16" vs 17" Wheels and Tires

Group:

1996 Mustang GT Convertible. Had factory 16" wheels and tires. Bought a
set of the factory 17" GT/Cobra 10 spoke wheels and added new tires. Will
the difference between the 16" versus the 17" wheels cause a difference in
the speedometer? Appears to be a 5-6mph difference (e.g. shows doing 80,
only really doing 74-75).

How would one go about getting something like this re-calibrated?

Thank you!
gjt

gjt wrote:
> Group:
>
> 1996 Mustang GT Convertible. Had factory 16" wheels and tires. Bought a
> set of the factory 17" GT/Cobra 10 spoke wheels and added new tires. Will
> the difference between the 16" versus the 17" wheels cause a difference in
> the speedometer? Appears to be a 5-6mph difference (e.g. shows doing 80,
> only really doing 74-75).
>
> How would one go about getting something like this re-calibrated?
>
> Thank you!
> gjt
>
>

As you know, wheel size has no effect on speedometer accuracy. Tire
size? Well, that is another matter. What size did you start with, and
what size do you have now? If you used the same tires on your new
wheels, your speedometer will be WAY off.

Use this calculator to compare your old tires w/ new ones:

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

and you will be able to calculate the PERCENTAGE difference (it won't be
the same # of mph everywhere on the speedo).

Rob

Not sure about 'stangs but in Jeeps we just buy a new speedometer drive
gear. Also called a pinion speedometer drive gear. It's calibrated to your
tire size and gear ratio. IE: I had 33" tires and 4.10' gears in my Wrangler
so I needed a 32 (guess) tooth pinion speeedomete drive gear.

HTH

Carl

"trainfan1" > wrote in message
et...
> gjt wrote:
>> Group:
>>
>> 1996 Mustang GT Convertible. Had factory 16" wheels and tires. Bought a
>> set of the factory 17" GT/Cobra 10 spoke wheels and added new tires. Will
>> the difference between the 16" versus the 17" wheels cause a difference
>> in the speedometer? Appears to be a 5-6mph difference (e.g. shows doing
>> 80, only really doing 74-75).
>>
>> How would one go about getting something like this re-calibrated?
>>
>> Thank you!
>> gjt
>
> As you know, wheel size has no effect on speedometer accuracy. Tire size?
> Well, that is another matter. What size did you start with, and what size
> do you have now? If you used the same tires on your new wheels, your
> speedometer will be WAY off.
>
> Use this calculator to compare your old tires w/ new ones:
>
> http://www.miata.net/garage/tirecalc.html
>
> and you will be able to calculate the PERCENTAGE difference (it won't be
> the same # of mph everywhere on the speedo).
>
> Rob

Carl wrote:
> Not sure about 'stangs but in Jeeps we just buy a new speedometer drive
> gear. Also called a pinion speedometer drive gear. It's calibrated to your
> tire size and gear ratio. IE: I had 33" tires and 4.10' gears in my Wrangler
> so I needed a 32 (guess) tooth pinion speeedomete drive gear.
>
> HTH
>
> Carl
>
> "trainfan1" > wrote in message
> et...
> > gjt wrote:
> >> Group:
> >>
> >> 1996 Mustang GT Convertible. Had factory 16" wheels and tires. Bought a
> >> set of the factory 17" GT/Cobra 10 spoke wheels and added new tires. Will
> >> the difference between the 16" versus the 17" wheels cause a difference
> >> in the speedometer? Appears to be a 5-6mph difference (e.g. shows doing
> >> 80, only really doing 74-75).
> >>
> >> How would one go about getting something like this re-calibrated?
> >>
> >> Thank you!
> >> gjt


A programming from a stealership or one of those programmers used in
OBD2 vehicles will do the job.

You could do the old 1+ method, I think it went something like this, for
every inch increase in rim diameter you add 10 to the width and subtract 10
from the aspect ratio. Example, take a stock 16 inch rim with 225 by 60
tires. i.e. R16 x 225 x 60. To go to a 17 inch rim but still maintain the
same overall diameter you would replace your tires with R17 x 235 x 50's.
This gives you a wider, lower profile tire with the approximate same overall
diameter as the ones you just replaced, note the operative term wider, your
fender wells need to have enough room to accommodate the new tire.

Everybody feel free to jump in here if I got this wrong

> wrote in message
ups.com...
>
> Carl wrote:
> > Not sure about 'stangs but in Jeeps we just buy a new speedometer drive
> > gear. Also called a pinion speedometer drive gear. It's calibrated to
your
> > tire size and gear ratio. IE: I had 33" tires and 4.10' gears in my
Wrangler
> > so I needed a 32 (guess) tooth pinion speeedomete drive gear.
> >
> > HTH
> >
> > Carl
> >
> > "trainfan1" > wrote in message
> > et...
> > > gjt wrote:
> > >> Group:
> > >>
> > >> 1996 Mustang GT Convertible. Had factory 16" wheels and tires.
Bought a
> > >> set of the factory 17" GT/Cobra 10 spoke wheels and added new tires.
Will
> > >> the difference between the 16" versus the 17" wheels cause a
difference
> > >> in the speedometer? Appears to be a 5-6mph difference (e.g. shows
doing
> > >> 80, only really doing 74-75).
> > >>
> > >> How would one go about getting something like this re-calibrated?
> > >>
> > >> Thank you!
> > >> gjt
>
>
> A programming from a stealership or one of those programmers used in
> OBD2 vehicles will do the job.
>

That's weird. It sound like the new tires/wheels combination is a smaller
diameter?
Fred
89 LX
Appears to be a 5-6mph difference (e.g. shows
doing
> > >> 80, only really doing 74-75).

