Tire profile comparison


Capt. Mike

Moderator
Someone forwarded me the following 14" tire rotation chart. It gives the distance a tire will roll per revolution. What's neat is that it basically allows you to compare expected speedometer error.

Aspect
ratio= 80 75 70 65 60 55
Width
165 622 --- --- --- --- ---
175 634 --- 606 --- --- ---
185 650 631 624 596 578 ---
195 --- 651 636 605 590 ---
205 --- --- 652 --- 602 582
215 --- --- 665 --- --- ---
225 --- --- --- --- 626 604
245 --- --- --- --- 650 ---

Note that the three sizes recommended for VW's, 185R14r, 195/75R14C and 205/70R14r all have the same rolling circumferance of aproximately 650. Therefore all should have the same vertical clearance and the same speedometer readings.

You can also see how much an off-size tire can mess things up. For example changing to a 185/70R14 would be a 624, or 4-5% decrease. Besides speedometer error, that means a direct change in gear ratios as well. That in turn translates into more revs (shorter engine life) for a given speed & an even higher drop in fuel mileage (also add additional rolling resistance). So that extra width is not free!

Thanks for the table.
 

mrdama2

New member
Thank for this informative post?

Do you know the unit of measure used? It can't be inches or centimeters. By my calculations, a 185 80R14 tire has a circumference of about 81 inches, or 205 cm. I'm not sure where the value of 650 in the chart comes from.
 

Capt. Mike

Moderator
This should teach me not to pass on informtion without double-checking. I think it is the circumfrance in feet without the decimal point. I remember the original source, as I posted, quoted it as the distance the tire rolls per revolution, i.e. circumferance. The other spreadsheet I have regarding effective gearing calculates the circumferance as about 6.6 feet for the three sizes common to Westies. And it calculates tire diameter as about 25.3" yielding 643 mm dia.

I suppose this is one of those cases were the results -- comparison of size effect -- is the same since circumferance is a direct function of diameter if my geometry hasn't failed me, too. The percentage change should be the same.
 

Capt. Mike

Moderator
Tire profile changes

A very rough rule of thumb is that if you change a tire size 10mm from one width, you have to change profile 5% to maintain the same rolling curcumfrance to retain the same gearing and speedometer readings.

For example, the standard Westy tire was 185R14. Standard profile is .78, .8 or .82. For convenience assume standard is .8 or 80% heieght to width.

Increasing to a 195 width would thus require a 5% reduction in profile, or 195/75R14. VW's option size for Vanagons was 205/70R14. The size was increased two (20mm) so the profile needed to be reduced 2 x 5% = 10%.

This rule of thumb is only going to be accurate for a couple of size changes, i.e. for 10 & 20mm width and 5 or 10% profile. Anything more will lose considerable accuracy.

Note that this rule is only to retain optimum gearing by rolling circumfrance. It does not insure fit or clearance. Obviously, extremes cause other problems in ride, handling and safety. Excessive width can reduce adhesion in wet or slippery cases. VW specified an optimum tire size (or two) based on the best compromise of handling, wear & safety. For the most part, their engineerss are a whole lot smarter than the average tire dealer.
 

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