rkj

I have conflicting reports: If I change my rims/tires on my A4 from 16" to
18" does it affect the speedo/odometer readings? My tire guy says the
dealer needs to make an adjustment, but the dealer says it will have no
effect.

Seems logical that a larger diameter wheel/tire would make the apparent
mileage lower (fewer rotations to travel a given distance), but I can't find
an authoritative source. Any help?

daytripper

On Sat, 20 Sep 2003 10:58:51 -0700, "rkj" <rkj@***.net> wrote:

>I have conflicting reports: If I change my rims/tires on my A4 from 16" to
>18" does it affect the speedo/odometer readings? My tire guy says the
>dealer needs to make an adjustment, but the dealer says it will have no
>effect.
>
>Seems logical that a larger diameter wheel/tire would make the apparent
>mileage lower (fewer rotations to travel a given distance), but I can't find
>an authoritative source. Any help?

what is important is the outside diameter of the tire. Choose a lower-profile
tire when you go to a larger diameter wheel...

Pete

"rkj" <rkj@***.net> wrote in message newsz0bb.678$vj2.528@fed1read06...
> I have conflicting reports: If I change my rims/tires on my A4 from 16"
to
> 18" does it affect the speedo/odometer readings? My tire guy says the
> dealer needs to make an adjustment, but the dealer says it will have no
> effect.
>
> Seems logical that a larger diameter wheel/tire would make the apparent
> mileage lower (fewer rotations to travel a given distance), but I can't
find
> an authoritative source. Any help?

When getting a larger rim, you need to get a lower profile tire, so that the
overall rim/tire combo diameter remains the same. For example, if what you
have right now is 205/55/16, then when you get your 18" rims, you'll get
225/40/18 tires in order to stay as close as possible to the OEM diameter.
This way, the difference in diameter will be so small, that you will not
affect your speedo/odo in any significant way.

You can play around wih this calculator to see what I mean:

http://www.miata.net/garage/tirecalc.html

Cheers,

Pete

Yoda

"daytripper" <day_trippr@REMOVEyahoo.com> wrote in message
news:ro5pmvoinc4moevs5np0voo4c418l2kvp1@4ax.com...
> On Sat, 20 Sep 2003 10:58:51 -0700, "rkj" <rkj@***.net> wrote:
>
> >I have conflicting reports: If I change my rims/tires on my A4 from 16"
to
> >18" does it affect the speedo/odometer readings? My tire guy says the
> >dealer needs to make an adjustment, but the dealer says it will have no
> >effect.
> >
> >Seems logical that a larger diameter wheel/tire would make the apparent
> >mileage lower (fewer rotations to travel a given distance), but I can't
find
> >an authoritative source. Any help?
>
> what is important is the outside diameter of the tire. Choose a
lower-profile
> tire when you go to a larger diameter wheel...

I agree I have changed my wheels on a Audi A4 from 15" to 17" RS4 copies
with no problems.
If you go for 18" expect to loose some comfort as the profile of the tyre
needs to be very low

Yoda

Mike Smith

daytripper wrote:

> On Sat, 20 Sep 2003 10:58:51 -0700, "rkj" <rkj@***.net> wrote:
>
>
>>I have conflicting reports: If I change my rims/tires on my A4 from 16" to
>>18" does it affect the speedo/odometer readings? My tire guy says the
>>dealer needs to make an adjustment, but the dealer says it will have no
>>effect.
>>
>>Seems logical that a larger diameter wheel/tire would make the apparent
>>mileage lower (fewer rotations to travel a given distance), but I can't find
>>an authoritative source. Any help?
>
>
> what is important is the outside diameter of the tire. Choose a lower-profile
> tire when you go to a larger diameter wheel...

Remember also that, if your total wheel/tire diameter changes, not only
will your speedo be off, but you will effectively be changing the
overall gearing. If your overall wheel/tire diameter increases, you
will probably have worse acceleration (with the stock tranny, anyway).

--
Mike Smith

Doug Ramage

"Mike Smith" <mike_UNDERSCORE_smith@acm.DOT.org> wrote in message
news:vmq43c9spvcl38@news.supernews.com...
> daytripper wrote:
>
> > On Sat, 20 Sep 2003 10:58:51 -0700, "rkj" <rkj@***.net> wrote:
> >
> >
> >>I have conflicting reports: If I change my rims/tires on my A4 from 16"
to
> >>18" does it affect the speedo/odometer readings? My tire guy says the
> >>dealer needs to make an adjustment, but the dealer says it will have no
> >>effect.
> >>
> >>Seems logical that a larger diameter wheel/tire would make the apparent
> >>mileage lower (fewer rotations to travel a given distance), but I can't
find
> >>an authoritative source. Any help?
> >
> >
> > what is important is the outside diameter of the tire. Choose a
lower-profile
> > tire when you go to a larger diameter wheel...
>
> Remember also that, if your total wheel/tire diameter changes, not only
> will your speedo be off, but you will effectively be changing the
> overall gearing. If your overall wheel/tire diameter increases, you
> will probably have worse acceleration (with the stock tranny, anyway).
>
> --
> Mike Smith
>

In theory, if the overall rolling circumference increases, acceleration will
suffer, but top speed increase. Unlikely to be noticeable. More significant
is the decrease in ride comfort and less resistance to aquaplaning with the
very lower profiles.
--
Doug Ramage

daytripper

On Sun, 21 Sep 2003 09:36:10 +0100, "Doug Ramage" <doug-ramage@lineone.net>
wrote:

