How do I correct for the 5MPH overage in our speedometer?

[short circuit]

All, I'm shopping for a new Winter tire/wheel set. This is what I understand our cars are, stats wise:

ET/EG/offset... +49
bolt pattern...114.3 x 5
center bore... 67.1
Base model: 17" x 7" and Style/Tech model 18" x 7.5"

I have the Tech model. So I'm considering 2 different sizes for my Winter setup based on the tire calculators also found in this section of the forum as well as what others are buying. Either 195/65 R15's -or- 205/55 R16's. The question is this: are these tire sizes "self-correcting" the 5 MPH overage or am I overthinking the issue? If not, what sizes in the 15" and 16" do I need to go to get as close to an accurate spedometer reading?

For folks who got different ET wheels, did you also have to get new bolts, or were you able to use the stock bolts? I'm thinking about an ET-40 wheel, which would push "out" the wheels slightly...wouldn't I need bolts which were 9mm longer?

TIA!

[x_Diesel_x]

I'm trying to think which tire selling website had this nifty tire/wheel diameter calculator that showed exactly how much (% wise) one size compared to OEM and such.

With that, it would be a breeze to find out what you'll need given those sizes.


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[MTD]

Tire Size Calculator - tire & wheel plus sizing

Bear in mind that even if you get a tire that is 100% perfect and "might" correct the speedo, over time when the tires wear, loss or increase or pressure, vehicle load, etc can and will through the speedo off. There really is no way of getting it 100% accurate and keeping it that way.

[x_Diesel_x]

That's the one MTD! Thanks a lot.

And yeah, it'll never be 100% accurate but you can get it to be more accurate than it is from factory.


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[Haywood Jablowme]

Little while back I installed 205-70-R15's on steel wheels. Fit perfect, no rubbing.
Over the Holiday I took a 700 mile trip to verify the changes.
Comparing my Veloster Milage to my GPS was as follows:

Miles per hour was within 1 mph at 50, 60, 70, 80
The odometer was now off by 5.6%. So when I had traveled 760 miles, it showed only 720 on my Veloster.
Keep this in mind when you canculate your milage, if you do.

[GreyGhost12]

I'm running 195/65/15's for snows on my NAV. They are exactly the same diameter as the OEM 215/40/18's and the speedo reads exactly -3 mph as it did the stockers. I'd say go with a 205/65/15 and you'll be a bit closer without "going over" in diameter.

[short circuit]

Ghost - going from 195/65/15's to 205/65/15's doesn't change diameter - it changes width of tire. It won't change speedo in the least.

My question is whether the "65" in your number needs to go to a 70 or a 60 in order to compensate for the slow speedo. And YES, I completely understand it will never be perfect. The idea here is to start off as near perfect as possible, that's all, and let it get worse from there. I have to laugh next as I type this, but member "JABLOWME" (HA!) runs 70's and has near perfect accuracy. I'll type this into the calculator and see...

Are the stock 17's and stock 18's both running slow, and both running slow as a % equally? Because when I need Summers, I'd want to do the same calculation and compensation too. Would be nice to know how much slower the 17's are as well as the 18's to the "real" speed. Has that ever been determined?

[short circuit]

Little while back I installed 205-70-R15's on steel wheels. Fit perfect, no rubbing.
Over the Holiday I took a 700 mile trip to verify the changes.
Comparing my Veloster Milage to my GPS was as follows:

Miles per hour was within 1 mph at 50, 60, 70, 80
The odometer was now off by 5.6%. So when I had traveled 760 miles, it showed only 720 on my Veloster.
Keep this in mind when you canculate your milage, if you do.

So speedometer was spot on but odometer was off by 5.6%. Correct? I'm seeking an accurate SPEEDOMETER reading. Please confirm. It would also mean that calculated (per car) miles per gallon with your setup was 5.6% off, meaning you really got 5.6% better mileage than car-calculated, correct?

