[Abboq]

I've been struggling with a 2000 Jeep Wrangler that I bought used three months ago. I've climbed up to 20.5 mpg from around 17 by inflating tires, cap, rotor, wires, plugs, and switching to full synthetic lubricants in engine, differentials, and transfer case.

What I'm not sure of is why I only get around 18-19 when I take the interstate bypass to work. I drive 55 on it and baby it. When I go through town for my commute, I drive 45-50 and get 20.5, despite stopping and starting through a dozen lights. I've done this repeatedly, devoting entire tanks to each type of commute.

My Jeep has the 2.5L four-cylinder, and came from the previous owner with 31x10.5 load range C tires. Those are 4.5 inches larger in diameter and 2 inches wider in width versus the stock 265/75R15, if my calculations are correct. I'm also guessing that the 6-ply rating makes them a little heavier than regular 4-ply tires.

The Jeep has always had sluggish acceleration, and in 5th gear, it really struggles to accelerate.

So, I added a vacuum gauge. What I notice is that I almost always get better vacuum in fourth gear (12-14 inches) than I do in fifth gear (8 to 10 inches). This holds true for speeds of 45-60. Under mild acceleration, vacuum drops much lower in fifth (can drop as low as 5" or 6") than in fourth (usually stays above 10"). I lose more vacuum going up small hills in fifth and I gain back less vacuum doing down hills in fifth.

I can't be positive, but it feels like it does take more pedal in fifth, but that could just be my paranoia.

I'm wondering if the larger tire diameter is lowering the engine's highway RPMs out of its useful power band? Could that, combined with heavier tires be making my overdrive inefficient, placing too much of a load for that engine speed?