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[Baylin]

I went into a local BMW specialty repair shop to get their advice on swapping the stock RFTs on my 135 with non-runflat conventional tires. My question was whether they could offer any advice on replacing the 215/240 runflat Bridgestones with Michelin Pilot Super Sport 225/255s. I was expecting to get an answer along the lines of possibly switching out some suspension parts, like stiffer springs. The answer I got surprised me. The shop manager told me flat out the it was a bad idea because the larger tire would cause the speedo to be way off. I've read all the threads on this forum on switching from runflats, so I knew there was some issue with speedo error, but was surprised that this would be perceived as such a major downside. Anyone have an idea how much error the larger tires would cause on speedometer reading ? My guess is a couple percent, so at 60 mph the reading would be off by a couple miles which to me is no big deal. Anyone know how to calculate the error exactly ?

Thanks

[BrokenVert]

Youre shop manager knows nothing about BMWs and German cars in general.

All BMWs leave the factory floor with an educed 3-5% low error in the speedometer. This is because it is german law that cars speedometers cannot read high under penalty of intense litigation.

So in reality switching to 255s would lessen the already present speedo error.


With 255s in the back your speedo will still read high, its usually 1-2%.

Just put on the PSS and have a good time. And dont listen to your shop manager if he says theyre too large for the stock wheels. Because as weve already established, he doesnt know a lot.

225/40 R18
255/35 R18 Those are the sizes you want.

[Dackelone]

I have 225's/255's PS3's. They larger tire size will cause the speedo to read slightly slower. How much slower? 1kph acording to my GPS! The factory speedo reads high by about 3% or 4%. I really do like the way the cars new stance with the larger tires.

At a real 160 kph my speedo reads 164 while my GPS reads 160 kph. My M3 digital speedo (coded in teh kombi) reads 159 kph now with the larger tires. before it read DEAD correct!).

I would not let your tire SA put you off of thise tire upgrade. In the end I just listened to all the tire guru's over here and went with my gut. I was very glad I did! I love my PS3's and the larger tire sizes AND not having RFT anymore!

[Baylin]

Guys - Thanks for your great advice ! Those run-flat Bridgestone bricks
will soon be history

[RPM90]

Originally Posted by Baylin

I went into a local BMW specialty repair shop to get their advice on swapping the stock RFTs on my 135 with non-runflat conventional tires. My question was whether they could offer any advice on replacing the 215/240 runflat Bridgestones with Michelin Pilot Super Sport 225/255s. I was expecting to get an answer along the lines of possibly switching out some suspension parts, like stiffer springs. The answer I got surprised me. The shop manager told me flat out the it was a bad idea because the larger tire would cause the speedo to be way off. I've read all the threads on this forum on switching from runflats, so I knew there was some issue with speedo error, but was surprised that this would be perceived as such a major downside. Anyone have an idea how much error the larger tires would cause on speedometer reading ? My guess is a couple percent, so at 60 mph the reading would be off by a couple miles which to me is no big deal. Anyone know how to calculate the error exactly ?

Thanks

Just goes to show you how much some service "advisors" and shop managers know about cars and car tech in general.
The answer he gave you about that size tire and the speedo is just plain WRONG.

The important to know about changing tire size, in terms of correct speedo info, is to keep the overall diameter of the tire the same or as close to the OEM spec.
A larger overall diameter will make the speed read low/slow, and a smaller overall diameter will make the speedo read high/fast.

When you change width you need to look at the tires aspect ratio, which is the middle number is the sequence example: 225/40/18.
The "40" means that the sidewall height is 40% of the width (255).

To find the proper sized tire, look up it's specs, and look for it's overall diameter, and get it as close to or exactly what the OEM tire is.
If the OEM tire's overall diameter is 25", then a tire with a 24.8" or 25.2" diameter will work just fine, and the speedo will give you about the same reading.

Most speedo's tend to read on the high side to begin with.
As the tire wears, and thus becomes smaller in diameter, the reading will read higher still, because a smaller diameter will rotate more revolutions per mile than a larger diameter.

Also, tire inflation pressure will have an effect on the overall diameter.
If the tires loose pressure the diameter will decrease and the speedo will read higher. If the tire pressure is raised the overall diameter can increase, and thus the speedo will read lower.

Another consideration is cargo weight. If it's just the driver, the tires squish to only a certain level. If you add 3 people and go with a full load, the overall diameter of the tire will decrease, and the speedo will read higher than if it's just the driver.

