For the mathematicians

[Funracer]

I have 4.10s and never corrected the speedo. If I go to 18 or 19"
rear wheels that would make my speedo nearly correct again, right?

[efnfast]

No, because the diameter of the tires will be about the same, you don't ride on the rims.

[XfireZ51]

No, because the diameter of the tires will be about the same, you don't ride on the rims.

Steve,

The 19" tires are about 1+" taller than stock 27 v 25.7". I have 3.73s and am using the stock speedo gears in the trans. My speedo error is ~3mph at 70 w the speedo reading lower than the GPS. 4.10s may put you right there w the taller tires.

[efnfast]

Ultimately it is all based on the rolling diameter of the tire regardless of rim size. Well, plus all the other ratios.

I'm also running 3.73's with stock tires and rims. I'm assuming I have speedo correction. Mine is off about 3 mph at 70. So when the speedo said 140, I like to say I was doin' a buck 45.

Dom, your 19's are 1" taller. By choice? Or is that the lowest profile available?

[-=Jeff=-]

with my 18" and a 4.10 I used the Speedo correction gears from Haibeck and my speedo seems to be within 1 mph or so (Have not checked it lately but easy enough to do)

[XfireZ51]

Steve,

Didn't choose a particularly taller tire. The Nittos I have are ones that a good number of Z06 owners use when replacing their RF tires. What rolling radius is and how it compares to stock size tires I am not certain. We just replaced the driven gear from a Purple 43 tooth to a Green 42 tooth. Probably 41 teeth would be closer although not sure whether one is available. At this point, it's close enough for me when it comes to daily use and highway cruise. When I first got the car, speedo was 10mph lower than actual.

[XfireZ51]

Steve,

Didn't choose a particularly taller tire. The Nittos I have are ones that a good number of Z06 owners use when replacing their RF tires. What rolling radius is and how it compares to stock size tires I am not certain. We just replaced the driven gear from a Purple 43 tooth to a Green 42 tooth. Probably 41 teeth would be closer although not sure whether one is available. At this point, it's close enough for me when it comes to daily use and highway cruise. When I first got the car, speedo was 10mph lower than actual.

BTW, here's a good resource for this calc. We used it to determine if my green gear would work.

http://www.novaresource.org/speedo.htm

[XfireZ51]

Looking at the Nova Resource site , my calc is

(15 tooth drive gear x 20.2)/ 27"= 2.79

Chart show use of 42 tooth green gear.

[scottfab]

No, because the diameter of the tires will be about the same, you don't ride on the rims.


Correct. Diameter of tires is the determining factor.
However, while we don't ride on the rims normally
sometimes when you take a $@(($ corner wrong
you DO ride on the #@#( rim.
$#)$( it.
ugh!

[-=Jeff=-]

Dom,

Looks to be avail
http://www.oehq.com/partpix/acdjpg//25513048.jpg

GM PN 25513048

[efnfast]

Correct. Diameter of tires is the determining factor.
However, while we don't ride on the rims normally
sometimes when you take a $@(($ corner wrong
you DO ride on the #@#( rim.
$#)$( it.
ugh!

Well, that sounds like you're speaking from experience

[WVZR-1]

There's actually not much of a need in the "guessing" unless it's a preferred "game". For the most part Internet calculating is based on "advertised" tire dimensions and not a "loaded" dimension that is required. The "guessers" get close, buy another anticipating maybe getting closer and it goes on.


Steve,

The 19" tires are about 1+" taller than stock 27 v 25.7". I have 3.73s and am using the stock speedo gears in the trans. My speedo error is ~3mph at 70 w the speedo reading lower than the GPS. 4.10s may put you right there w the taller tires.

This is a very WAG regarding tire dimensions and another WAG "hinting" an axle ratio might "compensate" !!!

OP - You first need to determine the tire sizes that will be involved if you made the change. If there's someone running a comparable tire and axle ratio it could be used for a base calculation BUT then again you need to take into consideration the accuracy of the one you're comparing it to. A comment such as "it suits me" isn't what I'd consider a "comparable" result. If someone has gone to the effort to measure the rolling/loaded diameter of a particular tire so a rolling/loaded radius can be determined it's the best, a measured loaded radius (center of rear axle to surface) is usually considered close enough but it isn't that difficult to chalk a tire and the surface a roll it a single revolution.

[Bob Eyres]

Isn't the circumference of the tire the determining factor, rather than the diameter?

The circumference, (distance around), is the distance you go in one revolution. Therefore it would be a more accurate measure, would it not?

This would take into consideration the rate of inflation....but now I'm veering off into economics, nevermind :rolleyes:

[WVZR-1]

Isn't the circumference of the tire the determining factor, rather than the diameter?

The circumference, (distance around), is the distance you go in one revolution. Therefore it would be a more accurate measure, would it not?

