Here is Jack Daniels recommended method for determining max heart rate. If you did any kind of math to figure it out, you probably don't have your real max heart rate.

As a runner, probably the easiest way to determine your maximum heart rate is to run several hard 2-minute uphill runs. Get a heart-rate reading at the top of the first hill run, and if your heart rate is higher the second time up, go for a third time and see if that is associated with an even higher heart rate. If it is not higher, you can be pretty sure that reading is maximum. If the third run is higher than the second, then try a fourth, or as many as needed before you do not see an increase in heart rate compared with the previous run. If no hill is available, you could just do a few 800-meter runs at a solid pace and do the same comparisons between repeated efforts.

Also, heat will absolutely raise your heart rate for a given speed. You have more blood going to your skin in addition to the amount going to your muscles.