[BFix] Gearing for Best Mileage

Sat May 3 13:43:53 MST 2008

If you're worried about mileage, you won't be going to 35's. There really isn't anything you can't do on 33's in a family mobile trail truck, IMO. dual lockers are far more important.
  ----- Original Message ----- 
  From: Quinn Dusenberry 
  To: bfix at broncofix.com 
  Sent: Saturday, May 03, 2008 11:49 AM
  Subject: RE: [BFix] Gearing for Best Mileage

  Thanks for the reply. That sounds about right to me. 4.56 for 33 with OD and 4.88 for 35 with OD. I'm tempted to go with 4.88's in the event that I end up with 35's in the future.

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  From: bfix-bounces at broncofix.com [mailto:bfix-bounces at broncofix.com] On Behalf Of paul wilson
  Sent: Saturday, May 03, 2008 8:25 AM
  To: bfix at broncofix.com
  Subject: Re: [BFix] Gearing for Best Mileage

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

  This brings me to a topic I wanted to discuss. Its 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 rpms 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 EBs, they went from 4.10s to 3.50s for about the same tire size in the early 70s 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 3.73s or could I go with something even deeper, like 4.56s or 4.88s?

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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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