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  #1  
Old 09-05-2012, 07:38 PM
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Speedometer Issue 20 inch Wheels

Hey Guys,

Just a quick question. On my Laredo X I originally had the 18 inch stock wheels and purchased a set of Overland 20's (off someone from the forums they are great btw) and they have Nexen High Performance size 275/45/20 which is a different size from the factory tires that come with the rims.

For some reason it seems that my speedometer is inaccurate. When it says I am doing 42 mph I am actually doing 40 mph confirmed by GPS and Radar ( it is off by 2mph faster than actual speed)

I have some questions.

1) Is this an issue that I should be experiencing or anyone else is?

2) If so, is there a way to recalibrate or fix it?

3) Is this adding more miles to my car than I actually have?

4) Is it affecting my EVIC mpg rating?

Thanks in advance for anyone's help on this much appreciated.

Regards,
Jared
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Old 09-05-2012, 09:06 PM
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Re: Speedometer Issue 20 inch Wheels

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Originally Posted by jdchamp31 View Post
Hey Guys,

Just a quick question. On my Laredo X I originally had the 18 inch stock wheels and purchased a set of Overland 20's (off someone from the forums they are great btw) and they have Nexen High Performance size 275/45/20 which is a different size from the factory tires that come with the rims.

For some reason it seems that my speedometer is inaccurate. When it says I am doing 42 mph I am actually doing 40 mph confirmed by GPS and Radar ( it is off by 2mph faster than actual speed)

I have some questions.

1) Is this an issue that I should be experiencing or anyone else is?

2) If so, is there a way to recalibrate or fix it?

3) Is this adding more miles to my car than I actually have?

4) Is it affecting my EVIC mpg rating?

Thanks in advance for anyone's help on this much appreciated.

Regards,
Jared
1) Yes, if you change the size of the tires without letting the computer know, you will affect all distance/speed related calculations.

2) I'm not sure which tuners are available for the new WK2s, but you should be able to recalibrate the tire size either through the dealer or through third party hand-held scanner/tuners

3) It is likely having the opposite effect. If your tires have a larger diameter, you would travel further every rotation than the vehicle computer would think. You would have to calculate the diameter of the tire and compare it to stock to figure this out more precisely.

4) Yes, but the MPG calculations are inherently noisier than the difference the tire size would introduce. I wouldn't be concerned about this.

That all being said, I have found speedometers of completely stock vehicles to have a MPH or two standard deviation when compared to GPS speed estimations. Even calibrated, you may notice it is a little off when compared to a GPS tracker.
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Old 09-05-2012, 09:14 PM
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Re: Speedometer Issue 20 inch Wheels

How's this for my first post?

Check out this tire size calculator: http://www.miata.net/garage/tirecalc.html

I've used it for years and found it very helpful in showing comparisons between tire diameter and how the speedo is affected.
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Old 09-05-2012, 09:44 PM
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I believe that the original tires are 265/45/20. Here are the results from the calculator.


Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
265/55-20 5.7in 15.7in 31.5in 98.9in 641 0.0%
275/45-20 4.9in 14.9in 29.7in 93.4in 678 -5.5%

It shows -5.5% difference.

Regards,
Jared
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Old 09-05-2012, 09:46 PM
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Actually I am not sure of the original tire size.

Can anyone confirm what the stock tire size is on the 20 inch overland rims?
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Old 09-05-2012, 11:25 PM
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Re: Speedometer Issue 20 inch Wheels

Quote:
Originally Posted by dr.lee.baugh View Post
1) Yes, if you change the size of the tires without letting the computer know, you will affect all distance/speed related calculations.

2) I'm not sure which tuners are available for the new WK2s, but you should be able to recalibrate the tire size either through the dealer or through third party hand-held scanner/tuners

3) It is likely having the opposite effect. If your tires have a larger diameter, you would travel further every rotation than the vehicle computer would think. You would have to calculate the diameter of the tire and compare it to stock to figure this out more precisely.

4) Yes, but the MPG calculations are inherently noisier than the difference the tire size would introduce. I wouldn't be concerned about this.

That all being said, I have found speedometers of completely stock vehicles to have a MPH or two standard deviation when compared to GPS speed estimations. Even calibrated, you may notice it is a little off when compared to a GPS tracker.
I tried to quote the OP and ^him.

Just from doing some easy (not 100% accurate) math in my head, it seems like all the formula did was subtract the new circumference from the original tires circumference. Which gives 5.5, which isn't really 5.5%, but is close enough.

You are saying that your MPH is 2 higher than 40 mph which is roughly 5.5%. So it seems fairly accurate. But this is where I disagree with Dr. on your #3 question. If the circumference your new tires is less than your old, your new tires revolution is faster (giving you your increased MPH), which also means your tires take less time to make a full revolution. So if your tires are taking less time to make a full revolution, you mileage will go up faster, just like MPH.

