lesz

You said it better than I did.

cafesilver

If you did even a little research you would quickly find out that tires are irrelevant in the vehicle determining how fast it is going. From the picture below you can see that the overall wheel/tire diameter is very small between wheel sizes. The difference between a 16 inch wheel and a 19 inch wheel is 3 inches but the overall wheel/tire diameter is only .6 inches difference.

The speed is calibrated in the computer based on a predetermined size of the wheel. If you start changing the wheel size the computer doesn't know and therefore gives you a false readout of the speed.

Put it another way, let's say your overall wheel/tire diameter is 25 for two different cars. On one car, the wheels are 1 inch and the other is 20 inches. For the two cars to go the same speed the rotations per minute of the wheels on each respective car would have to be different. The 1 inch wheel needs more rotations per minute than the car with 20 inch wheels to go the same speed.

1 rotation of the 1 inch wheel will travel less distance than 1 rotation of the 20 inch wheel even though both cars have an overall wheel/tire diameter of 25 inches

Mike728

You just don't get it. This is not a new thing. I'm going to go out on a limb and guess you have the wrong size tire on your 18" rims.

cafesilver

I think where you guys are getting held up is that you think the speed is being determined with the tires on the wheel but the tires are completely irrelevant. The wheel/tire diameter for different wheel sizes is nearly indistinguishable for the same vehicle (take the ES as the example) but the different sizes of wheels is distinguishable. The speedometer is determining the speed based solely on the size of the wheel. It's calibrated based on wheel size and not wheel/tire size. So if you start changing the wheel size, the overall wheel/tire size stays the same but with the wheels being bigger or smaller it effects the speed reading.

1 rotation of the wheels/tires with different size wheels will travel the same distance. But when you take the tires off and do 1 rotation of just the wheels the wheel with the larger diameter will travel more distance in 1 rotation that the smaller wheel. This is how the car's computer is calibrated and reading out speed. Without an adjustment to the computer programming or to the actual reading instrument (speedometer needle) when you put on larger wheels the speedometer reads a higher speed than the car is actually traveling.

lesz

I'm afraid that I don't know how else to say it, but I'll try one more time.

The only factor that is relevant is how far the vehicle travels with each rotation of the tires. If the 17 inch wheels with 215/55R17 tires have the same diameter as 18 inch wheels with 225/45R18 tires, the car will travel the same distance with both combinations per each rotation of the tires, and the speedometer reading will also be identical.

Talking about how far the car will travel per rotation if you take the tires off of the wheels makes no sense at all.


I suspect that your guess is correct. The only way that changing the wheel size will change the speedometer reading would be if, by putting the wrong-sized tires on the car, the diameter of the wheel/tire combination changed and, thus, caused the car to travel a different distance per rotation of the tire.

At this point, I don't expect that cafesilver is going to be willing to reconsider the mistaken conclusion to which he/she has come, but the only point in having others post is to make it less likely that others will believe an explanation that is completely wrong.

cafesilver

So confident that I was right and you were wrong I got back to work from lunch and started to do the same research I did two years ago. I remember doing hours of research before actually buying the larger wheels. I remember watching videos on how to re-calibrate the speedometer. And after seeing it was incredibly complex I just said to hell with it and I would live with the speedometer being slightly off (again, at 80, the car is actually going about 77). For the life of me though, I can't find what I found before, nor can I find the videos I watched. Did I dream this? I certainly didn't think so.

In fact, so sure was I that this was a giant load of crap back then I did actually do two experiments. With the 17 inch wheels and Michelin PRIMACY MXV4 - SIZE: P215/55R17 tires I drove the car on the highway for several miles at a constant speedometer reading in the ES 350 at 80 mph. Using the Speed Box app on my iPhone to compare it said I was traveling at 80 mph.

Once I had the 18 inch wheels and the new tires Continental EXTREMECONTACT DWS 06 - SIZE: 225/45ZR18 I conducted the exact same test. At 80 mph reading in the ES 350 the Speed Box app said I was traveling at 77-78 mph (it would constantly flicker back and forth so I assume I was going about 77.5 mph).

I also just looked up the actual specs of each tire because I thought I was going crazy. The Michelin's have an overall diameter of 26.3" and the Continental are 26". Based on this, the car speed should be nearly identical with these tires and the different size wheels.

So my question then is, why isn't it?

Bigsk8r

Folks,

All cars sold in the U.S. must conform to SAE J1226 for speedometer accuracy. Your cars all had built in error when you bought them. Here is an excerpt from a Car and Driver article about exactly this...

"...we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph."

