Oversized tires and speeding

I have a new Sequoia and the tires are bigger than what you would get on a normal car (supposedly came from company that way). The other day I got a speeding ticket for 5 miles over what I thought I was going (77, clocked at 82). I was shocked! I thought this was a myth. Could the tires have something to do with this?

The Sequoia is not a “normal car,” it’s a large SUV. The tires are supposed to be bigger. If it’s new, and the tires are standard factory equipment, they are not the reason for this discrepancy.

You check the speedometer by measuring the time it takes to cover a known distance using the mile markers on the side of the highway.

It’s not a myth at all. You’re OD is connected to the transmission. It determins how fast your going and how far you’re traveling based on how many revolutions the output shaft is turning. It’s all calibrated based on the size of the tires. If you change the height of the tires then your OD won’t be calibrated anymore. If you increase the tire size from 30" to 31"…that’s equates to a 3% increase in circumference…which means you’ll be traveling 3% faster then you were with the smaller tires.

Sorry - girl talking. “normal sequoia”. Also, math deficit. If I’m going 60 on the interstate, then it should take me 60 seconds to get from one mile marker to the other, right?

PS - thank you!

Thank you - I need to get this tested. It is only 4 weeks old and this is my first SUV. Beside timing mile markers, is there a shop or something that could test it? I know I can get someone to ride beside me or behind me…would that give me an acurate reading? BTW, is there a “chip” or something that goes in the computer that gets changed out when you get bigger tires? Could there have been an oversight at the factory?

Yes, 60 seconds per mile at 60 mph.

If you put larger tyres on a car the indicated speed will be less that with the smaller tyres, smaller tyres mean higher indicated speeds. You can try http://miata.net/garage/tirecalc.html for a calculator telling you how much faster or slower.

Most cars come from the factory with the speedometer adjusted to the size tyres the car was equipped with and adjusted to show about 3-5% higher numbers than actual, to help keep you from unknowingly speeding. A speedometer shop should be able to calibrate it for you if you like, but remember it will change as your tyres wear and anytime you buy new tyres.

If you like to check it, most freeways have distances marked off at every 1/10 mile with small white signs. Just crank it up to 60 mph and use a good watch to time how long it takes to travel a given number of those signs. I would suggest doing about five miles worth for a good accurate test. That should take you about 5 minutes. Divide the number of miles by the number of minutes and multiply by 60.

As tyres wear they will get smaller and tell you that you are going faster than you really are.

It is only the outside circumference that makes a difference.

I guess you should not have been all that shocked because after all you were speeding.

Added note: In many new cars the adjustment is made in the car’s computer to adjust for things like different tyres and that can be done by the dealer or anyone who has the right computer tools for that make and model.

Another way to calibrate the speedometer is to borrow a GPS unit that has a page with direction, speed, distance traveled, etc.; set it on the dash; get up to the speed point, 60, 65, 70,75,80,etc. on the GPS display; hold that speed; and then check to see how the speedometer matches. Your right seat rider can make up a calibration card for you, i.e. 57 speedo = 60 actual.

Okay, you’ve gotten plenty of information about how larger tires affect the speedometer readout, but we still haven’t determined if you really do have larger tires than the vehicle would have been equipped with at the factory.

Look at the sidewall of the tire and find the size information. It will be something along the lines of “P 235 / 75 R16”. Write down that info and post it here, we can find out if it really is the tire size throwing things off.

If the OP references the information on the sidewall of the tire and then compares that information with the data on the tire pressure label on the door jamb, that will tell her if the tires are the standard size. Alternatively, the Owner’s Manual will list the standard tire size for the vehicle. Either of these sources of information should resolve the issue of whether these tires are the standard size or not.

+On one of the door jabs will be a sticker listing the factory installed tires and their recommended pressures. Check to see if if your vehicle has the correct tire size installed. Then find one of those unmanned radar speed meters they set up to “inform” drivers of their speed. Make a couple passes past it and check your speedometer against the speed meter. Or simple go out on the road and pace a friend who is driving at a steady, known, speed. Trying to time mile-markers is almost impossible to do with any degree of accuracy…

Since this is a new vehicle, can you not have the dealer fix the calibration? Have you asked them?

Speedometers go bad after a while. Not all of them, but they are really bad sometimes. You should always check yours with a stopwatch on the highway. At least once every year.

The tire size is P265/65R17 and the tire size on the inside of the door (sticker) says the same (ugh!). In the book, there are a few different size tires listed and the book shows a corresponding hieght (sorry, left the book in the truck). I tried timing the mile marker and that’s not going to work. I will need someone riding “shotgun” to do this. I am going to call the local sherrif’s office and see where they have placed the unmanned radar speed meters.

Yes - I am shocked still. I saw the officer today (same place, same time for the last year) and drove right by him (while he had his radar gun pointed at me). I am beginning to think his radar gun is broke!

Yes, had to go by yesterday to pick up a part for the sun roof (truck only 4 weeks old). He said they do not “swap” tires because they don’t have the capability to place new “chips” in the computer. What is this “chip” he is referring to?

By law in most states, radar equipment has to be checked and calibrated periodically. If you are sure that you were not speeding, it might be worthwhile to retain an attorney who can subpoena the documentation for that particular radar device. After all, the total cost of a speeding ticket can include insurance surcharges in addition to the ticket itself, so it can actually wind up costing you really big bucks, thus possibly making an attorney’s help cost-effective. And, if you have AAA or another motor club, you might want to see if you are covered for legal representation. Sometimes your membership includes this type of benefit.

If you want to see how much of a joke police radar is, drive by one of those speed checking signs put out by the police. Even when I am the only vehicle in the vicinity, the it cant make up its mind whether I am going the speed limit or 10 mph over, it jumps back and forth. And that is with a fixed mounting. An officer holding a radar gun by hand is certainly going to give no better readings. Ive been pulled over a few times for this. One time it landed me a ticket.

About your car, first I would check the odometer accuracy, since the speedometer and odometer are both fed by the same chip, or vehicle speed sensor. Go to a freeway and see if the odometer is at the same fraction of a mile at each mile marker over several miles. It should be very close. If you go past 8 mile markers, your odometer will show that youve gone 7.5 miles if your car is really off as much as you think it is. At 16 miles, itll show youve gone 1 less mile. If your car is off like this, Id inform the dealer of this and have them check it out. It is very possible that the wrong chip, or vehicle speed sensor, could have been installed at the factory.

If the odometer test shows good, have a passenger check you with a stopwatch doing the same thing as above, but setting the cruise at 60 mph. Start the watch at a mile marker. After 8 markers, it should still be pretty much on minute. Itll show a half minute less if your car is off what you think it is. After 16 miles itll show a full minute less. If this is true, then have the dealer check out your speedometer.

If your car turns out good, then Id chalk it up to our tax dollars at work.

Well, I would not expect them to need a “chip” in a new car. I assume that the speedometer is electronic, which means a series of pulses is sent from the output of the transmission to the instrument cluster and there is a little electronic counter in there that counts how many pulses per second which is translated into a speed reading. I would expect that to be programmable on most new cars and not really require a chip replacement. In some cars the electronic counter, which is inside an integrated circuit chip, might be fixed and not programmable so changing overall tire size would require swapping out the physical chip. I find it hard to believe there is no adjustable calibration, either a programming change or a physical screw adjustment of some sort.

So, is the dealer saying the tires are actually the stock size? If not, who installed them?

Ah, so this says the tires are a stock size. They aren’t “bigger” than normal for this car. I still think the dealer should check the calibration for you. Get it in writing.