7 1/4 with 3:55's or even 3:23's
So how is a stock 225 with a 994 and 3:23 compared to 2:9/2.7 whatever rears? Assuming stick tire size.. is it that bad on the highway?
Every one percent change in tire size,and every one percent change in rear gear ratio, each produces the same one % change in rpm at a given speed.
the formula is;
(rpm times TC) divided by ( 1056 x R1 x R2) equals mph. This is for zero-slip
where TC is Tire Circumference, which is the roll-out, or diameter times 3.1416
and R1 is the trans gear, and R2 is the rear gear
For an automatic, you can expect about 3% slip, which at 2500 is 75rpm, sometimes it is more.
example one;
(2500 x 25.7 x 3.1416)/(1056 x 1.00 x 2.94)=65 mph
example two
(2500 x 27.0 x 3.1416)/1056 x 1.00 x 3.23)= 62.2 mph
from 2.76s to 2.94s is an increase of 6.5%.
From 2.94 to 3.23 is +9.9%
from a 25.7 tire to a 27, is +5%
therefore
Going from 2.94s and 25.7s, to 3.23s and 27s, is plus 9.9%ratio and minus 5%TC; for a difference of plus 4.9%.
Plus 4.9% of 2500 is 122 rpm =2622rpm@65mph
or minus 4.9% speed equals 62mph@2500
Whether this is good or is bad, only you can answer that.
For example,
going from 2.76s to 3.23s with the same 25.7s,
but at 75 mph is;
If the 2.76s cruise at 2720( zero-slip) then
with 3.23s it will be +6.5% plus 9.9% =+16.9% is 525rpm=~3250zero-slip, plus 3% slip=3340
For a Roadtrip, that's BAD in my books; and is shortening the life of your engine; but for
short hops I could live with it.
