PO had P205/70 R14s going. I need to replace at least two of them. Should I keep that size running or switch all four to a different size while i'm at it?
Kev
Thanks

If it's a 91+ with ABS brakes, keep the tire size the same, even if it's wrong. The ABS computer will generate a speed sensor fault if one wheel is the wrong size, or front to back are different diameters. It may not fault at low speeds, but a quick trip to highway speed will generate a fault that will not clear until the key is cycled off and on.

If it's an older non-ABS car, it won't matter beyond upsetting the handling dynamics of the car. Clearly the most predictable situation is 4 of the same size. The car can be driven with different sizes. If it's a wagon, the 205/70 is not really a bad choice. If it's a sedan, they're rather large and will give speedo error as described in another post: you'll be going faster than you think.

The car will work pretty well with quite a wide range of sizes from 175 to 215 in width and varying section heights. The only condition that should truly be avoided is having different sizes left to right. Sometimes different brands left to right will generate a pull - if you must mix and match brands, do it on the rear axle. Good luck with whatever you decide.
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Rob Bareiss, New London CT ::: 92 244-M47-208K ::: Bilsteins, Turbo swaybars, Virgos, Lowered, Group A wing, Tach, 6-Disc, Satellite Radio, Keyless Entry ::: Coming Soon: B-Cam!

If you don't want to have to go to a calculator every time, here's the way to do it manually, but just as precisely (for tire comparisons) as any website calculator:

Remember that:
1) the heights of the sidewalls is the section width (e.g., 205) times the aspect ratio (e.g., 0.70), right? (That because the aspect ratio is the height divided by the section width)
So, why do you need the height of the sidewall? To fill in a number for this second part:
2) the diameter (and circumference, pi, of the diameter) is just the distance across the diameter of the wheel plus the heights of two sidewalls, right?

Lay a ruler across a tire ... the ruler spans two tire section heights and the wheel's diameter, right? If you think of this, it will become so obvious, you'll never, ever forget it. It's a real "Doah!" (think Homer Simpson).

The only thing to deal with is the conversion of inches to mm, because while the wheel diameter is in inches (e.g., 14 inches), the other tire measurements are in mm, and you have to equate them. Remember that 1 inch = 25.4 mm.

Now, doesn't this make sense?!

So, to find the diameter (D) of any tire size, such as 205/70-14, here's the entire calculation in one shot:
D = (14 x 25.4) + (2 x (205 x 0.70))

Or, if you want to do it one step at a time, (14 x 25.4) = wheel diameter, in mm = 355.6 mm
and (205 x 0.70)) is the tire section height = 143.5 mm
so,
355.6 + (2 x 143.5) is the total diameter, or 642.6 mm

We don't need no stinkin' website calculator!!! Yeah!!!

sedan stock size = 185/70-14
wagon stock size = 185/R-14 = nearly impossible to find

I've settled on 195/75-14 as my own preferred size for 240 wagons when using a 14 inch rim. Summer or winter.

my preferred tire size calculator:
http://www.wickedbodies.net/Tire-Size-Calculator.htm

This one accepts "R" for the middle value (aspect ratio).
It uses 82 as a value for R.
I've also seen 80 used as a value for R so you can plug that in as the stock value if you like. [If you check 'specs' on tirerack.com's tires in size 185/R-14 you can find the tires' dimensions, calculate backwards, and learn that they all are actually 185/80-14's.] I've seen + heard 78 quoted as = "R" but I just can't buy that. Too far from the 80 and 82 I keep seeing elsewhere.

I used the above calculator to get its results re. 205/70-14 and decided it's a bit small (diam) for my taste. I did purchase a set in that size once, the car drove fine. It's larger in diameter than the 185/70-14's that many wagons are rolling on.

I've had no problems with mixed sizes. I've always put the size closest to stock diameter on the rear wheels - for speedometer accuracy and optimum load capacity. That is, load capacity increases with diameter so the smaller diameter tires aren't ideal for a loaded wagon. The smaller stock sedan size is obviously able to carry the car's front end and handles fine, so I'm OK with that in the front when I've inherited somewhat smallish tires on a new-to-me wagon.

--
Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.

I run that size, 205/70-R14, on the rears of my 1986 244GL. They produce a speedo error of 4.6% in the dangerous direction, speedo sez 70mph, radar trap will see 75 or so. So far no trooper problems.

What you want will depend on the stock size for your car. There should be a label on the rear edge of the RF door with that info.

To know that size without that label, the BB folks need the year and model of your brick. Tire stock sizes change between models and between years.

The 205's run fine on the rears. I put them on to raise the read end a little, the exhaust was scraping the pavement on a sharp drop-off driveway that was in my regular path to take Grandma home.

As a rule of thumb, speedo error will be minimized if you change tires such that a one step increase in the first number is matched with an one step change in other number, in the opposite direction.

Example: Suppose you stock size is 185/70-R14. A change to a 195/65-R14 tire gives an error of 0.9% fast; a change to a 205/60-R14 gives an error if 2.1% fast. Fast means that speedo reads faster than actual car speed. Safe.

Go to http://www.miata.net/garage/tirecalcold.html for my favorite tire size calculator. Note that if your stock size has no aspect ration number (the 70 or 75 or 65) it can be considered an 80 or 82.

Good Luck,

Bob

:>)