I hope that helps and is accurate.

edit: had -5.5 when I meant 5.5
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Old 09-05-2012, 11:38 PM
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Re: Speedometer Issue 20 inch Wheels

Stock 20s are 265/50 R20 or 265/60 R18, both of which have same diameter which is 30.5 inches. The 275/40s would have a 28.7 inch diameter, which is a 6% change. Your speedo will now read 6% faster than you would have with the OEM tires. So if your speedo was 40 mph with OEM tires, it would read 42.3 mph with the smaller tire plus or minus tolerances. Your odometer will also be 6% off (recording 6% more miles than actual). Not sure if the dealer can adjust the speedo for that much of a tire change. Yes it will affect the mpg reading, probably by 6%.

I like the wheel tire tool at http://www.rimsntires.com/specs.jsp?item=Wheel-Tire
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Old 09-06-2012, 01:50 AM
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Re: Speedometer Issue 20 inch Wheels

Usually a deiviation of the speedometer is only allowed in being too fast, never to slow, thus, as well to compensate for wear speedometers usually about 1 to 2 miles to fast..(helpful for speed controls as well...)

Happy Day

Achim
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Old 09-06-2012, 05:50 AM
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Re: Speedometer Issue 20 inch Wheels

Here are my computations on the stock size - 265/50-20 and OPs size 275/45-20. This is about a 2% difference, so there is probably some inherent inaccuracy in the spedo going in the same direction.

20+(((265*.5)/25.4)*2)=30.816
20+(((275*.45)/25.4)*2)=30.102
Wheel diameter
Tread width
Aspect ratio
Millimeters per inch
Gotta measure both sidewalls on opposite sides of the wheel
Diameter

Some have indicated you can recalibrate the spedo - how do you do this? If I could recalibrate the spedo easily, I would switch to the 275/55-20 in a heart beat. I appreciate your input.
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Old 09-06-2012, 08:41 AM
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Re: Speedometer Issue 20 inch Wheels

Oh, I used the wrong aspect ratio, size calculator says 275/45 -20s are 29.8 diameter which is a 0.7 inch smaller or about a 2.2% difference. Thats only about a 1mph difference a 40mph

To change the speedo cal, the dealer has to enter a new tire factor into the computer via his star scan system. There are limits, however.
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Old 09-06-2012, 09:09 AM
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Re: Speedometer Issue 20 inch Wheels

Quote:
Originally Posted by ChrisOC View Post
I tried to quote the OP and ^him.

But this is where I disagree with Dr. on your #3 question. If the circumference your new tires is less than your old, your new tires revolution is faster (giving you your increased MPH), which also means your tires take less time to make a full revolution. So if your tires are taking less time to make a full revolution, you mileage will go up faster, just like MPH.

I hope that helps and is accurate.

edit: had -5.5 when I meant 5.5


I think we are saying the same thing, just describing the two possible scenarios

To me, it sounded like the tire diameter was getting larger (moving from 18" rims to 20" rims), though I certainly did not calculate the radius. Larger tire = slower rpm = reduced speed/distance travelled.

You are describing the situation of if the tire diameter was smaller than the computer expects, which obviously would lead to the opposite results.

Depending on the actual differernce in tire size one of these and the inverse of the other will certainly be true. It sounds like the vehicle came with the stock 18's so would be calibrated for those. It looks like some of the above calculations are based on the difference between the tire originally on the stock 20" rim, and the 20" tire he has put on the 20" rims. The computer will still think it has the 18's...not sure if there is a difference between the stock 18's and the stock 20's...so it may or may not make a difference!

As everyone has pointed out, we need to know the difference in circumference between the rim/tires that were originally equipped and the new rim/tire combo.
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Old 09-06-2012, 09:28 AM
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Re: Speedometer Issue 20 inch Wheels

All the WK2 OEMs, whether the be 18s, 17s, or 20s, have the same overall tire diameter because they all use different aspect ratio tires. The 20 inch has bigger wheels but less tire side wall, the 17 inch wheels have less wheel but more side wall. The tire diameter comes out the same in each case (a 20 inch wheel plus 10.5 inch in sidewall is the same overall diameter as a 17 inch wheel plus 13.5 inch in sidewall which is the same overall diameter as a 18 inch wheel with 12.5 in sidewalls.). Depending on the tire, a 22 inch wheel could have a much smaller diameter than an 18 inch wheel.

Because all OEM wheels/tires have the same overall diameter, the star scan tool may not provide the tech with WK2 tire factor adjustments. Other chrysler vehicles have a wider variety of tire sizes but the tool only lets the dealer pick one of the optional sizes. Clever techs discovered some work arounds, but those may not work here.
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