"Ironrod" > wrote in message
...
> You could do the old 1+ method, I think it went something like this, for
> every inch increase in rim diameter you add 10 to the width and subtract
> 10
> from the aspect ratio. Example, take a stock 16 inch rim with 225 by 60
> tires. i.e. R16 x 225 x 60. To go to a 17 inch rim but still maintain the
> same overall diameter you would replace your tires with R17 x 235 x 50's.
> This gives you a wider, lower profile tire with the approximate same
> overall
> diameter as the ones you just replaced, note the operative term wider,
> your
> fender wells need to have enough room to accommodate the new tire.
>
> Everybody feel free to jump in here if I got this wrong
>
> > wrote in message
> ups.com...
>>
>> Carl wrote:
>> > Not sure about 'stangs but in Jeeps we just buy a new speedometer drive
>> > gear. Also called a pinion speedometer drive gear. It's calibrated to
> your
>> > tire size and gear ratio. IE: I had 33" tires and 4.10' gears in my
> Wrangler
>> > so I needed a 32 (guess) tooth pinion speeedomete drive gear.
>> >
>> > HTH
>> >
>> > Carl
>> >
>> > "trainfan1" > wrote in message
>> > et...
>> > > gjt wrote:
>> > >> Group:
>> > >>
>> > >> 1996 Mustang GT Convertible. Had factory 16" wheels and tires.
> Bought a
>> > >> set of the factory 17" GT/Cobra 10 spoke wheels and added new tires.
> Will
>> > >> the difference between the 16" versus the 17" wheels cause a
> difference
>> > >> in the speedometer? Appears to be a 5-6mph difference (e.g. shows
> doing
>> > >> 80, only really doing 74-75).
>> > >>
>> > >> How would one go about getting something like this re-calibrated?
>> > >>
>> > >> Thank you!
>> > >> gjt
>>
>>
>> A programming from a stealership or one of those programmers used in
>> OBD2 vehicles will do the job.
>>
>
>

There is a site that has the calculations done for you, it gives you the
percentage between your stock wheel/tire combo and what you are thinking of
putting on. It even gives you the speedpmeter reading at 60mph and what you
are actually going. the site is Miata Tire size calculator. Just type it
into Google and go from there

You are offsetting having a larger diameter rim by using a tire with a
narrower side wall. If placed side by side the new lower profile tire on
the larger rim should stand almost as tall as the original tires.


"Fred V." > wrote in message
m...
> That's weird. It sound like the new tires/wheels combination is a smaller
> diameter?
> Fred
> 89 LX
> Appears to be a 5-6mph difference (e.g. shows
> doing
> > > >> 80, only really doing 74-75).
>
> "Ironrod" > wrote in message
> ...
> > You could do the old 1+ method, I think it went something like this, for
> > every inch increase in rim diameter you add 10 to the width and subtract
> > 10
> > from the aspect ratio. Example, take a stock 16 inch rim with 225 by 60
> > tires. i.e. R16 x 225 x 60. To go to a 17 inch rim but still maintain
the
> > same overall diameter you would replace your tires with R17 x 235 x
50's.
> > This gives you a wider, lower profile tire with the approximate same
> > overall
> > diameter as the ones you just replaced, note the operative term wider,
> > your
> > fender wells need to have enough room to accommodate the new tire.
> >
> > Everybody feel free to jump in here if I got this wrong
> >
> > > wrote in message
> > ups.com...
> >>
> >> Carl wrote:
> >> > Not sure about 'stangs but in Jeeps we just buy a new speedometer
drive
> >> > gear. Also called a pinion speedometer drive gear. It's calibrated to
> > your
> >> > tire size and gear ratio. IE: I had 33" tires and 4.10' gears in my
> > Wrangler
> >> > so I needed a 32 (guess) tooth pinion speeedomete drive gear.
> >> >
> >> > HTH
> >> >
> >> > Carl
> >> >
> >> > "trainfan1" > wrote in message
> >> > et...
> >> > > gjt wrote:
> >> > >> Group:
> >> > >>
> >> > >> 1996 Mustang GT Convertible. Had factory 16" wheels and tires.
> > Bought a
> >> > >> set of the factory 17" GT/Cobra 10 spoke wheels and added new
tires.
> > Will
> >> > >> the difference between the 16" versus the 17" wheels cause a
> > difference
> >> > >> in the speedometer? Appears to be a 5-6mph difference (e.g. shows
> > doing
> >> > >> 80, only really doing 74-75).
> >> > >>
> >> > >> How would one go about getting something like this re-calibrated?
> >> > >>
> >> > >> Thank you!
> >> > >> gjt
> >>
> >>
> >> A programming from a stealership or one of those programmers used in
> >> OBD2 vehicles will do the job.
> >>
> >
> >
>
>