[BFix] Gearing for Best Mileage

[Quinn Dusenberry]

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?

 

  _____  

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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