Tires: Rolling circumference of tyres., inflation pressures, fuel consumption

Question
Hi,

1. I was wondering where one might find either a chart or a formula to determine the circumference of diferent size tyres.

2. What is the genereal rule regarding upsizing that results in an increase in circumference regarding acceleration, top speed and fuel consumption.

Thank you in advance

Answer
Herman,

There are a couple of things that complicate the "circumference" of tires.

1)  There are "calculators" that will calculate the circumference (diameter) of a tire based on the tire size.  Use a search engine with the key words "tire calculator"

2)  These "calculators" give you an answer based on the "size", which is different than the actual physical dimensions.  Said another way, a P205/65R15 does not actually have to be 205 mm wide and have an aspect ratio of 65%.  There is quite a bit of variability in the market

3)  The actual circumference (diameter) of a free hanging (not touching anything) tire is different than the rolling circumference (Rolling diameter) because the tire deflects under load.  Different inflation pressures and different loads will affect the rolling circumference, but as a general rule a properly inflation and properly loaded tire will have a rolling circumference about 97% of the free hanging circumference - a 3% difference.

4)  The difference in circumference between tires will have a minor effect compared to other factors.  Rolling resistance greatly affects fuel economy, so acceleration is also affected and rolling resistance varies from tire to tire.  However, a change in rolling circumference of a tire acts in a similar way as a change in final drive gearing.

Hope this helps.