If you really want to compare, I think you'd have to do a lot more to eliminate variables. Variations in speed due to traffic would far overshadow any other variable and make comparison impossible. You'd really have to do something like this:
Highway travel at night.
Pick a fixed path.
Drive the car prior to test to stabilize tire pressure and temperature.
Monitor and record tire pressure and outside temperature (throughout the test).
Start the car, at the entrance ramp to reset the mi/kwh.
Turn it off at the exit ramp so you can record the mi/kwh.
Accelerate and decelerate as closely as possible at the same rate at the same time.
Set the cruise control immediately.
do at least 10, better yet, 20 runs.
DISCARD any run where you have to step on the brakes or the accelerator, even for one second.
Then, after doing this 10 times, change out the tires and repeat another 10 times. New tests must be performed immediately afterwards, under the same road conditions and ambient temperature.
By the way, the effect of speed on efficiency is huge. I can get, say, 4-6 mi/kwh on the highway never going above 55 mph going to work, then get 7-9 mi/kwh just by driving 30 mph (and slower) on the service road along nearly the same path.
Of course, you can still probably get a general idea just by monitoring your efficiency over time before and after the tire change, but it's pretty hard to pin down without a real analysis.
Highway travel at night.
Pick a fixed path.
Drive the car prior to test to stabilize tire pressure and temperature.
Monitor and record tire pressure and outside temperature (throughout the test).
Start the car, at the entrance ramp to reset the mi/kwh.
Turn it off at the exit ramp so you can record the mi/kwh.
Accelerate and decelerate as closely as possible at the same rate at the same time.
Set the cruise control immediately.
do at least 10, better yet, 20 runs.
DISCARD any run where you have to step on the brakes or the accelerator, even for one second.
Then, after doing this 10 times, change out the tires and repeat another 10 times. New tests must be performed immediately afterwards, under the same road conditions and ambient temperature.
By the way, the effect of speed on efficiency is huge. I can get, say, 4-6 mi/kwh on the highway never going above 55 mph going to work, then get 7-9 mi/kwh just by driving 30 mph (and slower) on the service road along nearly the same path.
Of course, you can still probably get a general idea just by monitoring your efficiency over time before and after the tire change, but it's pretty hard to pin down without a real analysis.