Another thread has ventured into tire size and how it effects the speedometer.

I thought I would link to a site that has a very easy to use tool that will show what bigger or smaller tires will do to your indicated speed. Just input the standard TCH tire size, 215 X 60 X 16, then try different sizes of replacement tires after inputing a target speed. The indicated vs actual speed wil be displayed.

It's pretty cool!

http://www.paspeedo.com/calculator.htm

 

My actual is 57 MPH when the indicated speed on the speedometer is 60 MPH. If I switched from 215/60/r16's to 225/65/r16's my actual and indicated speeds would match at 60 MPH.

 

I can understand manufacturers calibrating their speedometers so they read a little high. But 3 mph seems a bit excessive.

 

Jeff,

You can look up the revs/mile ratings on specific tires at tirerack.com. Note that tires with the same size have somewhat different revs/mile and will therefore affect the speedo and odo. There is also the affect of wear on the tire which increases revs/mile slightly as the tire gets smaller with age.

JeffD

 

That's quite a deviation you got going. If that error is happening to your odometer too, that means your MPG is overstated too. It already sucks (just kidding).

I had the opportunity to measure my speedo with my GPS unit a couple of weeks ago over a long drive staying in the 50 to 70mph range. With my Miche's at 35psi, by GPS showed bang on to my speedo most of the time, wavering by at most 1 kph plus or minus (which is only 0.6 mph)

 

My speedo is also off by 3mph at 60mph (about 5%). For those who didn't already know, this is a common problem on the 07Camry. I plan on correcting my speedo with slightly larger wheels/tires, as per the calculator.

 

I need one of you "young" engineers whose brain still functions to answer this. Ken and I have had a dialog going and are in agreement with the problem but opposite in the solution. I was using the tire rack calculator and the Miata Tire calculator.

The problem: If your speedo reads 60 mph when you are actually only doing say 55 mph how do you correct it, taller tires or shorter tires.

My hypothisis, A taller tire/wheel combo will travel more distance per revolution, so if your odometer is calibrated to count revolutions to convert to miles then the only way to get the correction to get the reading closer (increasing reading on odo) is to put a smaller tire/wheel combo on so that you will record more revolutions.

This is where it's confusing. Maybe I'm making this more confusing than it is but you have to think "backwards" on this to get to the solution and if I'm thinking right on this (and I very well may not be) then below is my conclusion.

The calculator shows if you use a shorter tire (P225/50R16) it would show 57 MPH (5% LOWER) compared to your stock P215/60R16 tires that were reading 60 mph with the P215/60R16. However if you car by "GPS" or radar is actually only going 57 mph when your odo is reading 60 with the stock tires then you need to "make up" the extra. The Miata calculator says the non-stock tire reading is 5% too high assuming the actual reading is correct. Thus what you really what to do is put a tire / wheel combo on that will make up the error and actually read too high compared to the stock tire and the current odo reading. Therefore, as complicated as it is to my feeble mind it takes a shorter tire combo to correct the error.

If that is true then I can see why Toyota didn't correct the problem with tires because few people will want a TCH with 50 series wide tires. They should have corrected it with calibration of the spedo itself.

I have to admit when I first looked at this I thought it took taller tires to correct it but what you really need is a tire combo that reads faster than the current tires if you want your actual 57 at a spedo reading of 60 to actually be corrected to 60.

Help me so I can sleep tonight (I will admit that the right side of my brain says put on taller tires so that the odo reads slower)

Hey, I'm an old engineer that has had chemo treatments and my mind just doesn't do logic like it used to.

 

Joe:

I know that in years past, a speedometer shop could calibrate your speedo, by changing the speedo driven gear (don't know what it is realy called) at the transmission. It was very simple, they drove your car up on a set of rollers, and compared the real speed (from the rollers) with your speedometer, and calculated how far off it was, then just changed a very simple plastic gear. I would doubt that the TCH has this, as it doesn't have a "transmission" but one never knows if it is handled the same way.

I believe this would be a better approach than changing tire size to accomplish the goal of more accuracy in the speedo. Wouldn't it be nice if it could be addresed in the software? Has anyone brought this issue up with their dealers?

 

[QUOTE=WVGasGuy]I need QUOTE]

Don't we all


I saw your discussions, I think the two of you were confused over each other's references to actual road vs speedo indicated speed.

At an actual road speed of 60mph, changing to taller tires will show a slower speedo indicated speed.

At a speedo indicated speed of 60mph, changing to taller tires will result in a faster actual road speed.

 

... it takes a shorter tire combo to correct the error....


I also believe this is correct, if this is the avenue one chooses to take to correct the inaccuracy. A P225/50R16 would make the speedo then read 60mph vs 57mph at an actual 60mph.