OK,45 years as an auto mechanic.. I know the tires are 17 inch... regardless what you say they are..

 

Glad you don't work on my stuff. 245/70x17 is 30.5"

 

Yes, he's complaining about rpms at 80. That has exactly NOTHING to do with engine power. He has a manual transmission. His engine is 100% mechanically locked to his tires, and thus vehicle speed. Nothing slips, not the clutch, the transmission, the driveshaft, rear end, or tires. So his engine rpm WILL NOT change with more power. The only way to lower his rpm is to shift gears (can't shift higher cuz he's in 5th gear), change his rear end (if he already has the highest gearing AKA lowest numerical numbered gearset, he can't touch that), or get bigger tires.

 

Lol, the only thing thats 17 inch on your tires is the inside diameter where the rim/wheel goes.

 

if the tire does not fit the 17 inch rim.. the tire is useless. and the 30.5 inch is no where on the tire...

that is why 245/70x17 . aspect ratio and rim size = tire size to order. has been for longer than I have been alive.. but this is a FREE country.. call it what you want.
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I am Now retired. and no longer have to hear people tell me how to fix there car.
if you know how to fix your car.. fix it yourself.

 

Fortunately. It's amazing to me that in "45 years as a mechanic", you learned nothing about tires. Scares me to wonder what else you missed.

 

30.5" is the overall diameter of the tire. The 245 is the width in millimeters, so 9.65" wide. The aspect ratio is how tall the sidewall is as a percentage. 70% of 9.65" is 6.755", so the sidewall is 6.755" tall, double (2 sidewalls in tire diameter) it (13.51"), and add the rim diameter (17"), and you have the overall tire diameter (30.51"). Pretty simple math.

Anybody that wants to change tire size but not affect the speedometer has to figure it out the total diameter of the tire, and that's how. Been like that since 20 years before I was born.

And if the OP wants to lower his engine rpm, the simplest way is to have a larger diameter tire. He'd have to get a rather big tire to make a significant difference though.

 

Could it be my clutch starting to go out, or might need adjusting?

 

I, too, have an 06 V6 Manual 6 speed w/stock 3:21 gears. Off the top of my head, I know rpm's are right on 2000 at 70 but need to check for 75 and 80 next time I go out. I don't think they'll go up to 2500 just to go up to 75 but maybe. My first thought was 'are you still in 5th gear'? And your tires are correct. There is an issue with the tach on these model years but that problem is rpm's reading too low (due to stepper motor I think) like it will drop down to 200 at idle, not higher and only at rpm's below 2000 as mine has started occasionally doing this.
When I first bought my '06 new after having an '03 V6 5speed, I would catch my Dad cruising along in 5th, forgetting this is a 6 speed!..He was killing my mileage

 

its called "Roll-out" needed and used for racing.. like in drag racing.. and other forms..

or just changing for engine advantage or gearing advantage .

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a slipping clutch will only hold that slip for a mile or 2.. not miles and miles of slip.
it would be "Fried" in 5 miles. so No...