If someone puts on larger auto tires, than the vehicle was originally
equipped with, will the Speedometer read faster or slower than the
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.....
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.
On Thu, 29 Sep 2016 06:59:52 -0400, "David L. Martel"
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 08:41 PM, firstname.lastname@example.org wrote:
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, email@example.com wrote:
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
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
It happens that firstname.lastname@example.org formulated :
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, email@example.com wrote:
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:
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!
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
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...
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:
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