# re-calibrating speedo for tires?



## kghia (Nov 16, 2006)

When I had my mechanical speedometers restored and recalibrated on my Beetle and Karmann Ghia, knowing the Revolutions Per Mile for a given tire (also may be calculated from the exact rolling diameter)
It ran around $120-150 each, including new chrome rings and replacement of worn gears.
What is involved for the Eos in calibration of a speedo to the exact size of a different tire?
When you get different wheels from the beginning (i.e. a package), is it set up *for those tires*? Or is VW just distributing various levels of incorrect speedometers/odometers?
I have read the thread about higher mileage readings being mandated by law, but those threads also said that the odometer would be pretty much spot-on. Is this true with 17" and 18" wheels & tire combinations when originally ordered that way?
I also experience the almost-unavoidable tire spin when merging onto a road from stop, and if I changed my tires I would probably consider some 17" wheels too. I am just trying to determine what is would run to calibrate the system, and how much hassle it would be.
William


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## itsmejerry (Jan 31, 2003)

*Re: re-calibrating speedo for tires? (kghia)*

The factory 17 and 18" tire/wheel packages have a low profile tire. The 16 inch tire/wheel is a more "normal" sidewall. 
The thing that affects the speedometer is the overall diameter of the tire. With a low profile tire, and larger wheel, the overall tire diameter is pretty much the same. 
I've stood a factory 18 inch tire next to a 16 inch and they're within a hair's width of each other. The difference is the sidewall. The 18 has half the sidewall height. 
Check Tirerack.com for upsizing scales. You'll see that unless you make major changes recalibration of the speedometer isn't necessary.


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## minnvw (Oct 16, 2006)

*Re: re-calibrating speedo for tires? (itsmejerry)*

hi jerry, the over all height might be same , but if you line up the center holes of the wheel to match the other wheel, how do the tires compare in height then?










_Modified by minnvw at 5:47 PM 6-24-2007_


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## kghia (Nov 16, 2006)

*Re: re-calibrating speedo for tires? (itsmejerry)*


_Quote, originally posted by *itsmejerry* »_You'll see that unless you make major changes recalibration of the speedometer isn't necessary. 


You know, now that I'm looking at it, differences from 800 rev per mile to 850 rev per mile aren't really that much, and there are plenty choices that are closer to original than that span.
I guess when you are trying to put a narrow 15" tire on a Karmann Ghia the choices are more difficult to match...
Even said, do dealerships have the ability to re-calibrate to a given rev per mile, or is the lack of need the end of the story? Just curious now, since I believe you -- it doesn't _really_ seem necessary.
William


_Modified by kghia at 8:12 PM 6-24-2007_


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## just4fun (Oct 29, 2006)

*Re: re-calibrating speedo for tires? (kghia)*

I have been trying to find info on this in the Bentley manual, but no luck so far. My odometer is off by approx. 3.5 % (as compared to a GPS) and my initial thoughts were that incorrect tire sizing information input into the computer may be the reason.
I know all the NA built trucks we run in our fleet at work have the capability of inputting tire size information into the computer to compensate for speedo and odometer calibration. 
I would be very surprised if I were to find out the Eos doesn't have the same capability. I would think it is pretty standard in all modern computerized autos.
Check with your dealer. I'm sure they could program the correct tire size for you in a matter of minutes.
Kevin


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## kluski44 (Jun 11, 2007)

My Eos is 3 mph slower than the speedo. When I turned it in for air conditioning warranty repairs, I asked if they could adjust the speedo so it is better. The dealer responded that all cars are off and they would not adjust it. They said it would change as the tires worn down or I got a new set of the same tires. 
Kinda a bummer as this is the first (out of 7 previous cars) that is not right.


