I just replaced the tires with same size but different brand ones and the car speed feels like it's slower given the engine speed. Would someone with a 6-speed 997.1TT please tell me what the rpm is at 70 mph in 6th gear.

If the tires are actually same size....i.e. same circumference....each revolution will travel exactly the same distance. No way around that....everything else being equal (same trans. gearing...diff..etc.).

You can get the specs on old tires and new to ensure same size. In fact worn tires that came off should be slightly smaller due to treadwear.....so new ones with more tread would typically be slughtly larger....

DC

I understand and agree with everything you stated. I'm just looking for experimental (actual) data to confirm it.

I'll tell You. Tomorrow am

Looks like I'm right about 2500 in 6th at 70. Stock wheels and tires

Thanks Spmmorrow. That checks out with mine as well.

In any given gear and RPM combination, the car will always read the same speed. The vehicle speed is read off of a sensor in the transmission, and it doesn't know if you change the size of the tires. The easy way to verify this is to grab a GPS speedometer app for your phone and compare the readings to your vehicle speed. It's probably not going to be exactly the same, but should be pretty close and scale relatively evenly.

It might seem slower because of less noise.