Tyre size calculator to Rpm?

[D@VEY]

Anyone have a link or know how to work out the differences on what the difference tyre sizes make on the engine rpms.
At the the moment I have 185/65/15 running 1776 with twin 40's but motoway speeds arent great so was looking at changing to 185/80/15 to see if that would help?

 

[Dave.P]

http://www.tacomaworld.com/forum/tirecalc.php?tires=195-45r16-185-60r14" onclick="window.open(this.href);return false;
Change tyre sizes to suit

 

[D@VEY]

Cheers