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## WolfsburgerMitFries (Jul 4, 2005)

*Re: (kluski44)*

I'm really not in the mood to go to the tire rack and do the math from looking up all the gory technical specs on a tire, like diameter and revs per mile....and all those specs are available there for those with a regular job looking to burn up time on the clock but...
For those either changing rims and tires, or just simply upgrading from the stock rubber, it might be adventageous to adjust the aspect ratio up one setting and use a 235/50R17 or 235/45R18 tire. That might be the easiest way to correct a speedometer that's reading slightly fast, but again you'd have to do the math to see exactly what the difference would be. You'd also get the added benefit of putting slightly less miles on your car for the same distance traveled.







http://****************.com/smile/emthup.gif






















Oh also Kluski, if yor car is 3mph slower than the speedo (or in other words the speedo is reading fast) the problem is only going to get worse as your tire wears, gets smaller, and rotates more times per mile.



_Modified by WolfsburgerMitFries at 2:52 PM 6-25-2007_


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## jdl (Dec 7, 2006)

*Re: (WolfsburgerMitFries)*

The speedometer is set intentionally "fast" by VW, discussed in other threads. 
My car is 5 mph faster at 65 mph (so I drive 70 on the speedometer to drive 65. 
The actual speed can be found by reading the diagnostic output on the air 
conditioning LCD's when they are switched over to diagnostic mode. 
The dealer (Boardwalk VW in Redwood City, CA) told me that if I changed my tires 
to a higher profile tire (for a softer ride on my 18's), that I could bring it in the shop 
for a recalibration. So, it can be done. But can, or will they, take out the "saftey factor"
of the 5 mph????? 
Jack


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## WolfsburgerMitFries (Jul 4, 2005)

*Re: (jdl)*


_Quote, originally posted by *jdl* »_ But can, or will they, take out the "saftey factor"
of the 5 mph????? 
Jack

There's no actual safety factor, only a perceived safety factor. That's the same faulty logic of people who set the time on their alarm clock ahead "in case they sleep in". Or the same faulty logic of people who overpay taxes out of their weekly paycheck so they can "get more money back at the end of the year". The real way to get more money back at the end of the year is to have fradulent tax deductions or underreport your earnings, not pay more in taxes during the year then recollect your overpayment.




_Modified by WolfsburgerMitFries at 3:24 PM 6-25-2007_


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## jdl (Dec 7, 2006)

*Re: (WolfsburgerMitFries)*

Figures, I'm guilty of overpaying my taxes every year and excited to get a check back 
from the feds.








But seriously, I was told that it was a safety factor, just can't confirm this in documentation.
So the person who told me might also be wrong. But in this day and age, and in this car, 
why do they report two different speeds (speedometer and diagnostics). Other than 
changing tire size they should be able to be spot on. Especially with a GPS Nav system. 
Jack


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## chrisj428 (Feb 26, 2005)

*Re: (jdl)*

Here's the deal on the speedometer error.
The law in Europe mandates the speedometer may _never_ read slower than the actual speed the car is travelling. This is not so much for personal safety as, I'm sure, for the integrity of your bank account.
So, what VW and others have done is this: the speedometer is accurate when _the largest diameter tires possible_ are fitted to the vehicle. This way, no matter what size tires are fitted, the speedometer will never read "under".
As far as "adapting" the speedometer -- this is not something I'm aware of with the HEX+CAN vehicles. That doesn't mean it can't be done -- it just means I'm not aware of it being possible.


_Modified by chrisj428 at 1:42 PM 6-25-2007_


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## just4fun (Oct 29, 2006)

