If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
actual speed?

For example, Lets say a car came with 15" tires and I put on 16" tires
(and rims). When the speedometer should read 30mph (with the original
15" tires), will it read about 25 or about 35mph with the 16" tires?

I cant seem to comprehend how to determine this.....

On 09/29/2016 06:59 AM, wrote:
If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
actual speed?

For example, Lets say a car came with 15" tires and I put on 16" tires
(and rims). When the speedometer should read 30mph (with the original
15" tires), will it read about 25 or about 35mph with the 16" tires?

I cant seem to comprehend how to determine this.....

circumference = pi * diameter

Paint,

With larger tires your speedometer will read slower than your actual
speed. To make a calibration table for the larger tires find a measured mile
and record your speedo reading and the elapsed time.
Also, 15 " tires fit a 15"rim. The diameter of the tires not the rims
should be measured.

Dave M.

On Thu, 29 Sep 2016 06:59:52 -0400, "David L. Martel"
wrote:

Paint,

With larger tires your speedometer will read slower than your actual
speed. To make a calibration table for the larger tires find a measured mile
and record your speedo reading and the elapsed time.
Also, 15 " tires fit a 15"rim. The diameter of the tires not the rims
should be measured.

Dave M.

My little Ranger came with puny tires on 14 inch rims. The speedo was
pretty accurate. I switched up ro large (225/70 summer and 235/70
winter) X16 tires and wheels. My speedo is very accurate with the
235s. The secret? Replace the speedo gear with the correct one for
the gearing and tire size.

On 09/29/2016 06:54 PM, wrote:
Replace the speedo gear with the correct one for
the gearing and tire size.

Or that was the secret about 20 years ago...

http://www.hypertech-inc.com/product...alibrator.aspx

It's more difficult in this century for most vehicles.

On Thu, 29 Sep 2016 20:59:36 -0600, rbowman
wrote:

On 09/29/2016 06:54 PM, wrote:
Replace the speedo gear with the correct one for
the gearing and tire size.

Or that was the secret about 20 years ago...

http://www.hypertech-inc.com/product...alibrator.aspx

It's more difficult in this century for most vehicles.

If I was a deceitful guy I would set the speedo to read in KM and set
it up to think I had appropriately smaller tires (60% of stock). Then
it would rack up fewer miles. Set everything back to stock when you
sell it.

On Thu, 29 Sep 2016 20:59:36 -0600, rbowman
wrote:

On 09/29/2016 06:54 PM, wrote:
Replace the speedo gear with the correct one for
the gearing and tire size.

Or that was the secret about 20 years ago...

http://www.hypertech-inc.com/product...alibrator.aspx

It's more difficult in this century for most vehicles.
There are units you just connect to the VSS and tune with a
screwdriver - which makes it actually simpler than changing the gear.

On Fri, 30 Sep 2016 15:39:24 -0400, wrote:

On Thu, 29 Sep 2016 20:59:36 -0600, rbowman
wrote:

On 09/29/2016 06:54 PM, wrote:
Replace the speedo gear with the correct one for
the gearing and tire size.

Or that was the secret about 20 years ago...

http://www.hypertech-inc.com/product...alibrator.aspx

It's more difficult in this century for most vehicles.
There are units you just connect to the VSS and tune with a
screwdriver - which makes it actually simpler than changing the gear.

What is a VSS ???????

On Thu, 29 Sep 2016 20:54:08 -0400, wrote:

On Thu, 29 Sep 2016 06:59:52 -0400, "David L. Martel"
wrote:

Paint,

With larger tires your speedometer will read slower than your actual
speed. To make a calibration table for the larger tires find a measured mile
and record your speedo reading and the elapsed time.
Also, 15 " tires fit a 15"rim. The diameter of the tires not the rims
should be measured.

Dave M.

My little Ranger came with puny tires on 14 inch rims. The speedo was
pretty accurate. I switched up ro large (225/70 summer and 235/70
winter) X16 tires and wheels. My speedo is very accurate with the
235s. The secret? Replace the speedo gear with the correct one for
the gearing and tire size.

I heard that can be done, but dont that require tearing the tranny
apart?

On Thu, 29 Sep 2016 22:41:29 -0400, wrote:

Replace the speedo gear with the correct one for
the gearing and tire size.

I heard that can be done, but dont that require tearing the tranny
apart?

Back when they used a speedo gear, it was one screw.

On 09/29/2016 08:41 PM, wrote:
I heard that can be done, but dont that require tearing the tranny
apart?

No. I never changed the gearing but when you pulled the tranny it was
one bolt that secured the speedometer gear. Back it out, slip the keeper
out of the way, and the whole assembly pulled right out.

Alternately, you could forget to disconnect it and wonder what the hell
was hanging up the transmission when you tried to remove it.

On Thu, 29 Sep 2016 22:41:29 -0400, wrote:

On Thu, 29 Sep 2016 20:54:08 -0400, wrote:

On Thu, 29 Sep 2016 06:59:52 -0400, "David L. Martel"
wrote:

Paint,

With larger tires your speedometer will read slower than your actual
speed. To make a calibration table for the larger tires find a measured mile
and record your speedo reading and the elapsed time.
Also, 15 " tires fit a 15"rim. The diameter of the tires not the rims
should be measured.

