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Re: Tire sizes

Hairy green toads from Mars made Brooks Ellis say:

>     Earlier, I was asking about my wimpy tires and sideroll. Now, I am 
> wondering what I would have to do to bump up my tires from the current
> 205/60/15's to a 205(+?)/??/16. Is it possible to do without having to 
> get new rims? I would imagine that I need to also go with a wider tire to 
> not screw up the spedometer. 

If you want to go for 16", you have to get new rims, obviously.


>     Does anyone know of the WWW/HTTP 'Tire size calculator' address? That 
> would probably solve half my problem.

I have enclosed Mark Sirota's tire_size.c program below. I use it a
lot; compile and try it.


>     The car is a '90 100 Q. How hard will this feat be? Has anyone else 
> done it? 

I had slight problems with this size on my 100Q; a little
fender rub. I returned the wheels and went stock again. Sigh.

The offset is tough to match for this car. Anything I found had
too much, and stuck out far enough to contact the fender. Tire Rack
claimed these "fit like a glove" on the 100Q.

x-x-x-x-x-x-x-x-x-x-x-x-x-x tire_size.c x-x-x-x-x-x-x-x-x-x-x-x-x-x-x
/*

 * 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 - University of Rochester, Rochester, NY
 *  Internet: msir_ltd@uhura.cc.rochester.edu
 *  UUCP:     {decvax,harvard,ames,rutgers}!rochester!ur-cc!msir_ltd
 *
 * This program has been tested under SunOS 3.5 and 4.0, 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 <stdio.h>

#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", &section, &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  Difference\n");
	}

	/*
	 * 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);
}

-- 

Andrew L. Duane (JOT-7)			duane@zk3.dec.com
Digital Equipment Corporation		(603)-881-1294
110 Spit Brook Road			http://www.zk3.dec.com/~duane
M/S ZKO3-3/U14
Nashua, NH    03062-2698

Only my cat shares my opinions, and she can't work a calculator.