I know that larger size tires makes your speedometer read lower. (as in the wheels are turning slower at than smaller wheels would at any given MPH)

But by how much?

My stock tire size is 165-70-13, which is 22.09 " high.

The tires I want to get are 205-50-15 and are 23.07" high.

How much is my speedometer going to be off by having a tire that is .98" larger than stock? Anyone know or care to do the math?

 

Calculate the circumferences of the tires and compare them

 

http://tinyurl.com/6u8rw

wheels size/spedo calculator.

 

http://gs.tolan-hoechst.com/tirecalc.htm

 

Thanks for that link.

75 mph will be 78

 

the original, and best: http://www.miata.net/garage/tirecalc.html

 

Those tire calcs are not entirely accurate. Take into consideration that a tire, when loaded, isn't completely round. The loaded radius is actually flat. So if you really wanted to calculate the difference, you can go to the manufacturer's website, and they should have a loaded radius table.

Widths also vary from one tire manufacturer to the next.

I've just always used the Miata calc though.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kym &raquo;</TD></TR><TR><TD CLASS="quote">Those tire calcs are not entirely accurate. Take into consideration that a tire, when loaded, isn't completely round. The loaded radius is actually flat. So if you really wanted to calculate the difference, you can go to the manufacturer's website, and they should have a loaded radius table.

Widths also vary from one tire manufacturer to the next.

I've just always used the Miata calc though. </TD></TR></TABLE>

I disagree that they are not entirely accurate. They are accurate; you just need to know how to use them.

You are correct when you note that most of the tire calculators do not take into account the deflection of the bottom of the tire when calculating the outer diameter (or radius) of the tire. However, this will be true of both tire sizes when you do a comparison.

Thus, if you're looking at a tire whose size is 3 percent bigger than stock, the outer diameter is usually going to be 3 percent larger, regardless of whether you're comparing both sizes WITH deflection, or both sizes WITHOUT deflection. In this case, the speedometer will read approximately 3 percent lower than with the stock size.

Incidentally, there are some other variables that can affect the precision of these numbers. When you look at those manufacturers' specs, some of them take deflection into account, and others don't. And even when they do, the amount of deflection depends on the weight of the car and the inflation in the tires, so their specs are not precise. Another variable is the tread depth; the outer diameter of a tire worn down to the treadwear indicator bars is approximately 2 percent less than that of the same tire when new.

Again, though, the tire calculators are a good way to APPROXIMATE the differences that will result when you change tire sizes; the percentage differences simply assume that all other variables don't change between the two tire sizes.