[BFix] Gearing for Best Mileage

Sat May 3 08:25:02 MST 2008

I will try to answer the un-answerable Q.
I struggled and went with 4.56/33"/E4OD/0.71OD/5.8 and 
4.88/35"/AOD/0.675OD/5.0HO. Did I choose correctly?- who knows. Its 
something I can live with - barely.
Your 0.75OD is good compared to the AOD  and similar/better compared 
to the E4OD. What that means is you should have less OD hunting?? 
Provided the TQ of your 4.0 is adequate??
The tall OD is good in theory but the engine has to be able to pull. 
Not true normally, which is why you don't see the real tall OD 
gearing any more. This comment also applies to the taller axle ratios.
The 96 BB weighs upper 5s and the gearing is ok with respect to 
performance. The weight kills the economy. The 72 EB is OK with the 
economy on the flat/highway due to less weight and the performance is 
also OK. The thing hunts like crazy with the tall OD. This means in 
hill or mountain driving I am running in 3rd and that kills the economy.
No doubt big TQ motors can live with taller gears for both 
performance and economy (to a degree).

It is true that a smaller gear will be better for economy provided 
the engine can pull the gear. The example of the EB going from 4.1 to 
3.5 is a good example of how the engineers did it.

For a daily driver I would choose the stock gearing. and go back to 
stock tires max out the tire pressure and loose as much weight as 
possible. Figure out how to force up shifting so the engine is at it 
lowest rpm in each gear. Can you get an manual shift kit for the 
5R55E? Then you could lug the hell out of the engine and avoid the 
fuel eating downshifts by just letting the thing coast.  A manually 
locked up TQ converter is a blessing for economy and towing and just 
for use all the time (except when you have to stop). The big Ford 
trannys can be wired that way. We love it. (Being mechanical the AOD 
is hopeless).

Not familiar with the 4.0. What is it peak TQ peak? I wonder where 
you are with respect to that value. I agree with TS shoot for a 
couple of hundred rpm below the TQ peak in direct. Of course the Ford 
TQ curve may be just an approximation. So some test driving would be 
required (or a dyno test).

So there you go -- No solution just a lot of ideas. Spend on mods 
with uncertain result or spend on gas.

IMO, If it was me here are my steps:
1) manual TQC lockup
2) manual shift kit
3) evaluate engine vs existing gearing to see if gear change will work
Regards and LOL, Paul
===================

At 06:31 PM 5/2/2008, you 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>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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