If you have room, an 11R24.5 will give you even more speed. You will need longer studs. If you go to a Euclid distributor, they can probably fix you up. Euclid is a brake and suspension part distribution network owned by Meritor.
In the trucking industry we find it much easier and accurate to use revolutions per mile to calculate road speed. Here is the calculator I use.
http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf

For some reason it will not post as a link, but I think it will copy and paste. If not, Google Roadranger speed calculator.