's way off. Fronts stayed at 255/35. Doesn't seem to add up real well but that's my experience. I don't have any tuning/coding for gas or speedo. Radar detector is Uniden R3.

[MJBO]

Mine is off by about the same, but I’m almost certain its because the dealer fitted 30 rather than 35 aspect tyres on the rear. As soon as some Pilot Sport 4 S are in stock that’ll be rectified.

Even with correct tyres it will still likely be out of 5% (rather than the current 10%). When coding my iDrive with Carly there were options for speedo accuracy, haven’t changed any of them though.