I want to increase the overall diameter of my tires by about 2" in order to drop my highway rpms.

I'm running BFG P215/70SR 14 that I measure 25" o/a diameter.
The BFG site says these are 25.9".
I'm not sure why there's a .9" difference in measurements (the tires are still in excellent condition, so they're not worn)

Should I go with a taller 14" tire?
The tallest I can see is a 225/70R14 at 26.5"
Only 0.6" taller than mine if I go by the BFG website.
But by my measurements, they would be 1.5" taller and may work.

Am I nuts trying to get the taller tire or should I just go ahead and switch to 15"s?

I just went to 15s (covered in another topic). I went with BFG 225/70/15 and I am very happy with the look, ride, feel etc. I think they fill out the wheel openings nicely. Just a note, the right front does rub just a bit if wheel is cut all the way to the right. The car rides much nicer IMO with the 15s. As you can see in the pic below I went black wall out and no trim rings (for now)...

Same experience for me with the 15's. Nice overall and a little rub on the front right.

Are you measuring them on the car?

The difference between the two sizes you mentioned is about 2.3%. So depending on your rpms at highway speed, the difference will be less than 100rpm. You're going to need 15s to make a dent in the highway rpm.

If you get the right backspace you could probably fit a 255/60R15 on the rear and a 215/65R15 on the front. Still if you do the math, that's only a 4% difference compared to what you have today. So for example if before you were cruising at 3000rpm, it's only dropping to about 2875 (a 225 rpm drop).

Mike,
I measured them on the car.
I'm only looking to drop 200 - 300 rpm, so going to 15's should work.

I've been using this link for calculations;
http://www.hotrodpitstop.com/tools.html#1

I used 15's on the rear and 14's on the front.

As seen in pic the front sits high with 15's(235-60-15)on the front.

second pic is 14's on front(24.5 in tall)and 15 on rear(26.6 Tall).

Helps keep the nose down at higher speeds.

As others point out, increasing a size up tire or increasing rim size will not effect the RPM's that much. The best way would be to change differential ratio.

I agree, I don't beleive I noticed any drop in RPMs to be honest. And I went from 215/60/14 to 225/70/15, I guess it probably did drop a bit, but not enough to notice...

Thanks for this info guys. Gives me something to think about.
Since I just did a ratio swap in the fall and have barely driven the car since, I can't bring myself to switching ratios again.

I drive 90% in the city but do like to head out for an extended road trip once in awhile.
I'm just nervous about running 3000+ rpms at highway speeds for extended periods.
I think I'm still gun-shy from my overheating NIGHTMARE last summer!

If I had to do it again, I wouldn't have gone with 3.55's. Should have dropped down a ratio.
Although I am really happy with the "get up and go" off the line now!

So it sounds like the added expense of 15" wheels and tires may not be the answer. Hmmmm...........

I've found the 3.23 is a great ratio for both acceleration and top end. The 3.55 is a good one for off the line, but top out to quick for freeway use... unless you have a OHC-6.

Yep, that's what I'm finding.
3.23 would be the way to go.

I am running 3.55 and at 80mph I am turning 4k RPM. So all in all I am very happy with my ratio/combo. the 3.55 w/ the 2.56 first gear is an awesome combo off the line. Love it!