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Discussion Starter #1
The speed on the speedometer is not the actual speed; it's less. When the needle indicates 70mph, I am acutally going about 68ish. This also means the odometer is racking up more miles than it's actually gone.

Kia won't do anything; the automotive industry is allowed up to a 5% error.

When it comes time for a new set of wheels, I am going to go one (1) size larger, which should effectivly bring the speedometer back to where it should be. The larger size wheel goes a further distance on each revolution, so that should fix it.

I wonder if the larger wheel will impact other things like speed sensors, computer input etc.

Anyone have any thoughts on this?
 

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All the Kias I have owned have the same problem -- faster speedometer than actual speed. Your idea appears sound...slightly higher wheel size to compensate. Shouldn't affect the computer either.
 

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Discussion Starter #4
It has worked in the past with cars with cable driven speedometers. I think the larger size tire would give the Forte and even more aggresive stance!
 

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The speed on the speedometer is not the actual speed; it's less. When the needle indicates 70mph, I am acutally going about 68ish. This also means the odometer is racking up more miles than it's actually gone.

Kia won't do anything; the automotive industry is allowed up to a 5% error.

When it comes time for a new set of wheels, I am going to go one (1) size larger, which should effectivly bring the speedometer back to where it should be. The larger size wheel goes a further distance on each revolution, so that should fix it.

I wonder if the larger wheel will impact other things like speed sensors, computer input etc.

Anyone have any thoughts on this?

The % error in the auto industry is actually 10% ... it's normal.. My 03 Tahoe and my 95 Honda Accord also have this discrepancy..
 

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The speed on the speedometer is not the actual speed; it's less. When the needle indicates 70mph, I am acutally going about 68ish. This also means the odometer is racking up more miles than it's actually gone.

Kia won't do anything; the automotive industry is allowed up to a 5% error.

When it comes time for a new set of wheels, I am going to go one (1) size larger, which should effectivly bring the speedometer back to where it should be. The larger size wheel goes a further distance on each revolution, so that should fix it.

I wonder if the larger wheel will impact other things like speed sensors, computer input etc.

Anyone have any thoughts on this?
Before upgrading to 215/40 on 18 inchers (from 215/45-17 on OEM Koup SX wheels), my speedometer was a shade over 5% off (GPS cross-checked); actual was 94.7 Kph at an indicated 100 Kph. I'm now just over 1% off, actual 98.8 Kph at 100 Kph indicated.

If you stay within 5%, no problems. Even if you go larger than 5% difference, the car and its sensors won't know and won't care as long as all 4 wheels are the same circumference (no front-to-back stagger). Just means your speedometer error gets larger and potentially a problem if the constabularies in your area are particularly fascist...

Use 2PiR (2 X 3.1416 X total radius of wheel and tire) to calculate theoretical circumference for comparison.

Ex: Koup baseline, 195/65 on 15 inch:
Radius: 15 inches x 25.4 mm/inch = 381 mm + (195 X .65 = 126.75) / 2 = 253.875 mm

Pi is a constant 3.1415929... but 3.1416 is good enough so:

2 X 3.1416 X 253.875 = 1995.15

OEM SX wheels+tires 215/45-17 is 1936.16, which is 2.86% less than baseline;

My 215/40-18s are 1976.69 which is 0.8% less than baseline.

The closer to baseline, the smaller the speedometer error.

Those numbers are theoretical; actual numbers will differ slightly because the tires are not exactly and precisely XXX mm wide with a perfect XX% sidewall height. Go to Tire Rack on line and click to compare the tires you're considering; one of the criteria in the comparison chart is the number of turns per mile for each tire. You can then arrive at some pretty close numbers to base your decision on.

******
 

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I actually like this discrepancy. I ignore it and speed just a bit less that I normally would and am therefore just a bit less likely to get a speeding ticket.
 

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Yea I fairly like the discrepency as well, I cruise control around 70 and even though cops here don't really pull you over for 70, I have that extra feeling of confidence thinking I'm probably really going like 68 or so , but its better to have it be over what you're actually going than under imo... also depending on the tire pressure in your tires would also affect the speedometer accuracy..
 

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When I was winter tire shopping I was 'late' as I bought the car in December, and was limited in sizes available in the tire I wanted. I ended up going to a 205/50 from the 215/45 (increase of +1.8%) and that made my speedo exactly right. And yeah, the wheel well is 'filled out' a little nicer (yes I do see the difference 1/4" makes), as well as the 205 in the brand I went with is actually wider in the tread than the 215 LS2's are... go figure.

Just got lucky, I guess.
 

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@******, you need to divide the wheel size by 2, not the sidewall width by 2 when calculating the radius. For example, the theoretical radius calculation for 15 inch and SX tires are:

baseline: 15 inch x 25.4 mm/inch /2 + 195 x .65 = 317.25 mm
SX: 17 inch x 25.4 mm/inch /2 + 215 x .45 = 312.65 mm

Thus, SX radius is 98.6% of the baseline, which means ==> on SX, 70 mph display means you are actually going at 68.98 mph or 69 mph - assuming that your speedometer has been perfectly calibrated for the baseline tire. In reality, however, the speedometer is never so perfectly calibrated for each car, and each car may have slightly different discrepancy ranges.
 

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Discussion Starter #11
KiaTech, I am not sure what size is actually the next size larger. I take it from your post the 205/50 is the step up from the 215/45.

I was looking at the front strut, and saw that the larger tire would be getting close to the strut component. I assume you have no problems with tire rub.

Can you post a picture of how your car looks with 205/50’s?
 