These are all small incremental things that can change a tires overall diameter, and they don't really make that much of a difference, unless a number of these factors comes into play.
If your tires are over half worn, and they are 5psi light, and your car is full of your friends, the overall tire diameter can squish down quite a bit.
With all those things together I would suspect that your speedo will read higher, and it already reads higher to begin with.

Speaking of high reading speedo's, it just occurred to me a potential reason why some speedo's, like in BMW's, tend to read high, and higher as speed increase; tire pressure.
BMW, and most car makers, recommend a higher tire pressure for sustained higher speed driving. This type of sustained very high speed driving occurs daily on the autobahn.
So, if a driver increases their tire pressure the overall diameter of the tire becomes larger. Add to that the increase in diameter as the tire is spinning very fast, and it expands, the speedo may at that point begin to read a more correct speed. Hmmmm.....

Possible I guess. But, some autobahn drivers have said that their speedo reads high still, even at autobahn speeds.
I wonder though if those drivers increased their tire pressures as is recommended for that type of sustained high speed running?

[RPM90]

Originally Posted by BrokenVert

Youre shop manager knows nothing about BMWs and German cars in general.

All BMWs leave the factory floor with an educed 3-5% low error in the speedometer. This is because it is german law that cars speedometers cannot read high under penalty of intense litigation.

So in reality switching to 255s would lessen the already present speedo error.


With 255s in the back your speedo will still read high, its usually 1-2%.

Just put on the PSS and have a good time. And dont listen to your shop manager if he says theyre too large for the stock wheels. Because as weve already established, he doesnt know a lot.

225/40 R18
255/35 R18 Those are the sizes you want.

Actually most speedo's tend to read on the higher side.
You would want to err on the high side as reading too low could get the driver in trouble.

With a larger diameter your speed will read lower, not higher.
A larger diameter will have less revolutions per mile.

Tire width isn't the determining factor for speedo reading.
it's the overall diameter of the wheel/tire.
You can go with wider tires and as long as the sidewall ratio results in an overall equal diameter, or close to it, the overall speed reading will remain the same.

[BrokenVert]

Originally Posted by RPM90

Actually most speedo's tend to read on the higher side.
You would want to err on the high side as reading too low could get the driver in trouble.

With a larger diameter your speed will read lower, not higher.
A larger diameter will have less revolutions per mile.

Tire width isn't the determining factor for speedo reading.
it's the overall diameter of the wheel/tire.
You can go with wider tires and as long as the sidewall ratio results in an overall equal diameter, or close to it, the overall speed reading will remain the same.

Whoops. I flipped em without realizing it. And for all intents and purposes, when we talk about one size wider tires it inevitably leads back to larger diameter.

[RPM90]

Originally Posted by BrokenVert

Whoops. I flipped em without realizing it. And for all intents and purposes, when we talk about one size wider tires it inevitably leads back to larger diameter.

No prob. I've done it.

I don't get the "all intents....." comment.

Do you mean that many people tend to confuse and/or not understand the difference?

[BrokenVert]

Originally Posted by RPM90

No prob. I've done it.

I don't get the "all intents....." comment.

Do you mean that many people tend to confuse and/or not understand the difference?

No. I mean that if you go up one size in width you will go up in overall diameter. when going from a 215/245 to a 225/255


So in this context they are one in the same.

[VMRWheels]

225/40 and a 255/35 is a common upgrade for 1-Series owners. You'll be perfectly fine.

-Charles@VMRWheels

[tracer bullet]

Agreed just do some basic math.

A 225 / 40 / 18 tire is 225mm wide, has a height off the wheel of 40% of that, and the wheel itself is 18". So do a little calculating and unit conversion to go (40% of 225mm ) + 18" + (40% of 225mm ) = overall tire diameter (don't forget unit conversion). Take diameter and turn it into circumference. Then compare stock to what you are considering aftermarket and find how far off they are from each other.

Basic math. Not scary.

[BrokenVert]

^ill do it for him. Stock overall D-outer is 24.7" D-outer' for 225 is 25.2"

So a difference of +.5" or about +1.25 cm in overall diameter.

[JimD]

My 128i speedometer reads about 3 mph faster than I am really going at 60 mph. Or about 5% high. If it had rear tires with 0.5 inch more diameter - or about 2% more diameter - the speedometer would still read about 3% faster than I was really going.

From a speedometer error standpoint, the bigger tires actually help. They cut the error roughly in half.

Jim

[GaryS]

I have 265/35 in back and the speedo still reads a little high.

[JimD]

It should read high. A 265/35 is only 25.3 inches in diameter or about 2.5% taller. The speedometer should still read about 2.5% faster than you are going.

Jim