Kizashi Club

Changes to the overall tire diameter also changes the MPG due to the changes to the drive ratios.

Does anyone know if the CVT in the Kiz compensates for a slightly smaller overall diameter tire? I am contemplating going to a 245/40ZR18. Shorter by about .25 inches resulting in about 14 more revolution per mile.

It won't be able to make any adjustments for the tire size. With the 245/40 you will have only about a 2% difference so I wouldn't even worry about it. Your driving style and tire pressures will have more effect on mpg than those tires.

Thanks puckaveli. You sound like a tire guy like myself.

Changing from a 195/65R15 to a 215/45ZR17 made me lose about 2 MPG average in my '09 Corolla with a 5-speed. 34 vs 32 MPG. About a quarter of an inch difference in diameter.

I figured any MPG loss would be attributed to the amount of freeway speeds (55MPH +) traveled on a slightly smaller diameter. I'm hoping the CVT is more forgiving than a conventional 5 speed. Not impressed with the OE Dunlops.

You should get slightly improved acceleration.

twoqttsdad wrote:
Changing from a 195/65R15 to a 215/45ZR17 made me lose about 2 MPG average in my '09 Corolla with a 5-speed. 34 vs 32 MPG. About a quarter of an inch difference in diameter.

What this shows is that a tire's width has the biggest effect on mileage. In this case, going to a 215 from a 195 tread width. Wider tread increases friction, as well as increasing frontal area which negatively affects aerodynamics.

Another thing to consider: even if you keep the same width, a taller tire will weigh more.

1) Do NOT decrease tire diameter UNLESS you live someplace flat, 'cause the decrease in ride height could be devastating (like when yer rig hits tall road-crap).

2) Handling can be adversly affected (for myriad reasons, all unfortunately valid).

3) You might accelerate more quickly, but you might not; testing required.

4) A careful study of tire weight (at www.tirerack.com) can provide one with brand choices that can significantly lower unsprung weight! Good thing.....

5) It really does matter most the style and environment of operation. Ay?

A low weight tyre might not necessarily be a good thing. It could mean softer sidewalls, or the tyre mightn't have as many ply layers (and be more prone to punctures or other damage.)

Check the math versus mine. I like to think I'm a tire guy too. I know that I have to keep reminding myself that the second number is a percentage (aspect ratio) of the first and not an actual measurement number in millimeters. Also I understand it to be, that when you decrease the sidewall you need to multiply times two the reduction (or increase) to get the effect on diameter.

According to my calculations assuming you currently have 235/45/18

Proposed tire 245/40/18 equals sidewall (40% of 245) of 98 mm

Current tire 235/45/18 equals sidewall (45% of 235) of 105.75.mm

Difference in sidewall is 7.75mm but you need to multiply that times two (right???) because from top to bottom of tire you are losing 7.75 mm in each area, thereby reducing the overall diameter by 15.5 mm

This equals .610 inch smaller diameter which is a little over double your estimate. It will make acceleration slightly quicker but you also will be turning higher rpm at your highway speeds most likely leading to lower mileage.

I know you are a tire guy, but double check what I "think" the calculations are with a tire shop.

If I got this right, Is this really worth a 10mm tire width change??? Is it really going to make the appearance, handling, etcetera that much more noticeable or any better??

Honestly, unless you drive your car at the edge of it's traction capabilities a lot you will not notice a difference in tire weight, a wider tire of the same brand providing a bigger contact patch, or a gain in acceleration from smaller diameter. And that isn't spirited driving on public roads, that's hours upon hours on tracks or auto-x.

And the wider tire and smaller diameter might make a difference small difference in mpg's but unless he drives the same route the exact same way everyday he won't notice much of a difference.

To the original poster - get the 245/40 if it makes you happy. The science everyone is using is correct, you seem more concerned with mpg than performance so unless you micro manage your fuel dollars down to pennies get the tire that you want.

Thanks for all the input guys. It helps to see the picture.

I wouldn't recommend calculating a tire's overall inflated diameter based on it's service description. Try looking up the specs on the manufacterer's web or book.

For example a Toyo Proxes 4 Plus 245/40ZR18 is spec'd at 25.9" overall diameter.
The Toyo Proxes T1 Sport of the same size is spec'd at 25.7"

If anyone is really interested about learning about tires try reading up on the subject on this website. He's a tire engineer and goes by the name of Capri Racer on other websites because he used to race back in the day. Very smart and polite and takes the time to explain ina way that everyday guys like ourselves can understand. If you can understand all of his material consider yourself a tire techie! I manage a tire warehouse and am still learning everyday!

http://www.barrystiretech.com/index.html

You can also take a look at this web which is one of many that is helpful, but again I would always go to the manufacturer to the actual specs.

http://www.miata.net/garage/tirecalc.html

I am probably going to go to a 245/45 or 245/40, just wanting a little extra width. Just curious if the CVT is able to keep the same highway cruising rpms when switching to a tire that's slightly shorter vs a conventional 5 speed with a fixed final gear ratio.

Thanks!