Different Tire screw up my computer

[mslacool]

I own a set of 205 60 15 studded snows and I wonder if replacing them in the winter for my stock 195 65 15 is a problem. The local gas shop owner says that changing the tires screws with hybrid computers, but this sounds a bit whacko... I know that these tires will leave me traveling 59.3 mph when the speedo says 60 but will that harm anything? I can't imagine how....

But please group, let me know.. .only if you know... ?

thanks
Paul

[apriusfan]

<div class='quotetop'>QUOTE(mslacool @ Nov 14 2007, 07:57 PM) [snapback]539789[/snapback]</div>

I own a set of 205 60 15 studded snows and I wonder if replacing them in the winter for my stock 195 65 15 is a problem. The local gas shop owner says that changing the tires screws with hybrid computers, but this sounds a bit whacko... I know that these tires will leave me traveling 59.3 mph when the speedo says 60 but will that harm anything? I can't imagine how....
[/b]

59.3 sounds awfully like 60. Certainly within the margin of error. Did the local gas shop owner offer any specifics as to how "changing the tires screws with hybrid computers"? Like which computers and what will be the result of the screwing with the computers....

Personally, I think you are fine switching tires as regards the computers in the Prius.

[kenmce]

I believe what he was trying to say is that the car keeps track of how many times the axle goes all the way around, makes an asumption about how far the car travels per revolution, and uses this to give you figures for your speed and MPG. If you put on a bigger or smaller tire you will go a slightly farther or shorter distance every time the axle goes around once. If the computer does not know about this, it will give you figures that are consistently slightly off. This will do the computer no harm, but may well annoy the driver.

I would think Toyota would have left a way to tell the computer you have changed to a different size, but don't know what it would be. It can't just "look" at a new rim and know what size it is, someone would have to tell it.

[Boo]

I see you probably used the Miata Tire Calculator in order to come up with the over-read of the speedometer.

Curious though, how is your stock or OEM tire size 195/65/15? If this was an error, then I would just suggest plugging the correct OEM tire size into the calculator in order to come up with a more accurate over-read or under-read, as the case may be.

[apriusfan]

<div class='quotetop'>QUOTE(kenmce @ Nov 18 2007, 06:06 AM) [snapback]541201[/snapback]</div>

...I would think Toyota would have left a way to tell the computer you have changed to a different size, but don't know what it would be. It can't just "look" at a new rim and know what size it is, someone would have to tell it.[/b]

Not really. Providing the ability to 'program' different tire sizes into the onboard computer is rarely done, unless the manufacturer uses the same computer for different models and each model utilizes different tire sizes. The only car that comes to mind that fits this description is the Corvette - Z06 vs. non-Z06. The Corvette's Electronic Braking Traction Control Module has two options for the different wheel & tire settings depending on whether Z06 wheels & tires are being fitted or not.

Absent a provision for different wheel & tire sizes in the car's computer, you have two choices - 1.) Find a wheel & tire combination that has the same (+/- 1 or 2%) circumference (expressed in the rotations/mile specification for the tire) as the stock tire, or 2.) Mentally adjust for the different circumference of the replacement tire.

The one thing you absolutely do NOT want to do is fit different wheel & tire combinations on the front and rear axles (e.g. 16 inch wheels & tires on front axle with 17 inch wheels & tires on rear axle). Absent a manufacturer provision for the difference (Corvette), you will have a very nasty surprise in store when the stability control computer tries to adjust for the different rotations it is measuring from the front and rear. Keep the same circumference on all four tires.