If this is not the right place to ask this, I apoligize. I am fairly new to this web site. I recently purchased a 2001 SuperCrew 4x4 that came with Goodyear Wrangler tires and 17in wheels. I am not sure of the exact tire size but it is what comes standard on them. Prior to buying, the dealership replaced these with Goodyear Wrangler AT/S LT305/70 R16 and 16x8 wheels. They said they did this so it would not affect the speedometer. Is this correct?

[This message has been edited by skinny (edited 01-23-2001).]

 

Hi...Welcome to the site... I maybe wrong but the speedometer is calibrated at the factory for the 17" tire....

If the dealership changed the tires to a 16" and did not have the speedometer re-calibrated I believe it would indeed effect the speedometer reading... Again as I said I maybe wrong....

John from NC

 

And if it affects the speedometer, then it will also affect the odometer, right? Let's see if I can remember my geometry... Smaller tires means smaller circumference which means more revolutions to cover the same distance which means higher (inaccurate) speedometer reading which means more miles on the odometer than actually travelled which means shorter warranty period (expires sooner due to miles).

I would check into it just to make sure.

 

The only thing that matters on tire size in the overall outside diamter.The trucks computer really doesn't care if its on a 16 or 17 inch wheel.And if the diameter of the aftermarket wheel/tire combo is different than the stock one , then yes your speedometer and odometer are both off.

 

I remember a trick question that was asked around the office about this. Supposed you were looking at two identical used vehicles, deciding which to buy. Both had 50,000 miles the only difference was each had the tires swapped out, one with a smaller tires than orginal the other with larger tires.
Which would you buy?
If the tires on each caused a 10% error in mileage then neither vehicle actually had 50K on it.
The smaller tire vehicle would have 45K and the larger tire vehicle would have 55K.

Most people not thinking it thru would would go with the larger tire vehicle,
'bigger means turning slower' means 'lower miles' when the opposite is true.

To clarify, I mean fewer 'actual' miles not less showing on the odometer.



[This message has been edited by Raoul (edited 01-23-2001).]

 

Hi Skinny,

As long as the wheel & tire package they installed has the same installed tire height, or circumference, then your speedometer will still be accurate, if the new package has a difference height, or circumference, then the speedo & odo will be off.

If you have an area on a local highway where your state/local police use to check their odometers, with marked marked signs on the highway, you can check that yourself to make sure. Or, you can team up with another vehicle that has an accurate speedometer, and then compare at various speeds, starting at about 30 mph and then going up in 10 mph increments up to say, 70 or so, and see just what your speedometer indicates when the other vehicle with the accruate speedometer indicates 30, then 40, 50, and so on, so you can determine the error.

If you have the exact sizes of both the original and the new tires, then you can plug those numbers into any of a number of tire & gear charts on the Internet, to see if there is a difference in height. I don't have a URL handy to send you directly to one, perhaps someone else will post a URL for you.

Good luck!

------------------
Mike Troyer
Performance Products, Inc.
National Distributor of Superchips
(540) 862-9515
Email: mtroyer@compuserve.com
Performance Products F150Online Superchip ordering system: F150Online Superchip Ordering System
First National F-150 Online Rally Event Organizer

 

I found out the original tire size was 265 70 R17 and now it is 305 70 R16. I do not know the height difference of these two tires. I would like to know of a URL on this subject if anyone knows of one.


[This message has been edited by skinny (edited 01-23-2001).]

 

Mike,
I don't know if you can answer this or not. I called Goodyear and found out the 265 had a 32.0 overall diameter and the 305 has a 33.0 overall diameter. The 265 has a revolution per mile of 635.13 and the 305 has a revolution per mile of 630.30. Will this make a big difference with the speedometer, if so, any idea how much? Also, I have been thinking about possibly getting a Superchip in the future. Not knowing alot about it would the chip be able to correct the tire difference?

 

Hi Skinny,

OK, so then according to your numbers, your new tires *are* in fact 1 inch taller, so your speedometer *is* going to be off.

Depending on exactly which computer code, or software revision, your computer has, we may be able to correct that for you, this is an on-going R&D project and we can correct for tire size & gear ratio changes in some of these codes. We'll need to discuss several details with you, so I would suggest just giving us a call to go over all of this with you. We're here M-F from 10 am to 7 pm, Eastern, and our number is just below.

------------------
Mike Troyer
Performance Products, Inc.
National Distributor of Superchips
(540) 862-9515
Email: mtroyer@compuserve.com
Performance Products F150Online Superchip ordering system: F150Online Superchip Ordering System
First National F-150 Online Rally Event Organizer

 

Hi skinny, here is a calculator for you:

http://www.dsm.org/tools/tiresize.htm

From what I've read in the Supercrew forum the 17" tire size that came on them was

265/70/17

So if you plug that into the calculator, it spits out the 16", 70 series with no speedo error would be = 283mm. So, it looks like a 285mm would have been perfect.

[This message has been edited by Raoul (edited 01-23-2001).]

 

Just so you know the Good year 265/70 tires are more in the 31" range than the 32".