>
>"Mike Smith" <mike_UNDERSCORE_smith@acm.DOT.org> wrote in message
>news:vmq43c9spvcl38@news.supernews.com...
>> daytripper wrote:
>>
>> > On Sat, 20 Sep 2003 10:58:51 -0700, "rkj" <rkj@***.net> wrote:
>> >
>> >
>> >>I have conflicting reports: If I change my rims/tires on my A4 from 16"
>to
>> >>18" does it affect the speedo/odometer readings? My tire guy says the
>> >>dealer needs to make an adjustment, but the dealer says it will have no
>> >>effect.
>> >>
>> >>Seems logical that a larger diameter wheel/tire would make the apparent
>> >>mileage lower (fewer rotations to travel a given distance), but I can't
>find
>> >>an authoritative source. Any help?
>> >
>> >
>> > what is important is the outside diameter of the tire. Choose a
>lower-profile
>> > tire when you go to a larger diameter wheel...
>>
>> Remember also that, if your total wheel/tire diameter changes, not only
>> will your speedo be off, but you will effectively be changing the
>> overall gearing. If your overall wheel/tire diameter increases, you
>> will probably have worse acceleration (with the stock tranny, anyway).
>>
>> --
>> Mike Smith
>>
>
>In theory, if the overall rolling circumference increases, acceleration will
>suffer, but top speed increase. Unlikely to be noticeable. More significant
>is the decrease in ride comfort and less resistance to aquaplaning with the
>very lower profiles.

Eh? Ride comfort, sure, but I'd argue that less resistance to aquaplaning is
not truly attributable to aspect ratio.

On a given vehicle (so the performance differences are strictly due to tires)
resistance to aquaplaning is primarily a function of tread *width* and
pattern.

That width is a factor in calculating aspect ratio does not somehow convey
cause to aspect ratio...

/daytripper
'00 s4 6spd

C.R. Krieger

daytripper <day_trippr@REMOVEyahoo.com> wrote in message news:<vm0smvke2hr6v3gce3ku711htkks7gmvfa@4ax.com>. ..
> On Sun, 21 Sep 2003 09:36:10 +0100, "Doug Ramage" <doug-ramage@lineone.net>
> wrote:
> >
> >In theory, if the overall rolling circumference increases, acceleration will
> >suffer, but top speed increase. Unlikely to be noticeable. More significant
> >is the decrease in ride comfort and less resistance to aquaplaning with the
> >very lower profiles.
>
> Eh? Ride comfort, sure, but I'd argue that less resistance to aquaplaning is
> not truly attributable to aspect ratio.
>
> On a given vehicle (so the performance differences are strictly due to tires)
> resistance to aquaplaning is primarily a function of tread *width* and
> pattern.
>
> That width is a factor in calculating aspect ratio does not somehow convey
> cause to aspect ratio...

Well, hell's fire, boys, if we're goin' to go gettin' all
*theoretical* here, let's go all the way! The other thing that the
wide aspect tire (usually, unless you're getting the same width as the
original tires)

daytripper

On 22 Sep 2003 09:14:31 -0700, warp2_shadow@yahoo.com (C.R. Krieger) wrote:

>daytripper <day_trippr@REMOVEyahoo.com> wrote in message news:<vm0smvke2hr6v3gce3ku711htkks7gmvfa@4ax.com>. ..
>> On Sun, 21 Sep 2003 09:36:10 +0100, "Doug Ramage" <doug-ramage@lineone.net>
>> wrote:
>> >
>> >In theory, if the overall rolling circumference increases, acceleration will
>> >suffer, but top speed increase. Unlikely to be noticeable. More significant
>> >is the decrease in ride comfort and less resistance to aquaplaning with the
>> >very lower profiles.
>>
>> Eh? Ride comfort, sure, but I'd argue that less resistance to aquaplaning is
>> not truly attributable to aspect ratio.
>>
>> On a given vehicle (so the performance differences are strictly due to tires)
>> resistance to aquaplaning is primarily a function of tread *width* and
>> pattern.
>>
>> That width is a factor in calculating aspect ratio does not somehow convey
>> cause to aspect ratio...
>
>Well, hell's fire, boys, if we're goin' to go gettin' all
>*theoretical* here, let's go all the way! The other thing that the
>wide aspect tire (usually, unless you're getting the same width as the
>original tires)

"Postus Interruptus" notwithstanding, I believe you reiterated my point:
the size of the contact patch, not the aspect ratio, is what's in play wrt
hydroplaning...

/daytripper
'00 s4 6spd w/low aspect S03's, quite good in the wet...

C.R. Krieger

daytripper <day_trippr@REMOVEyahoo.com> wrote in message news:<1tsumvsstifeq411mbmgn4nl1340gbf7es@4ax.com>. ..
> On 22 Sep 2003 09:14:31 -0700, warp2_shadow@yahoo.com (C.R. Krieger) wrote:
> >
> >Well, hell's fire, boys, if we're goin' to go gettin' all
> >*theoretical* here, let's go all the way! The other thing that the
> >wide aspect tire (usually, unless you're getting the same width as the
> >original tires)
>
> "Postus Interruptus" notwithstanding,

That's *exactly* what it was. As soon as I realized I was repeating
what you'd said, I hit the "Cancel message" button. Only it said,
"Post message". Unfortunately, AFAIK, there's no 'overs' in Usenet
....
--
C.R. Krieger
(no more embarrassed than usual)