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I see two large sources of error in your experiment:
- Torque Converter must be locked up for any meaningful RPM numbers - it is obviously NOT locked up in one or both of your first 2 pics.
- You are reading RPM at an INDICATED 80 mph, not ACTUAL mph. As tire size changes, so does the ACTUAL mph at any given INDICATED mph.
Wouldn't it just be better - and more accurate - to compare the revs/mile for any given tire?
Just my $0.02.
You guys sure are nice about telling me my math and science are OFF - ha
Your question is a good one - how do different tire sizes affect RPM at any given speed?
The only problem is the method.
I found out all I need to know about speedometer accuracy, how the speedo differs from the odometer, what speed I'm actually travelling at when I set the cruise control to a given speed, etc. by using a standalone GPS unit.
If you have, or can borrow, an automotive GPS unit like a Garmin, Tom Tom, Magellan, etc. that shows speed based on GPS position (they all should) then you will have an accurate reference speed to compare everything else to.
Easy test #1: Set your cruise control to 60 (or 80 if you feel the need for speed) and see what speed the standalone GPS unit says you're travelling. The difference is your speedo error - and don't be surprised when it shows your speedo is more accurate with larger than stock tires.
Easy test #2: Reset the trip odo on both the LC and the GPS unit and drive around for a few hundred miles. If you have larger than stock tires, you will find that you actually drove farther than your onboard odo thinks you did.
Phone-based GPS apps are not as accurate as dedicated standalone GPS units, but might give you a rough idea.
Speedo @ 80, Tach @ 2k on the nose
GPS @ 80, Tach @ 1,9k on the nose
If I have a chance, I'll set the cruise when my GPS reads 80 and see what my speedo says. Traffic was too much to try this earlier today.
Hard to look at so many things at once, but it appears when my speedo was reading 80, my GPS confirmed I was going almost 83.
Or maybe I have that backwards.