Gear Ratios & Economical IH Engine RPMs

["Thomas J. Harais" <tharais@domain.elided>]

There has been a lot of discussion on the digest lately about gear
ratios and slowing the IH engine down on the highway. The mention of
picking up a five speed OD tranny brought to mind an old question I've
always had.

What is the minimum RPM that you can run an IH V-8 at freeway speeds and
still have it effectively work? I ask this in terms of seeking better
fuel economy.

My Scout has 3.54 gears. It runs damn near 3000 RPM at 65 MPH. A lot of
the late sixties and early seventies detroit V-8s ran this way.

But, after the Arab oil crisis, OD transmissions, higher rear end ratios
and lock up torque converters were the norm along with different valve
timing to boost economy.

My girlfreinds Trans-Am with the 305 TBI Chevy and OD automatic turns
only about 1700-1800 RPM at freeway speeds. My guess is the the IH V-8
probably has more torque than the Chevy in this range and would
therefore work.

But, 3000 down to 1800 is a 1200 RPM drop and that seems significant. Is
anyone on the Digest running 2.73s (or maybe one of you five speed
converts) such that their engine is turning less than 2000 RPM at
freeway speeds? Does this work? Is there a significant improvement in
fuel economy? After all, wheight and air resistance on any IH road
vehicle already put two strikes against it in the fuel economy area.

An improvement from 11MPG to 13MPG isn't enough to justify the cost and
hassle of diff gear changes or transmission swaps. But, 11MPG to 16MPG
might be. I'm seeking input from someone that has actually been there.
But then again, that is why we all use the Digest in the first place!

Tom Harais
'76 Terra/304/727