Does Tyre width reduction "badly" effect Speedo accuracy ?

[TILTNTURN]

Ok Front tyres on ,my E Class Coupe were 235/40/18. I had 2 new tyres , 225/40/18 .
Dropped car off at my local garage to have them fitted and he phoned me to say they were the wrong tyres and would give a pretty inaccurate speedo reading and therefore wasnt happy fitting them .
I asked him to go ahead and Id take responsibility if the boys in blue stopped me.

I appreciate the tyre ratio will be altered but being that speedos are already out by upto 10% then surely its not going to make that much diff >>>>> Or Is It?
Thanks in advance .
Tilt .

 

[M80]

I compare to GPS speed,
but this would give an idea of speedo difference.

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.

www.willtheyfit.com

1.26% speedo difference. Hardly worth mentioning really.
Insurance 'may' have concern in the event. But if your manual details that size I wouldn't think they could say anything.

 

[rorywquin]

Diameter should be the only thing that affects the accurcy of the speedo.

 

[M80]

Yep, but the profile is a % of the diameter of the wheel.

I had to think about that.

And still got that wrong.
18" is the wheel diameter,
the profile here is 10mm less, so rolling diameter is less, by 1.26% apparently.

Have I got that right yet?

 

[JBell]

Should keep to the same size tyres, not sure why you ordered the wrong size

 

[AnthonyUK]

It's only 10mm (<1/2") but I wouldn't want to argue with my insurance company about it if I was involved in an accident.

 

[Blobcat]

Yep, but the profile is a % of the diameter of the wheel.

I had to think about that.

And still got that wrong.
18" is the wheel diameter,
the profile here is 10mm less, so rolling diameter is less, by 1.26% apparently.

Have I got that right yet?

Almost...

The 235 and 225 are tyre widths and the 40 is the aspect ratio or percentage of the width as a height

So 40% of 235 is 94mm and 40% of 225 is 90mm

It will under read on the speedo by ~1mph on the new sizes

Speedos are allowed to be out by 10% but only to over read - they're not allowed to under read - Mercedes are generally 2-3mph over so you'll be closer to accurate on the new sizes

Less comfortable though as less sidewall to absorb the road imperfections

 

[TILTNTURN]

Should keep to the same size tyres, not sure why you ordered the wrong size

Excuse me J BELL I didnt order the Wrong Size . I had 2 New tyres already sitting there and decided to have them fitted . Ask the question youll get the correct answer sir .

 

[TILTNTURN]

Thanks to all Barring Mr Bell .
Much appreciated .

 

[Naraic]

It may or may not be too detrimental for the speedo...but, BUT...you may find your crusie control knocks off at higher speeds because you have mismatched diameters at each end of the car.

 

[Naraic]

Excuse me J BELL I didnt order the Wrong Size . I had 2 New tyres already sitting there and decided to have them fitted . Ask the question youll get the correct answer sir .

There's no need for that. What he said was reasonable.

 

[Grumblepuss]

also be aware that the difference in size will get worse with wear so it may only be 8mm to start with but then add full tyre wear onto that and you could have a another 12mm of difference so around 2CM height difference from a stock new tyres (based on most new tyres having over 7MM of tread and legal minimum being 1.6MM)

 

[Craiglxviii]

Thanks to all Barring Mr Bell .
Much appreciated .

Naughty @JBell ! Bad @JBell !

 

[LostKiwi]

Almost...

The 235 and 225 are tyre widths and the 40 is the aspect ratio or percentage of the width as a height

So 40% of 235 is 94mm and 40% of 225 is 90mm

It will under read on the speedo by ~1mph on the new sizes

Speedos are allowed to be out by 10% but only to over read - they're not allowed to under read - Mercedes are generally 2-3mph over so you'll be closer to accurate on the new sizes

Less comfortable though as less sidewall to absorb the road imperfections

Um.....
He'll be less accurate as the wheel will do more revolutions for a given distance so if it read high previously (which by law it must) it will be worse now.

 

[brandwooddixon]

Isn't the speedometer reading taken from the gearbox drive to the driven wheels? So if only the front tyres are being changed then the speedometer will still read correctly.
The only issue may be, as stated in another post, that the cruise control may drop out or a fault flagged if the ABS sensors detect a large discrepancy in speed of rotation between the front and rear wheels.

 

[Gazwould]

Almost...

The 235 and 225 are tyre widths and the 40 is the aspect ratio or percentage of the width as a height

So 40% of 235 is 94mm and 40% of 225 is 90mm

It will under read on the speedo by ~1mph on the new sizes

Speedos are allowed to be out by 10% but only to over read - they're not allowed to under read - Mercedes are generally 2-3mph over so you'll be closer to accurate on the new sizes

Less comfortable though as less sidewall to absorb the road imperfections

But real world this doesn't happen , all theory.... , wheel weight is the ride quality definer .

GPS is King .

 

[andys320]

Isn't the speedometer reading taken from the gearbox drive to the driven wheels? So if only the front tyres are being changed then the speedometer will still read correctly.
The only issue may be, as stated in another post, that the cruise control may drop out or a fault flagged if the ABS sensors detect a large discrepancy in speed of rotation between the front and rear wheels.

Have to say this would be my understanding, happy to be corrected.

 

[EmilysDad]

Isn't the speedometer reading taken from the gearbox drive to the driven wheels? So if only the front tyres are being changed then the speedometer will still read correctly.
The only issue may be, as stated in another post, that the cruise control may drop out or a fault flagged if the ABS sensors detect a large discrepancy in speed of rotation between the front and rear wheels.

I've no idea what my Merc or the Merc in question uses to determine speedo reading, but the Vauxhall Omega I used to have used the ABS sensor on one of the front wheels ....

 

[Flyinspanner]

Diameter should be the only thing that affects the accurcy of the speedo.

The aspect ratio is and width are the variables, the wheel diameter is a constant, but the other two affect the circumference, and thus the figure displayed on your Speedo for any given rotation…..

 

[mioba]

Thanks to all Barring Mr Bell .
Much appreciated .

Easy tiger.

Aside, why fit the wrong tyre.