Terry Morgan / firstname.lastname@example.org / June 6, 2001
The Mercedes-Benz Ponton shop manual indicates that 190 models came with either 6.40-13 or 6.70-13 tires and says the "effective radius, dynamic" for a 6.70-13 at 60 km/h is 307 mm and at 100 km/h is 313 mm. This is equivalent to a diameter of 24.17" to 24.65." (The 6.40-13 tires are 8 to 6 mm smaller in diameter) This is equivalent to the 185/80-13 size, according to the calculations in the table below.
The width of the tires under both old and new system is the width of the tire: in inches for the old system and millimeters for the new system. I believe the old-system tires had a varying aspect ratio near 80% (?). Under the current system, if there is no aspect ratio given, it is 80 by default; these are slowly becoming extinct.
I measured the circumference of my new 185/80-13s with a tape and I got exactly the same as the calculated circumference. However, there are probably small differences between tire brands. I also measured the section width (widest point of sidewalls) and it corresponds with the width (185 mm).
The "% diff" column shows the difference in circumference from the 185/80-13 size (first row) and indicates the change you will get in engine rpm, odometer, and speedometer readings.
Tire Size Guide
|width||aspect ratio||size||diameter||circumference||% difference||revolutions/mile|
Some of these tire sizes may not be available
How these values were calculated
a) Diameter (in): width (mm) / 25.4 mm/in * (aspect/100) * 2 + size (in).
b) Circumference: diameter * pi
c) % diff: percentage of increase or decrease (-) from the 185/80-13 size.
d) [circum - (circum of 185/80)] / (circum of 185/80) * 100
e) revolutions/mile: (12 in/ft * 5280 ft/mi) / circumference
By today's standards, the 80-aspect-ratio tires give very mushy handling and lower road-holding limits. One of the first upgrades for better handling is to go to larger wheels and tires with a shorter sidewall. I am planning to install 14" wheels and I might want to increase the overall gearing for more relaxed cruising while keeping up with modern traffic. Use the table above as a guide for selecting a size.