Miata Mailing List: June 1993, Message #113


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   From: miq@wv.MENTORG.COM (Miq Millman)
Subject: Re: Tire & Tire Transport
   Date: Mon, 28 Jun 1993 22:10:58 -0400



Herb Samuels says:
> I have one question concerning the gearing change between a 185-60-14
> vs. the 205-55-14 which was noted. Don't both tires have the same
> circumference.  The 195-55-14 is a little smaller than the 185-60-14
> while the 195-60-14 is a little bigger while the 205-55-14 should be
> almost the same.  In the real world, however, the circumference might
> differ in the "same sized" tires from different manufacturers.  Any
> thoughts about this possibilit?
>
> Herb Samuels

Here is what the "Net Tiresize Program" has to say:
Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile Difference
185/60-14         4.4in  11.4in    22.7in         71.4in        887 0.0%
205/55-14         4.4in  11.4in    22.9in         71.9in        882 0.6%
195/55-14         4.2in  11.2in    22.4in         70.5in        899 -1.3%
195/60-14         4.6in  11.6in    23.2in         72.9in        869 2.1%

However real world events concur that the 205/55/14 is about .5 seconds
slower in the 0-60 benchmark.


Here is a copy of the program:
-------------------------snip snip-----------------------
/*
* This program calculates the actual size of a tire given its size
* specification.  Any number of tire sizes may be specified on the command
* line.
*
* For each size specified, the percentage difference from the first size
* specified is also given.  This is useful for the difference in speedometer
* reading or gearing.
*
* Tires are specified in the form "185/60R14", for instance.  The delimiter
* characters are not important.  Specifications containing speed ratings (as
* in "185/60HR14" are handled correctly.
*
* TRX wheel sizes (which are measured in millimeters instead of inches)
* are handled correctly.
*
* Note that the output of this program assumes that the size figures in the
* specifications are extremely accurate.  As this is seldom the case, it is
* best to check with each tire manufacturer for more precise information.
*
* Mark Sirota - Department of Electrical Engineering
* University of Rochester, Rochester NY
*  Internet: msirota@ee.rochester.edu
*  UUCP:     {decvax,garp,harvard,hombre,rutgers}!rochester!ur-valhalla!msirota
*
* This program has been tested under SunOS 3.5 and 4.x, BSD4.3,
* Ultrix 3.0, Symbolics Genera 7.2, and MSDOS 3.2 (MicroSoft C 5.1).
* Thanks to Richard Welty for the Symbolics port; it was quite a learning
* experience.
*
* This program uses getopt() to parse the command line arguments.  getopt()
* is a part of the standard C library for many implementations; if you don't
* have it, it is available for free from numerous archive sites.
*
* From: Neville D. Newman (neville@ads.com)  Oct 2, 1989
* - made default output in English units, added switch to select metric
* - use sscanf() for a more robust input format
* - output diameter to parallel manufacturers' charts
*/

#include 

#define M_PI	3.14159265358979323846

#define ENGLISH	0
#define METRIC	1

/*
* Place to store argv[0]
*/
char	*progname = (char *) NULL;

void
usage()
{
fprintf(stderr, "Usage: %s [-me] tirespec ...n", progname);
}

main(argc, argv)
int	argc;
char	**argv;
{
int		c;
extern int	optind;
extern char	*optarg;
int		system = ENGLISH;

/*
* Store the program name for error messages
*/
progname = *argv;

/*
* Parse the arguments.
*/
while ((c = getopt(argc, argv, "me")) != EOF)
switch (c) {
case 'm':
system = METRIC;
break;
case 'e':
system = ENGLISH;
break;
case '?':
default:
usage();
exit(1);
}

/*
* If there are no specifications to process, give the usage message.
*/
if (optind == argc) {
usage();
exit(2);
}

/*
* For each argument, print the parameters of a tire of the given
* specification.
*/
for (; optind < argc; optind++)
printparams(argv[optind], system);
}

/*
* Print the parameters for the given tire specification.  All internal
* storage is done in metric.
*/
printparams(spec, system)
char	*spec;
int	system;
{
int		section,
profile,
wheel;
float		wheeldiameter;
float		sidewall,
radius,
circumference,
revspermile;
static float	controlradius = -1.0;

/*
* Parse out the numeric sections of specification
*/
if (sscanf(spec, "%d%*[^0-9]%d%*[^0-9]%d", §ion, &profile, &wheel) != 3) {
fprintf(stderr, "%s: "%s": Incorrect tire specification.n", progname, spec);
return;
}

/*
* If the wheel diameter is more than 100, assume it's already
* in millimeters.  Otherwise, convert it.
*/
wheeldiameter = (float) wheel;
if (wheeldiameter < 100.0)
wheeldiameter *= 25.4;

/*
* Calculate the parameters in question
*/
sidewall = ((float) profile / 100.0) * (float) section;
radius = sidewall + wheeldiameter / 2.0;
circumference = 2.0 * radius * M_PI;
revspermile = 1609344.0 / circumference;

/*
* If this is the first size, remember it so we can make comparisons
* later.  Also, print a header for the chart.
*/
if (controlradius == -1.0) {
controlradius = radius;
printf("Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Differencen");
}

/*
* Display the results
*/
if (system == METRIC)
printf("%d/%d-%-3d     %6.0fmm  %4.0fmm  %6.0fmm  %11.0fmm  %9.0f  %9.1f%%n",
section, profile, wheel,
sidewall, radius, radius * 2.0, circumference,
revspermile,
((radius - controlradius) / controlradius) * 100.0);
else
printf("%d/%d-%-3d     %6.1fin  %4.1fin  %6.1fin  %11.1fin  %9.0f  %9.1f%%n",
section, profile, wheel,
sidewall / 25.4, radius / 25.4, (radius * 2.0) / 25.4,
circumference / 25.4, revspermile,
((radius - controlradius) / controlradius) * 100.0);
}



--

Miq Millman       miq@wv.mentorg.com         miq_millman@rainbow.mentorg.com
Mentor Graphics 8005 Boeckman Rd, Wilsonville OR 97070  503 685 1492   C2148
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