[short circuit]

Stock wheels are 215/40/18 for the TECH model. Striving for a 5MPH faster tire to get the speedometer to be near-accurate, we need to change the "profile" # of the tire to compensate for the speedo difference. Using the online calculator, it seems that a proper 15" wheel would/could be 195/60/15, coming in 2.310% too fast compared to stock. That's pretty close.

The 205/70/15 wheel suggested by HAYWOOD JABLOWME, when compared to the stock 215/40/18 came in 6.143% too slow, exacerbating the issue of the speedo and opposite of what I want to achieve and at odds with his GPS.

So what is correct???

[Haywood Jablowme]

Right now a stock speedo is approximately 6% fast. Do a search, there are plenty of threads on it.
So at 70 mph it shows you at 74mph. The tires fixed that because they are close to 6% bigger.
I verified this several times at each speed. 50, 60, 70, and 80 mph.
i.e. one revolution of the tire now takes you 5.6% further. The speedo runs off of the transmission, not the tires.
This is for the SPEED indicator, the needle, not the odometer, which is the distance traveled, not the speed.

When the tires are bigger, they go further then they do if they are smaller, so, at 2500 RPM you are now traveling 5.6% further, but the car does not know it. That is why the odometer is off. The distance traveled. So when you have driven 300 miles on the odometer, you have actually driven 316.8 miles. You need to take this into consideration when you calculate your gas mileage because you have driven 5.6% further than the odometer indicates.


Each data point was taken three times.
Unless my GPS was off, the numbers jive perfectly.

[short circuit]

Right now a stock speedo is approximately 6% fast. Do a search, there are plenty of threads on it.
So at 70 mph it shows you at 74mph. The tires fixed that because they are close to 6% bigger.
I verified this several times at each speed. 50, 60, 70, and 80 mph.
i.e. one revolution of the tire now takes you 5.6% further. The speedo runs off of the transmission, not the tires.
This is for the SPEED indicator, the needle, not the odometer, which is the distance traveled, not the speed.

When the tires are bigger, they go further then they do if they are smaller, so, at 2500 RPM you are now traveling 5.6% further, but the car does not know it. That is why the odometer is off. The distance traveled. So when you have driven 300 miles on the odometer, you have actually driven 316.8 miles. You need to take this into consideration when you calculate your gas mileage because you have driven 5.6% further than the odometer indicates.


Each data point was taken three times.
Unless my GPS was off, the numbers jive perfectly.

HAYWOOD, I believe you've cleared it up for me - the speedometer registers too fast for the "real" actual speed of the car. That means I need to get a tire which pushes the speedo SLOWER, not faster. That would mean that the 205/70/15 wheel suggested by you, when compared to the stock 215/40/18 (the 70 profile tire came in @ 6.143% too slow) would be ideal. And therefore, for a 16" tire, in case anyone is interested, lets say compared to the 215/40/18 stock size, the 205/55/16's are "only" 0.406% too slow. Not good enough... You would need to get a tire with a "profile # of 65, not 55 (for 16" tires). That would give you a "6.940% too slow" reading. A profile # of 60, according to the online calculator, provides for a "3.679% too slow" reading - also NOT BAD and a doable choice as well.

In summary, for a TECH tire of stock 18", if you're interested in correcting for the speedo inaccuracy, one should get a PROFILE of 70 for a 15" wheel and a PROFILE of 60 (or 65) for a 16" wheel.

Somebody, please confirm these specs as I've not had enough coffee this morning yet.

[GreyGhost12]

Ghost - going from 195/65/15's to 205/65/15's doesn't change diameter - it changes width of tire. It won't change speedo in the least.

You may want to revisit how a tire size is determined. A 205 will in fact increase diameter as compared to a 195 of the same aspect ratio. Why? Because the aspect ratio of 65 is a percentage of the section width of 205. Last I checked, 65% of 205mm is a larger number than 65% of 195mm. This will indeed increase the over all diameter of the tire...