*Re: (chrisj428)*

Just to throw a little fuel on the fire.
I'm not worried about the speedo being out a bit. I have confirmed my speedo reading against actual speed from a GPS (which is _supposed_ to be accurate, by all accounts)
So I know that I can simply drive the speedo 3.0-3.5% higher and I'm still ok.
What I find interesting is that my odometer is also reading low by approx. 3.5%. The general concensus here on the forum is that the odometer should be accurate, even if the speedo is out.
Also interesting was a post by one of our Ontario members (Grinder I think? but it may have been Hotmoose out of Quebec, I don't recall for certain) that stated his odometer was bang on until he took his Eos in for a routine service, and now it is reading approx. 3.0-3.5% low as well.
This makes me wonder if this is in response to the successful class action suit against Honda for odometers that were reading high (effectively reducing the warranty mileage). Better to give up 3% on the odometer than find yourself in a class action law suit.
If anyone has a GPS, and has had their Eos in for service recently, double check the accuracy of your odometer to see if this is becoming common practice.
Kevin










_Modified by just4fun at 12:59 PM 6-25-2007_


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## neweosowner (Nov 25, 2006)

*Re: (just4fun)*

My odometer was fine; now reads about 3% (3.5%?) off. It's been in the shop a few times.


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## PanEuropean (Nov 3, 2001)

*Re: re-calibrating speedo for tires? (kghia)*


_Quote, originally posted by *kghia* »_What is involved for the Eos in calibration of a speedo to the exact size of a different tire?

There is an adaptation value within the instrument cluster that can be changed (using a diagnostic scan tool) to adapt the speedometer for significantly different tire sizes.
In practice, this value does not need to be changed unless the user is at one extreme or the other (e.g. 14" or 22") respecting wheel sizes. The difference in *total diameter* between 16, 17 and 18 inch wheels that are fitted with the proper specification of tires is negligible, because the tire usually becomes thinner as the wheel size increases, thus keeping the total diameter pretty much the same.
Legislation in both America and Europe requires that the car manufactures set the speedometer *needle *up so that it never, ever indicates less than the actual speed of the vehicle, regardless of what size of approved tires are fitted to it. Because of this, most automobile speedometers read about 7% higher than actual vehicle speed. This difference will be slightly less if you have larger wheels fitted - perhaps 5% rather than 7%.
Note that I emphasized the word *needle *in the above paragraph. The vehicle speed sensing system knows exactly what speed the car is traveling at, usually to an accuracy that is within half a percent of actual. Outputs from the speed sensing system are used for various purposes - for the ABS brake system, the ESP stability system, the cruise control (which is part of the engine controller), and for the electronic top speed limitation of 210 km/h (about 130 MPH) that is imposed on every VW brought to North America. All of these different systems know what the true speed of the vehicle (accurate to within the half of one percent) is. Only the needle of the speedometer is intentionally designed to read high. The needle cannot be adjusted - in other words, there is nothing you can do to get rid of the overstatement. If you changed the adaptation values in the instrument cluster, you might be able to get the needle to read accurately, but all the other outputs would then be inaccurately low.
The subject has been extensively discussed (translation = _beaten to death)_ in the Phaeton forum - have a look at the table of contents for that forum if you would like additional information.
Michael


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## PanEuropean (Nov 3, 2001)

*Re: re-calibrating speedo for tires? (PanEuropean)*

Note also that there was an error made in the adaptation value set in some of the very first Eos vehicles that were brought to North America. VW became aware of this error and issued a Technical Bullitan to the dealers asking them to make a software tweak to correct the error next time these vehicles came in for service.
Because it is very likely that all of the affected vehicles have since been sold and had their first oil change (or, were corrected before they were even sold, if they had not been sold at the time the TB was issued), this is nothing more than a historical footnote. You can read more about it here if you are curious: TB: RVU - Instrument Cluster, Update Speedometer Coding (OR) - Includes TB 00-06-10.
Michael


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## Higgs Boson (Feb 4, 2007)

*Re: (neweosowner)*

I took my Eos in for 5k service at the end of April then went on a road trip from Chicago through the southwest. As I went the odometer and my GPS began to drift apart. (I hadn't really tracked milage closely before, so I can't say if this was new.)
I performed the following exercise two times:
1. pull over at a mile marker.
2. reset GPS and trip odometer.
3. drive 100 mile markers, pull over.
4. compare the three. (odo, GPS, road)
Both times the road and GPS agreed within 0.02 miles and both times my odometer read 96.3 miles.
By the way, the Eos was a fantastic way to tour New Mexico, Arizona, Utah and Colorado!






















