# Putting on larger auto tires / Speedometer readings

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.....
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On 09/29/2016 06:59 AM, snipped-for-privacy@unlisted.moo wrote:

circumference = pi * diameter
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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.
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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.
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On Thu, 29 Sep 2016 20:54:08 -0400, snipped-for-privacy@snyder.on.ca wrote:

I heard that can be done, but dont that require tearing the tranny apart?
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On Thu, 29 Sep 2016 22:41:29 -0400, snipped-for-privacy@unlisted.moo wrote:

Back when they used a speedo gear, it was one screw.
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On 09/29/2016 08:41 PM, snipped-for-privacy@unlisted.moo 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.
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On Thu, 29 Sep 2016 22:41:29 -0400, snipped-for-privacy@unlisted.moo 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 tailstock.
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On 09/29/2016 06:54 PM, snipped-for-privacy@snyder.on.ca wrote:

Or that was the secret about 20 years ago...
http://www.hypertech-inc.com/products-speedometer-calibrator.aspx
It's more difficult in this century for most vehicles.
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wrote:

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.
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wrote:

There are units you just connect to the VSS and tune with a screwdriver - which makes it actually simpler than changing the gear.
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On Fri, 30 Sep 2016 15:39:24 -0400, snipped-for-privacy@snyder.on.ca wrote:

What is a VSS ???????
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On Wed, 05 Oct 2016 16:12:54 -0400, snipped-for-privacy@unlisted.moo wrote:

Vehicle Speed Sensor
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It happens that snipped-for-privacy@unlisted.moo 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.
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On 09/29/2016 12:59 AM, snipped-for-privacy@unlisted.moo 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.
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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!
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Turn signals? You were driving a VAN, whaddya need turn blinkers for? lol
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On Thursday, September 29, 2016 at 4:03:11 AM UTC-4, snipped-for-privacy@unlisted.moo wrote:

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.
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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...

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
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