This information can be found all over the web...

[MTD]

My brain hurts reading all that.

[GreyGhost12]

My brain hurts reading all that.

Since I have no brain, I have NOOOO pain. Life is good!!

[short circuit]

GREY, with age comes wisdom. You are indeed correct! When doing the online tire calculator compares, I used stock size of 215/40/18. I then compared that to 195/65/15 and 205/65/15. The 195/65/15 comes in @ 0.852% too slow (not good for what I want to achieve), and the 205/65/15 comes in @ 2.904% too slow (much better for what I want to achieve)! So its the ratio, and percentage of eachother that matters.

Thanks for teaching me something new today. Lots of options and choices, and of course, needs to be bounced against tire costs. Tweaking tire sizes may very well save you a few dollars while accomplishing the goal of making the speedometer more accurate to the driver. I'll next head over to tirerack and compare costs of these different options.

My head hurts too!!!

[jrhartman]

Has anyone determined why the speedo is so far off from the factory? Or brought it back to the dealer for them to "fix"?

I have only had my VT two weeks and noticed it was off by quite a bit at 70mph and was going to bring it in for the dealer to explain/correct. I shouldn't have to buy brand new tires to correct an inaccurate speedo IMHO.


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[Haywood Jablowme]

manufacture of cars always have it show a little high.
Mostly because it can vary from tire to tire.
If they show it high, you slow down. you don't get a ticket. you get better gas mileage.
If they tried to set it dead nutz, and it was low, you would speed up, get worse gas mileage, and probably a ticket.
Then try to blame the car company cuz your spedo was off.

Tire size = first number is width in mm, second number is % of the width, third number is wheel size in inches.
I have no f#cking idea why the outside diameter and width are in mm and the inside diameter is in inches.

example given 205/70/15
Tread is 205mm wide, from the rim to the od is 70% of 205mm, they fit on a 15" rim.

[GreyGhost12]

GREY, with age comes wisdom. You are indeed correct! When doing the online tire calculator compares, I used stock size of 215/40/18. I then compared that to 195/65/15 and 205/65/15. The 195/65/15 comes in @ 0.852% too slow (not good for what I want to achieve), and the 205/65/15 comes in @ 2.904% too slow (much better for what I want to achieve)! So its the ratio, and percentage of eachother that matters.

Thanks for teaching me something new today. Lots of options and choices, and of course, needs to be bounced against tire costs. Tweaking tire sizes may very well save you a few dollars while accomplishing the goal of making the speedometer more accurate to the driver. I'll next head over to tirerack and compare costs of these different options.

My head hurts too!!!

You're welcome.

Now, to add to all the head-hurting... Not all manufactures adhere 100% to these standards as there really is no "set standard" when it comes to tire sizes. I'm sure you're scratching your head now...

Ok, just because a 205 is a 205, it doesn't actually "make it" a 205. Some OEM's use the actual section width (inside to outside belly of the tire) for their 205 denomination and some use the actual foot print or tread width and some use a value somewhere in between these two. So then the aspect ratio (and actual virgin tire diameter) becomes somewhat skewed in relation to this first dimension. So if you compare brand X, Y, and Z, all are 205/65/15's, they may or may not all be the same exact starting diameter and footprint.

I've researched tires for 20 years and found many anomolies along the way. What you will need to do is dig down into the nitty-gritty to see if one brand of given tire size is closer to what you're actually looking for in diameter. I've seen a variance as much as +/-.5" overall diameter between several tire manufacturers all claiming to be of the same exact tire designated size.

Online calculators are great but don't give 100% of the story here...

Just a heads-up for more of a headache generator than you were previously expecting...

[jrhartman]

manufacture of cars always have it show a little high.

All of the cars that I have owned this side of 2000ish is model year have been pretty damn spot on.....this one is is terrible.

[MTD]

Unfortunately there is no way of making it 100% accurate and stay that way. There are way too many factors that can change the accuracy.