Sizing Tyres - not for the mathematically challenged

[Martyn Ridley <ridleymj@bis.on.ca>]

The question is what tyres will fit my LBC?  The simple answer is to fit the original size and go motoring, or you can (try to) follow and perhaps get just a little more for all your agro.

1. Get a calculator, pen/pencil/pad, wine or beer.

2. Send everybody else elsewhere

3. Get the following info about your car:  	- wheel diameter
- rim width
- original/current tyre size

4. To determine tyre size:

If you have a modern car this is easy. The maker tells you on the tyre. So a 145/80R-13 is

145 millimeters wide
80% aspect ratio
R adial
13 inch wheel

That is to say that the wheel and rim are measured in inches and the tyre is measured in millimeters. Fortunately the French didn't reinvent percentage, so aspect ratio is the same for both system.

How to convert from a 5.20-13 cross ply

5.20 in = 132.08 mm (25.4) wide		year of mfg:  46-57 = 94% aspect  ratio
89% aspect ratio						57-64 = 89%
cross ply							64 on	= 78%
13 inch wheel

Now you can't buy a 132/89R-13, that would be too easy.  Now the British (who like the Yanks can't seem to figure out base 10) measured the width in tenths so 5.2 is 5 1/4 inches.  To add to this confusion, tyre manufactures weren't too worried about rolling diameter so it varies, as did air pressure and hence actual rolling diameter. (We are going to stay theoretical for this discussion.)

5. Now you have to match rolling diameter or O/D, and find a tyre that will stay on the rim.

O/D (oh dear!) is now standard. To get your rolling diameter:

145 x .80 = 116 (height) x2 (top and bottom) + {13/25.4} 330.2 = 552mm ( 21.73 in)

OK, so for the cross ply:

5.20 x .89 = 4.63  x2 + 13 = 22.26 (565mm)

6. But what is my radial doesn't say 145/80R-13?  Then, it is most likely an 82% aspect ratio, but could be 78, 72,69,74 91,92,87,86 or 83, and that's just Michelin.

so:	145 x .82 = 119 x2 + 330 = 568mm (22.36in)

The idea is to stay at the same O/D as this will have minimal effect on our speedometer.  A larger tyre will make the speedo read less. Plus or minus 5% is not going to have much effect as the constabulary are generally tolerant (New York State excluded).

5A. There are three issues to be considered with rims, rim width, offset and diameter.  I am not going to deal with offset or changing the wheel diameter (not enough beer or wine at present).

So your Sprite rim is: 	3.5in (88.9mm)  - original Frog, Midget
4.0in (101.6mm) - optional wire
4.5in (114.3mm) - Rostyle 

The 'safe' tyre will width will vary with many factors.  Radials can be have a greater percentage than cross ply, the higher the aspect ratio, the wider the parameters, and of course the manufacturer will also state limits.  The object here is to keep the tyre on the rim.  If you look at old photos of car under heavy cornering you will appreciate what I mean.  So in general you can have an 80 series tyre between 110% and 160% of the width, while a 50 series should be no more that 120% to 140%.



3.5in (88.9mm) 	4.0in (101.6mm) 	4.5in (114.3mm)	  5.0 (127mm) 

135			152%			133%			118%

145			163%			143%			127%

155			174%			153%			136%

165		   forget it!			162%			144%			126%

175						172%			153%			138%

185									162%			146%

195												154%		140%

I remind you that the tyre width is not the tread but the widest section (the bulge) in the middle.  Too small or too large will have problems.  Now I expect numbers of you to tell me that the 155R-13 fits just fine on the Frogeye wheel.  It may do, but I don't advise it.  Even the 165/70R-13 at 162% is pushing the limit.  If you plan to drive the car hard, look at wider wheels.

6. Now finally you are ready to do the work. (BEER BREAK!!)


Start with: 5.20 x .89 = 4.63  x2 + 13 = 22.26 (565mm) 

145-13		= 568mm	good for pressed steel wheels, wires Rostyle
145/80R-13	= 552mm	a winner?, perhaps

155-13		= 584mm	a tad tall for all
155/80R-13	= 578mm	tall but popular choice!

165/70R-13	= 561mm 	good but too pudgy for pressed steel

175/70R-13	= 575mm	a tad tall, but a I bet a much better handling car

185/60R-13	= 444mm	tooo small

195/60R-13	= 564mm	Ah!, perfect, but only if you have a 5.5 inch rim! 

The last statement is not quite true.  Cross ply tyres vary more than radials and were 'underinflated' by our standards (we all run our radials at 40lbs/sq.in. don't we!! - [I'm serious]).  So in reality you want the tyre a tad smaller.  Remember the 5% variance, 565-28 = 537.  So, for those of us with pressed steel or wire wheels, the clear winner is the 145/80R-13, for those or you with Rostyle or fancy wheels, the 165/70R-13 is probably a better choice in that it is more 'sporting'. Rubber bumper guys, stay with the 145/80R-13, to avoid altitude sickness!	

I hope this helps some of you out there.  I now know which trye I will buy.

Cheers,
Martyn



It's the going, not the getting there, that's good.
(Harry Chapin - Greyhound)