Dave M.

My little Ranger came with puny tires on 14 inch rims. The speedo was
pretty accurate. I switched up ro large (225/70 summer and 235/70
winter) X16 tires and wheels. My speedo is very accurate with the
235s. The secret? Replace the speedo gear with the correct one for
the gearing and tire size.

I heard that can be done, but dont that require tearing the tranny
apart?
Nope. 1 little 6mm bolt holds the speedo drive unit in the trans. On
older vehicles it eas a speedo cable. On newer ones (like my 20 year
old ranger!!!) it is a pulse generator. If you go TOO far you may need
to replace the gear on the output shaft, which requires removing the
tailstock.

It happens that formulated :
If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
actual speed?

For example, Lets say a car came with 15" tires and I put on 16" tires
(and rims). When the speedometer should read 30mph (with the original
15" tires), will it read about 25 or about 35mph with the 16" tires?

I cant seem to comprehend how to determine this.....

Paul gave the answer. I remember a somewhat surprising answer to a math
question. It stated theoretically if you could add 3 feet of rope to a
rope which encircled the Earth touching all along the equator,
approximately how far off the ground would it hover at all points if
the added three feet caused it to do so.

On 09/29/2016 12:59 AM, wrote:
If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
actual speed?

For example, Lets say a car came with 15" tires and I put on 16" tires
(and rims). When the speedometer should read 30mph (with the original
15" tires), will it read about 25 or about 35mph with the 16" tires?

I cant seem to comprehend how to determine this.....

Like Paul said, it depends on the rolling diameter of the tire. For
example 18" wheels with those rubber band tires may not have a greater
diameter than 16" wheels with a higher profile tire.

I run 15" in the summer and 14" studs in the winter and the studs show
about 10% slower. I just go by the GPS. The car's speedometer is
centrally mounted which introduces for parallax so the GPS display is
more convenient anyway.

On Thursday, September 29, 2016 at 9:37:55 AM UTC-4, rbowman wrote:
On 09/29/2016 12:59 AM, wrote:
If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
actual speed?

For example, Lets say a car came with 15" tires and I put on 16" tires
(and rims). When the speedometer should read 30mph (with the original
15" tires), will it read about 25 or about 35mph with the 16" tires?

I cant seem to comprehend how to determine this.....

Like Paul said, it depends on the rolling diameter of the tire. For
example 18" wheels with those rubber band tires may not have a greater
diameter than 16" wheels with a higher profile tire.

I run 15" in the summer and 14" studs in the winter and the studs show
about 10% slower. I just go by the GPS. The car's speedometer is
centrally mounted which introduces for parallax so the GPS display is
more convenient anyway.

I recently borrowed a friend's "church van" sized van to move some stuff.
50 miles from home, the speedometer stopped working. Sometimes it would
pick a speed and just sit there, other times it would bounce back and
forth over a 20 MPH range. I downloaded a speedometer app for my
smartphone and used that for the rest of the trip.

When I returned the van I told my friend about the issue and volunteered
to pay/help pay for the repair. He replied: "Oh, yeah. I guess I should
have told you about that. I always use a GPS as my speedometer."

(He also didn't tell me that the turn signals didn't work unless the
Hazard button was depressed. The Hazards didn't work at all, but they
had to be "On" in order for the turn signals to work. I stumbled upon
that one myself while checking the lights for the trailer harness.)

Hey, it was big and it was free. No complaints!

DerbyDad03:

Turn signals? You were driving a VAN, whaddya need
turn blinkers for? lol

On Thursday, September 29, 2016 at 4:03:11 AM UTC-4, wrote:
If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
actual speed?

For example, Lets say a car came with 15" tires and I put on 16" tires
(and rims). When the speedometer should read 30mph (with the original
15" tires), will it read about 25 or about 35mph with the 16" tires?

I cant seem to comprehend how to determine this.....

It will read lower than actually. If you google you can find tire
speed calculators online, where you can put in the original tire
size and the new and it will tell you the error.

If someone puts on larger auto tires than the vehicle was originally
equipped with, will the speedometer read faster or slower than the
actual speed?

Yes.

Usually, when you increase the size of the wheels (often called "plus one"
or "plus two") the new tires have a lower profile. In other words, the side
wall of the tire is shorter than the original tire, so the outer diameter
is almost the same.

Basically, you end up with more wheel and less tire. This can improve
performance as there is less side wall flex in the tire during cornering.
The flip side is it also makes the ride firmer since there's less tire to
cushion bumps.

Of course, you see people pushing this idea to ridiculous extremes now days
with huge wheels and extremely low profile tires. Personally I think it
looks stupid, but to each their own...

I cant seem to comprehend how to determine this.....

There are plenty of calculators online that will let you determine the
outer circumference of various wheel and tire combinations. Usually you
want to stay within 2-3% of the factory size to prevent issues with the
speedometer reading faster or slower.

Here's one you can try:

http://www.1010tires.com/Tools/Tire-Size-Calculator

Anthony Watson
www.watsondiy.com
www.mountainsoftware.com