Mixing Tire Sizes

[TomoHawk]

I'm looking at getting some sportier tires for the 280Z. The ones on there are great for touring (s-rated), but I think I rolled one during autoX.:hurt: and I wasn't even pushing it (first time out.)

The OEM size is 195/70-14

On the fronts I'd like to put 195/60-14, and on the backs 205/60-14

(Fuzion HRi, which is aFirestone/Bridgestone composite)

The front diameters would be 1.5 inch smaller, which will help with a clearance issue, and the backs should be the same diameter, so the speedo is still correct.

I bfigurethat the front might be a tad lower, but that would be a good thing, andthere shouldn't be a difference in weight distribution, balance, etc.

opinions?

thx

[Arne]

205/60-14 are considerably shorter than the 195/70-14. A typical 195/70-14 has an overall diameter of 24.8" for about 838 revs per mile. By contrast, a 205/60-14 runs about 23.7" in diameter, or about 876 revs per mile. That's about a 9% difference in rpm, so that's how far off your speedo will be. (It will be reading faster than actual speed.) Proper size 14" 60 series to maintain speedo accuracy is 225/60-14.

Something else to consider is that having different widths front and rear will alter the handling balance of the car. If nothing else is changed to compensate, narrower tires on the front will make the car understeer more.

[ZSaint]

What is the diameter of a 225-45X16?

[TomoHawk]

here is a web page that will calculate all the particulars to compare tires:

http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp

[ZSaint]

Thank you. I placed it in my favorites. I checked out the 175/65-14 tire that came on the 240Z. I put on 245/45-16 tires which are smaller than the original tires. This does not show that. It shows the 14" tire as being shorter. I wonder why?

[Arne]

Factory size on the 240Z (and 260Z too) was 175-14, not 175/65-14. When there is no aspect ratio listed, the implied ratio is 80%. So in those charts, you'd want to use 175/80-14 to get the factory sizes.

[ZSaint]

My '71 came with a new set of 175/65-14 tires. I have not idea why but that is what it came with.

[TomoHawk]

I checked out the 175/65-14 tire that came on the 240Z. I put on 245/45-16 tires which are smaller than the original tires. This does not show that. It shows the 14" tire as being shorter. I wonder why?

To begin with, your 16 inch rims are 2 inches bigger than the 14s. everything else being equal means the overall diameter (or height) is more too.

the height is calculated by adding the diameter of the rim, plus twice the sidewall, which is a percentage of the tread width. for example, if your tire is a 245/45-16, then the treadwidth is 245mm, and sidewall is 45% of 245. You'll need to convert that part to inches either first or after.

total height is

(245/25.4) x .45)*2 + 16 = 24.7 inches compared to the other one which is 23.0 inches height.

fwiw

[ZSaint]

I understand the formula. I also know the 16" tires are 1" shorter than the tires I took off the Z. (I have pics of this. Pics don't lie!) The source quoted to me states the 14" tire is shorter. I am wondering why?

[TomoHawk]

Air pressure? Tires inflated with higher pressure are usually biger than the same ones with less pressure.

[sblake01]

Air pressure? Tires inflated with higher pressure are usually biger than the same ones with less pressure.

Up to a point.

[ZSaint]

Actually, the new 16" tires were inflated to 35#. The old radials were at 32#. I think various tire companies have different specs. I remember the Michelin tires were always larger than Goodyear, Firestone, Bridgestone, etc. These 14's are some offbrand tire. Maybe they are just "large".