Roy
I should add that the GPS is not stock, it is a garmin GPSMap 60CSx


_Modified by Higgs Boson at 10:09 PM 6-25-2007_


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## BigFoot-74205 (Jan 26, 2006)

I have a question about wheel sizes and ESP. Does the wheel & tire size have any influence on the way ESP works? For instance someone suggested using 235/50 R17 tires instead of 245/45 R17. Would that make a difference?
And no, I don't actually plan to do that on my Eos, I'm just curious.








Damir


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## BigFoot-74205 (Jan 26, 2006)

So, can anyone help with my query?
Damir


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## de7158 (Feb 8, 2006)

*Re: (BigFoot-74205)*

This should help with your query
http://www.miata.net/garage/tirecalc.html
Cheers 
Peter


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## BigFoot-74205 (Jan 26, 2006)

Peter,
I know that site already and I find quite useful. It doesn't really explain whether or not tire & wheel size influence ESP though.
Damir


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## WolfsburgerMitFries (Jul 4, 2005)

*Re: (BigFoot-74205)*


_Quote, originally posted by *BigFoot-74205* »_Peter,
I know that site already and I find quite useful. It doesn't really explain whether or not tire & wheel size influence ESP though.
Damir

ESP is looking for differences in tire rotational speed in relation to each other, that's how it detects loss of traction. Another component of ESP is the yaw rate sensor which is unaffected by wheel and tire size. So long as all your tires are the same size, all will be well with your ESP. Your original question stems from my idea of going one tire side profile taller which is a very minor change. Look at the people who won't be happy until they've got 19 or 20 inch rims on their car. That's a huge difference compared to my idea, but neither idea is going to conflict with the ESP so long as all the tires are the same size.



_Modified by WolfsburgerMitFries at 9:37 AM 7-2-2007_


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## BigFoot-74205 (Jan 26, 2006)

Jeremiah,
Thanks for the reply. That makes quite a bit of sense actually.








Damir


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## WolfsburgerMitFries (Jul 4, 2005)

*Re: (BigFoot-74205)*

Alright so i did the math. using Pirelli P6 tires on a 17" wheel as an example...
A 235/45R17 rotates 822 times per mile
A 235/50R17 rotates 791 times per mile
You end up with 3.77% reduction in rotation as new, and would drop the speed indication (of course) 3.77%. At 60mph, the indicated speed would drop 2.26MPH to 57.74MPH. Sounds like an interesting option to me, especially since the added sidewall height would make the tire slightly more compliant, slightly reducing the probability of rim and tire damage if you hit a large obstruction in the road. Ride comfort may also be a touch better for those interested in that, however I think the Eos rides quite well as it is.
235/50R17 has significantly fewer choices (in the all season category), and there is some Porsche specific tires listed in the summer performance category. Interesting.
There's a really interesting all-season Yokohama tire available (called the Advan S4) in 235/50R17 that is "AA" rated for traction, has a 400 treadwear ratiing, and is one of the least expensive tires in its category and size at $186. Its also rated #1 in the ultra high performance all season category. I'd go with this in a second.
http://www.tirerack.com/tires/...ce=17












_Modified by WolfsburgerMitFries at 9:48 AM 7-4-2007_


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## WolfsburgerMitFries (Jul 4, 2005)

*Re: (WolfsburgerMitFries)*

I also looked at the Tire Rack recommended installer list for my area...
http://www.tirerack.com/instal...40502
and found that most places calculate tire mounting fees based on sidewall height. Most places appear to charge 5-6 dollars more for 40-45 series tires than 50-55 series tires. The more I look at using 235/50R17 size tires on an Eos, the better of an idea it becomes.


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