If you get in any vehicle manufactured and marketed in the US and has stock (the 17" original wheel) or stock replacement (18" wheel with scaled tire) and measure the speed with an accurate GPS, I guarantee you will see 2.5 - 3.5 MPH difference between it and the stock speedo...

Mike728

Yes, after that post I decided I should probably just step away.

fishnlab

lol. this was great!

jgscott

iTrader: (1)

Ok just seeing this thread. Did not read back past this page. But I have the same problem. I bought a used 2013 ES 350 recently. Felt it was not going as fast as the speedo says. So I ran a iPhone GPS app. Showed Speedo says 3-4 miles faster the actual. Also had someone track my car with their car on cell call with them. My ES was showing 3-4 miles faster ranging the 55-80mph range.

So we then took the iphone GPS speed app to their car. It was dead correct only .5 - 1 mph late to pick up sometimes, but when car held at exact speed the app was correct to. My ES is showing 3-5 mph faster on the speedo. I have a 2013 ES350 UL with the 18" wheels. It had crappy Kumho tires on it. I just replaced them today with a set of 4 New Michelin Primacys. Not had a chance to check it again yet, but The Michelins are quitter, smother, and handle way better.

Is there a way for me to correct the speedo myself without taking it to the Dealer?

cafesilver

They all think I am crazy but I know for a fact that what I said is true: wheel size changes the readout of the speedometer. The dealer will not recalibrate the speedometer for you. The speedometer is calibrated at the factory for the size WHEEL that is factory installed. Putting on larger diameter wheels will make the speedometer read faster than the car is actually going and putting on smaller diameter wheels will make the speedometer read slower than the actual speed.

Videos that I watched that show how to fix this almost all had to do with adjusting the calibration of the speedometer. It involved pretty much taking the entire dashboard apart and there is a lot of guesswork to get it correct. Ultimately I decided it wasn't worth the effort and I know that my speedometer reads 3mph faster at 80mph (speedometer reads 80mph but car is actually going 77mph). The faster you go, the bigger the gap. For example, when the speedometer reads 30mph the car is going 29-30mph and when the speedometer reads 100mph the actual speed is probably more like 95-96mph.

jgscott

iTrader: (1)

Do you have the Factory 18" wheels? What Year is your car? The 1st day I got the car I had to drive back 4 hours on the highway. I knew after about a hour something felt wrong about the speed vs the speedometer reading. That was when I downloaded the Speedbox, and Speedometer App on my iphone and saw the difference. After that I have side by side even speed paced 2 other different car. Same results.

Anyone who has the 18 wheels should run a GPS speed app or pace another accurate car speedo and check. I am well aware of the tire measure height and revolution blah, blah, blah, sizing calculations. Don't know why mine is off, but it is off? No question about it.

cafesilver

I have a 2015 ES 350 that came with 17" wheels. I put 18" wheels on after I bought it. You can buy the ES with 18" wheels from the factory and then it is calibrated correctly. This is the same for every car manufacturer.

jgscott

iTrader: (1)

No I have a Ultra Luxury Package that came with the Factory 18" wheels from the Factory. Its still incorrectly giving the wrong speed. Though I just changed to different tire tho, and have not has a chance to check it again yet.

Also if I remember correctly not all tire Brands of the size like size measure exactly the same. There are slight measurement differences sometimes in tires from Brand to Brand..

Ratchey

I hate to tell you, but unless the sensor it at the outer edge of the wheel, there is no possible way for the car to know if you have 17, 18 or 24 in wheels.
So unless you are saying the speed is provided by the TPMS (and we know it isn't because you will still show speed if you remove the TPMS) your statement is completely incorrect.
It is based on revolutions per mile, probably captured at the axle half shaft. That speed is determined by the tire size, not the wheel size.

If you have the exact same height tire, a 17 and 18 inch wheel will rotate at the exact same speed at the 8.5" mark. Since you can swap wheels straight across, the sensor can't be measuring at the outer edge of the wheel. If you are getting variances in speed between 2 different sets of wheels, it is because your tires are not the exact same height.

If you have stock wheels and old tires, your reading will be off as the tread is lower.

If you have stock wheels and new stock tires and your reading is off, it was set incorrectly from the factory.

https://tiresize.com/tyre-size-calculator/

If you really want to test this, get two 17" wheels and two 18" wheels and mount tires on them that are exactly the same height. Even 3/32 can throw this off.
Now mount those tires on the car in any configuration you want. (17s on front, 18s on rear; 18s on front, 17s on rear; 17s on left, 18s on right; 18s on left, 17s on right; 17s on front left/rear right, 18s on front right/rear left) and you will get the same exact speed reading every time. It's physics, not programming.