I'm not a tire engineer, but wouldn't the size increase to over 90mm once the tire warms up and it is spinning at speed?

 

http://pw1.netcom.com/~sgalaba/tiresize.htm

 

What formula do you use?

 

Simple fact is that you need the rolling diameter of both tires to accurately come to a conclusion. This would be measured on the same wheel at the same air pressure.

 

The short answer to your original question is that the larger tyre is less than 1% bigger, so the difference is not big enough to worry about. Both tyres will have more than 1/2" of tread, on diameter, so the difference in diameter is much less than the tread depth. At some time during its life a 140/90 will be the same diameter as a 140/85.

 

85 divided by 90 equal .9444. 70 mph times .9444 equals 66.11 mph actual speed. Speedo would still read 70 mph. To be going 70 mph your speedo would need to read 73.962 or about 74 miles per hour then your actual speed would be 70 mph.

 

Correct. I did the math using the numbers provided. Not knowing actual diameter of both tires leaves room for error. Math was more for an explanation of how to do the calculations for one's own use at a later time when the actual variables are known.

 

With all due respect, that math is a mile out! The tyre cross section is either 90% or 85%, but the diameter isn't included. A typical 140/90 tyre is 25.6" in diameter and a 140/85 is about 1/2" smaller.

So 25.1 divided by 25.6 equal .98. 70mph times .98 equals 68.6mph. What's a mph or so between friends!

 

I did the math using the numbers provided. Not knowing actual diameter of both tires leaves room for error. Math was more for an explanation of how to do the calculations for one's own use at a later time when the actual variables are known.

 

@J2X ... calculated based on diam of 16" plus 2x'height' of tire. HOWEVER, this is only an approximation, but should be 'close enough' to get a feel for what the section height change is going to do to RPM, and is what I think the OP was after.

As many have pointed out there are several factors that need to be taken into account to get a really accurate answer. The 'specs' tire makers supply are nominal at best so you would really need to get the tires (both) up to normal temp and then measure rolling circumference (we do this on bicycles to get the speedo accurate) by marking a line on the tire, lining that mark up with a mark on the pavement and rolling the bike forward until the mark is again perpendicular with the pavement and measuring the distance.

As you can see this process has lots of room for error and includes the hassle of having to do it for both tires, so the approximation used initially is probably 'good enough' to make a decision on a tire change.

The GPS (while not perfectly accurate for an instantaneous reading) will give you 'real' speed at a given RPM. But I think the OP just wanted an idea of how much of a change would he see.

Cheers