^^^^ Severely worn tire-tread could also account for a small bit of speedometer error (maybe 1 MPH or less) by making the tire-diameter slightly smaller and increasing the number of RPMs the wheel makes relative to true road speed. It is the same effect (but to a lesser extent) as a lower-than-stock profile on the tire/wheel assembly.

 

Tires only have about 8/32nd of an inch of usable tread. That’s not going to be enough to alter the speedometer.

 

I’m not making it up. I’ve read all about it over the years in MT, C&D,
forums, etc. Give people the benefit of the doubt more often. These ones I’m talking about are before digital.

To be fair, it’s high speeds 100+ when they start to read noticeably faster. It’s honestly probably not a bad thing.

 

You have several BMW owners who have commented here…reading about something and having it be true are different things.

 

It would, but only by a small amount.....probably less than 1 MPH. Also, the smaller the tire, the more that worn tread would affect it. It would affect it more on an older 15 or 16" tire than on a larger 20 or 22" one.

In fact, that is why Subaru, almost all of whose American-market vehicles are full-time AWD, used to tell owners to replace their tires in sets of 4 at once, not piecemeal or one or two at a time....to keep all four tires the same diameter and rotation-speed, which lessens heat and wear on the center-differentials. I know...I owned an Outback for six years.

 

Seeing that speedometers don’t display speed lower than 1 MPH, it’s Not enough to alter the speedometer.

All cars with AWD systems tell you to only replace tires in sets.

 

All due respect to people here but do they have the (I’m sure insanely expensive) equipment like the big car mags do? I’m surprised nobody else has heard of this, regardless if they believe it or not.

I don’t personally believe apps on phones are as accurate as their stuff, those were mentioned. I’ve used them before; they’re okay.

 

Somehow I was able to tell that my Lexuses were all about 2 MPH slow. You pass radar things on the road, look at Waze, etc. GPS for speed is actually extremely accurate.

Bear in mind too that speedometers are off by a %, the faster you go the more MPH they will be off. So if someone is seeing a 3MPH gap at highway speeds, at 100+ it would be more.

 

I find that the radars on the roads that tell you to slow down are pretty accurate, but that’s city driving in my personal cars.

 

Draggy, costs around $100 and gets you through metrics you want for any speed/acceleration testing

 

My experience is the exact same, 3 mph faster on the speedometer than how fast I’m going based on GPS units and these units are pretty accurate these days. The only reason I tested my BMW is because when taking a trip once and setting the cruise control I was getting passed by other cars and I was going relatively fast and sure enough I was right.

I have a Draggy and it works really well and pretty darn accurate. I haven’t used it in a while though.

 

How do you know your speedo is off?

 

That's pretty common. Every car I've owned, and I mean every, has read 1-2 MPH higher at 60 MPH than what those speed signs say. I think this is done intentionally for safety.

 

My Chryslers are dead on those signs.

 

How I know my speedo is off by 2 MPH.
Well I might as well tell the whole story just for fun. lol

My dad is in his 70's and likes to wear baggy cargo shorts with big low pockets.
He puts his I-Phone in the big pocket so he can reach it easy but the pocket has no flap.

I warned him about going to the store like that and reached down and pulled his phone to show him how easy it is to get your phone stolen.

Well...
Last year he calls me and says someone stole his I-Phone out of his pocket while he was standing in line to get checked out at Walmart.
Surprised anyone??? haha

He said he called the police and they tracked the phone until it lost signal and was turned off somewhere in a bad neighbourhood across town.
So, since he had insurance he called and made a claim and the wait was 2 days to get one shipped expedited.

Very next day he calls me and says he needs a ride to a police station about 100 miles north.
Turns out whoever stole it- their father turned it in to a church and the church turned it into the police dept.

On the way back of approx 100 mile trip, I started my stopwatch on my Timex watch(don't laugh) and counted the mile markers back with cruise set to exactly 60 mph.
Did some math and came up with 2 MPH "over error" for the trip.

Timex watch with Indiglo technology.
Plz try to contain your excitement! haha