This would take into consideration the rate of inflation....but now I'm veering off into economics, nevermind :rolleyes:

You need the "rolling" circumference to calculate the measurements needed for the formula or "short-cut" it using the "loaded radius" I mentioned. The rolling circumference of a tire with appropriate inflation is "assumed".

[scottfab]

Isn't the circumference of the tire the determining factor, rather than the diameter?

The circumference, (distance around), is the distance you go in one revolution. Therefore it would be a more accurate measure, would it not?

This would take into consideration the rate of inflation....but now I'm veering off into economics, nevermind :rolleyes:

Circumference, radius and diameter are locked functions since
C=d * Pi and 2r = d
Therefore the determining factor(s) are linked and one and the same for purposes of the current thread.

[Blue Flame Restorations]

Sixth grade math. My daughter and I just did this last week. Lol

[Hog]

3.14159265 off of the top of my head. Wow been a long time. The ratio of a circle's diameter to its circumference is PI:1 or 3.14159265_:1.

Like 3 times the circumference of ones head is roughly equivalent to ones height. Go get a pice of string and check it out. I'm 6'4" and it works for me. The piece I wrap around my head walks down my body 3 times and gets to my ankles from the top of my head.

peace
Hog

[scottfab]

Actually it's 4th grade math where I went to school.
I think that's covered in a TV program called "Are You Smarter than a 5th Grader" cause that's when you learn the volume of a cone and out smart adults with it. :)
It's a hilarious program. Some of those kids are real whizzes.

[Bob Eyres]

3.14159265 off of the top of my head...... 3 times the circumference of ones head is roughly equivalent to ones height.
peace
Hog

Off the top of your head indeed.

Hey, the tape hit me at the ankles too. So are our heads too small? or are our legs too long?

Thanks for the math lesson guys.

Speaking of tire sizes, am I the only one who doesn't like lower profile tires and bigger wheels on the ZR-1?
I know it probably improves handling somewhat, but aesthetically I just can't handle the big wheel look on the 1980's C4 design.
Being straight line oriented, I like the look of 255/50x16" BFG DR's on the back. I wish there was a stock 16" wheel in the turbine, or spoke style that I could use on the Z.

[scottfab]

Well, that sounds like you're speaking from experience

Yup and is why I now have two sets of wheels.
Happen 16yrs ago when I first went from a C3 to a C4.
Something about turning radius :-D
There's nothing like the sound or rim on top of curb to
get your attention.

[Hog]

Off the top of your head indeed.

Hey, the tape hit me at the ankles too. So are our heads too small? or are our legs too long?

Thanks for the math lesson guys.

Speaking of tire sizes, am I the only one who doesn't like lower profile tires and bigger wheels on the ZR-1?
I know it probably improves handling somewhat, but aesthetically I just can't handle the big wheel look on the 1980's C4 design.
Being straight line oriented, I like the look of 255/50x16" BFG DR's on the back. I wish there was a stock 16" wheel in the turbine, or spoke style that I could use on the Z.
No I'm with you about the over sized rims on the C4's. Some of the 16" rims with 50 series rubber look allright, but I do think that the 17" rims with 35-40 series rubber do look good on the Y and F-bodies.

I remember when Viper went from 17" rims to 18" rims and lost a marked amount of handling prowess. Rim diameters have gotten crazy and it seems the bigger is better, but not in my book. The Shelby Series 1 rims would be far better looing IMO if they were 17" rims.
More rims, more sprung mass hurts many aspects of performance, although if larger brakes are utilized, some are overcome.

With you being straight line biased, your shorter rim, taller sidewall tire makes huge sense in regards to contact patch. If it werent for fitting over brakes, a 15" rim/tire combo would be even better for supervised acceleration trials.

So far as the height/head circumference, reaching our ankles is about average IIRC.

peace
Hog

[scottfab]

... snip....

Speaking of tire sizes, am I the only one who doesn't like lower profile tires and bigger wheels on the ZR-1?

Nope, I'm with ya on that. In fact I don't care for any "wagon wheel" looking wheels either. Prefer the turbine type for looks and functionality.

I know it probably improves handling somewhat, but aesthetically I just can't handle the big wheel look on the 1980's C4 design.

The smaller the sidewall the more likely there'll be a loss of wheel when hitting a pot hole or whatever. The wheel mfgrs love it. Sales of replacements and all.


Being straight line oriented, I like the look of 255/50x16" BFG DR's on the back. I wish there was a stock 16" wheel in the turbine, or spoke style that I could use on the Z.

Oh and one's height is also aprox the distance from finger tip to finger tip of outstretched arms.

[Funracer]

So, head circumference and finger widthe aside :), generally speaking, 4.10 gears and bigger tire diameter would somewhat make the speedo more accurate vs only 4.10's or only larger
diameter tires. They affect the speedo oppositely (is that a word?).

Cancel each other out to some extent maybe a better way to say it.

Correct?

[PhillipsLT5]

Does this help?
http://www.miata.net/garage/tirecalc.html