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actually a 215 50 would be slightly bigger.. a 205 50 would be about the same as a 215 45
 

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I don't have a picture to post from work, so if I remember tonight I'll toss one up when I'm home.

Here are the sizes that will fit our stock rims, and be considered 'safe' fit.

215/45-17 is stock - 77.31 in
205/50-17 is my winter - 78.75 in (Car goes 1.9% faster)
215/50-17 a logical choice - 79.98 in (Car goes 3.5% faster)
225/45-17 my personal choice for summer rubber - 78.44 in (Car goes 1.5% faster)
225/50-17 WAY too big... HUGE! - 81.21 in (Car goes 5.0% faster)

So you can see that the 205/50 and 225/45 are both 'just a hair' bigger, but not by much. Neither has any issues with fit, rubbing, etc. There is actually a lot more room under there than it seems. The advantage of the 225 is it is a little wider in the tread (depending on tire) as I find the Goodyears to be a rather narrow tire compared to other 215's.

The 215/50 is actually a considerable size larger, and not recommended by me, but you can do what you like.
 

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Wheel circumference

Why would you not just measure the tire width, tread surface to tread surface across center and multiply that times Pii.
Pii times diameter = circumference. No more theory, actual values.





Use 2PiR (2 X 3.1416 X total radius of wheel and tire) to calculate theoretical circumference for comparison.

Ex: Koup baseline, 195/65 on 15 inch:
Radius: 15 inches x 25.4 mm/inch = 381 mm + (195 X .65 = 126.75) / 2 = 253.875 mm

Pi is a constant 3.1415929... but 3.1416 is good enough so:

2 X 3.1416 X 253.875 = 1995.15

OEM SX wheels+tires 215/45-17 is 1936.16, which is 2.86% less than baseline;

My 215/40-18s are 1976.69 which is 0.8% less than baseline.


******
 

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To get diameter of a tire you don't have in front of you, you need to do the math like ****** did. If he had a selection of tires I'm sure he just would have grabbed a tape measure and gone nuts.

As for theory vs. reality... as soon as a tire is loaded, it's diameter changes. Theory goes out the window and reality sets in. This is where inflation, wear, loading, the actual tire's design and even alignment can all play into each car's unique tire circumferences, which ****** could not estimate. If you are swapping tire sizes on your own car, then most of those unique variables remain constants, and his theoretical numbers become the reality based solely on the actual replacement tire fitting the design standards for it's size (which most actually don't).
 

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Alternate method

Again, I tend to go with imperical data.

I have also marked a spot at bottom dead center on a tire, make a mark on the driveway, street, whatever, straight down from that mark, roll forward one revolution till the tire mark is again at BDC, make a second mark on the pavement, move the vehicle, measure the distance between the two marks. With the exception of the tire expanding from centrifugal forces at highway speeds, this would also give you a simple way to measure without all the math..... whatever works for you is the right way.

I do have one question. Where is the rim measurement taken? Is it at the outside edge of the rim and is truly the finished diameter of the rim, or is there some overlap between the rim and tire that would affect a stacked up measurment? I really don't know and don't have a tape measure with me here at work.





To get diameter of a tire you don't have in front of you, you need to do the math like ****** did. If he had a selection of tires I'm sure he just would have grabbed a tape measure and gone nuts.

As for theory vs. reality... as soon as a tire is loaded, it's diameter changes. Theory goes out the window and reality sets in. This is where inflation, wear, loading, the actual tire's design and even alignment can all play into each car's unique tire circumferences, which ****** could not estimate. If you are swapping tire sizes on your own car, then most of those unique variables remain constants, and his theoretical numbers become the reality based solely on the actual replacement tire fitting the design standards for it's size (which most actually don't).
 

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An interesting question. My trusty tape measure and a dismounted LS2 tell me that the 17" is measured across the tire, from the inner-most edge of the bead, to the other side. Basically, if you had a cylinder exactly 17" in diameter, it would just barely fit in the tire, but it would fit.

Measuring a 18" on a Sorento that happens to be sitting here, it's 19.5" top to bottom of the rim, only.

Fun facts: an LS2 215/45-17 tire is 8" across, and has 3.75" of absolute sidewall. So 3.75x2+17= 24.5" tall. Out of that 3.75" of sidewall, only 3" is actually visible once the tire is mounted.
 

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That's the part I was wondering about and didn't think to look at it this weekend. The problem with stacking up measurements is that it can cause errors when there are overlaps. Sound like about 3/4" or so of overlap between rim and tire bead. So, maybe the best way is to measure the distance the tire travels in one revolution with the weight of the car on it. Should provide a very accurate comparison from tire to tire.





An interesting question. My trusty tape measure and a dismounted LS2 tell me that the 17" is measured across the tire, from the inner-most edge of the bead, to the other side. Basically, if you had a cylinder exactly 17" in diameter, it would just barely fit in the tire, but it would fit.

Measuring a 18" on a Sorento that happens to be sitting here, it's 19.5" top to bottom of the rim, only.

Fun facts: an LS2 215/45-17 tire is 8" across, and has 3.75" of absolute sidewall. So 3.75x2+17= 24.5" tall. Out of that 3.75" of sidewall, only 3" is actually visible once the tire is mounted.
 

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Sorry to bump a old topic but I think its important for others to know because I was about to go to Kia.

I been driving my car past a your speed check and it has been off. Finally tonight I broke out the GPS and I was still off. Then had my buddy go next to me and was still a little off.

I searched it and found this thread know I know im not crazy and it saves me the hassle of them telling me there is nothing I can do about it.
 
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