You will need a correcting adapter with a ratio of 1.281779661 or as close as possible. Now if your 33" tires run +/-0.5 of an inch, you could still see up to a 3% error. Which would be +/- 2 MPH at 65 MPH.
Here is a GoogleDoc I made that you can play around with tire size and gearing. The formula is from
Speedometer Cables USA under "Need a correcting adapter".
CORRECTING_ADAPTER_CALCULATOR