[BFix] Gearing for Best Mileage

Sat May 3 11:56:25 MST 2008

Here's a little I found out about my 4.0L SOHC engine:

"Besides the venerable 4.0L 160 hp V-6, Ford has added two new engines, a
5.0L 215 hp V-8 (introduced in 1996) and a SOHC version of the 4.0L V-6
(introduced in 1997) that pumps out a neck snapping 205 hp @ 5,000 rpm and a
seismic 250 lb-ft of torque @ only 3,000 rpm. This new engine sports a 9.7
to 1 compression ratio, which is part of the reason for its high efficiency.
My test car had this new V-6 and it was connected to a class exclusive 5
speed electronically controlled automatic."

Poorly written sentence, but I think the HP and torque numbers are for my
4.0L SOHC engine. The rest of the article has some other good Explorer stuff
too if anyone is interested in reading it.

http://www.theautochannel.com/vehicles/new/reviews/1999/lweitzman_ford_explo
rer.html

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From: ts [mailto:ntsqd at yahoo.com] 
Sent: Friday, May 02, 2008 8:01 PM
To: Quinn Dusenberry; 'EBE'
Subject: Re: [BFix] Gearing for Best Mileage

How fast are you now going at ~2800 RPM and at ~2100 RPM while in direct? At
what RPM does it really start to pull? I think the first thing to try to
estimate is where the Torque peak is. Then gear it to run there or a bit
under at your normal fwy speed.

The main reason I prefer the 700R4 over the AOD is that the AOD's have too
tall of an OD ratio. IMO anyway.

Quinn Dusenberry <q.dusenberry at comcast.net> wrote:

This brings me to a topic I wanted to discuss. It's about my Explorer, but
it applies to Broncos too. My "new" 97 Explorer came from the factory with
3.73 gears, 235/75 tires (29" tall). Since I converted to 33" tires, a
common gear table (http://www.4lo.com/calc/geartable.htm) says that I should
go to 4.27 gears (or so) to maintain factory gearing which is 2809 at 65
mph. Taking the OD into account, the rpm's should drop to 2106 (0.75 OD
ratio).

So here is my question. When the factory wants to go from power to mileage,
they tend to use lower numerical gear ratios. In the case of EB's, they went
from 4.10's to 3.50's for about the same tire size in the early 70's when
the gas crisis hit. So on the one hand, it seems to me like my mileage
should improve some by throwing on larger tires which effectively lowers the
gear ratio. But as you point out, it seems that I probably have dropped out
of the optimum range for my engine. My question is what gear ratio should I
run for peak mpg? The new factory ratio of 4.27, keep it at the existing
3.73's or could I go with something even deeper, like 4.56's or 4.88's?

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From: bfix-bounces at broncofix.com [mailto:bfix-bounces at broncofix.com] On
Behalf Of ts
Sent: Friday, May 02, 2008 11:02 AM
To: EBE
Subject: Re: [BFix] replacing 170 with a 200? 4.0??

Yes and no. It does matter how hard that engine is working. The harder that
you work an engine the shorter it's life will be, but also the less likely
that it is operating at it's best rpm (unless on a governor).

Say it takes 35HP to move down the road at 65 mph. A 40 HP engine is going
to be basically WOT for the duration, where a 100 hp engine will be running
with higher manifold vacuum and at a leaner mixture than the 40 HP could
stand.

Effective total gearing is huge. For best efficiency the engine needs to be
operated at it's best Brake Specific Fuel Consumption rpm (BSFC, least fuel
for most power produced), which happens to be the same rpm where peak torque
occurs. 

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