Engines with lack bottom end power?

well
lets look at the numbers
a D78-14 is about 24.8 tall
A 225/70-15 is about 27.4
This represents a change in torque multiplication to .905
the 2.93s compared to the 2.71s is a loss to 0.925
So together, the loss is .905 x .925= a loss to ~84%
lets look at the change in rpm.
With the first combo, your hiway rpm should have been around 65=2564 at zero slip, perhaps a lil more with an auto.
In the second combo, 65=2161@zero slip
Now if your engine makes say 200ftlbs@WOT,in this range, this is a maximum of
98 horsepower@2564 but only 82 hp at 2161 Yeah so even without the change in TM(torque Multiplication), yur gonna feel this 16% loss.
Lets run that thru the previous gearing numbers, to get on-road Torque numbers;
With the small-tire/2.91s, it looks like 200 x 2.91 x 24/24.8 =563 ftlbs
With the big-tires/2.71s, it looks like 200 x 2.71 x 24/27.4 =475 ftlbs
thus the loss of TM comes to 563 less 475= 88 ftlbs. which is a loss of 88/563=15.6%
Thus overall, your new combo, at WOT, may feel like it has ; 2.91 less 15.6%= 2.46s compared to the small tire combo. But the Horsepower loss is also gonna show up
Of course at any particular cruising speed, the throttle will not be WOT, but instead, the big-tire combo may in fact be a little further open than it was with the small-tire combo. Yet the fuel economy may improve due to the lowering of the rpm, by some 400rpm.
But don't be fooled; your engine has not lost power, the roadspeed at which it occurs has just been changed. Sometimes this can be a good thing, like say for passing.. Or like at what speed, top of Second gear occurs, or even First.