[BFix] Gearing for Best Mileage

Thu May 8 09:35:15 MST 2008

What is the OD gear on the AOD?

Ben

----- Original Message ----- 
From: 
To: bfix at broncofix.com
Sent: 5/8/2008 9:33:00 AM 
Subject: RE: [BFix] Gearing for Best Mileage

You are correct that 1:1 or direct drive (the best) have less parasitic loss than OD.  But, you still need a wide over-all ratio from a transmission to allow the engine to operate in it's most efficient range.  (This is especially true with big diesels as the engines efficient operating range is only ~500 RPMs.)  So, if you can find a transmission that has a wide enough ratio spread with a direct drive top gear ratio and put in fairly high ring and pinion you might gain a couple percent gain in MPG... 

I would recommend running the 4.88 gears with 35" tires and the AOD.  I was running a mild 351W and 4.56 gears but the AOD downshifted too much.  With a built 5.8L and the 4R70W (slightly lower OD) the 4.56 gears may work fine but it would be close in my opinion.

Tony

From: bfix-bounces at broncofix.com [mailto:bfix-bounces at broncofix.com] 
Sent: Monday, May 05, 2008 6:39 AM
To: broncokak at yahoo.com; nvrstuk at elltel.net; bbronko at msn.com; bfix at broncofix.com
Subject: Re: [BFix] Gearing for Best Mileage

I've always thot OD was a crock. And yes, I've owned several, one that I even like.

Here's my thinking:
We're trying to keep the engine at it's ideal speed for least fuel consumption.
We're trying to maximize efficiency so that the power made goes to moving the vehicle and not into making more heat.

So which is more efficient, a 1:1 mechanical coupling or a gear-set spinning under load?
Looking at it another way, the engine is turning some RPM, in OD, so now the drive shaft is turning faster than the engine, but wait! we've used a R&P ratio that slows this back down so that the tire is turning a speed below that of the engine. Every one of those speed changes costs efficiency proportional to the speed difference. Could we have designed a less efficient system?

If they'd design first gear to be lower, then we could use a taller R&P (lower numeric ratio = more efficient) and a top gear of 1:1, mechanically coupled.
Instead we get a taller first gear, a lower numeric R&P (less efficient), and an OD.
I've pondered this since I put an AOD in my '67 Ranchero in 1989 and have yet to find any justification for this silly band-aid approach other than "it's the way we've always done it."

Ken <broncokak at yahoo.com> wrote: 
All good points. I really wasn't considering a 5 mile long hills but they are out there. One thing I liked when I had my 33" tires was I really didn't need to pull it out of OD for most hills. But then I got thinking that OD is for flat open travel. In every other vehicle I've ever driven I needed to down shift out of OD on really big hills (steep / long grades). So I figured I'd set the Bronco up to do the same. I've pulled some good long grades in PA, NY, NH & CT and yes, lost some speed but all in all held my own in OD. If I wanted to gain speed I'd down shift. Isn't that what OD it's for? To give you a little more gearing for the flat open road and drop the RPMs to try and conserve a little fuel and ware and tear?

Ken

 68 EB 5.0L / ZF 
35" BFGs / 3" Lift
4.56 ARB Front & Rear

----- Original Message ----
From: Brian Cooke <nvrstuk at elltel.net>
To: FRANK w <bbronko at msn.com>; bfix at broncofix.com; Ken <broncokak at yahoo.com>
Sent: Sunday, May 4, 2008 7:43:12 PM
Subject: Re: [BFix] Gearing for Best Mileage

Depends if you want to go a bit slower off road AND if you want to be able to climb any freeway hill in OD.

I ran a 351W with 4.10's and 35" M/T's for close to 8 yrs. I'd shift down (out of OD into 4th) on 6% grades or more to accelerate at all.  Since I have a doubler for low gearing it really doesn't matter what rear end ratio I run for trails but it really does for street.

I ran a 351W with 4.10's and 37" SSR's for 2 years. I still had to shift down on 6% grades or more just to maintain 70mph.

I just put in 4.56 gears with the 37" SSRs (6months now) and I still have to shift down on 6% grades or more!  :)

My buddy runs a very stout 351W stroker, now a 408 with 4.56 gears and 37" M/T's and has to shift down on the same 5 mile long 6% grade. He's dyno'd over 425 HP, so gearing makes a HECK of a difference.

Long story short....     I'd suggest the 4.88 gearing with the ZF and 35" tires.  Especially for lower gearing off road if you aren't running a doubler or equiv.

Brian @ Columbia River Broncos...20 forward gears & 4 reverse to choose from.
----- Original Message ----- 
From: FRANK w 
To: bfix at broncofix.com ; Ken 
Sent: Sunday, May 04, 2008 3:07 PM
Subject: Re: [BFix] Gearing for Best Mileage

So if you were to keep the 35's all the time would you stay with the 4.56 or would you go with the 4.88. I have a 5.8, ZF 5 speed, 35" tires and getting ready to buy gears. I thought of going with the 4.88's. Thanks.

Frank 

----- Original Message ----- 
From: Ken 
To: NHieger at aol.com ; q.dusenberry at comcast.net ; bfix at broncofix.com 
Sent: Saturday, May 03, 2008 4:21 PM
Subject: Re: [BFix] Gearing for Best Mileage

I have a 5.0L Explorer motor and the ZF 5 speed, 4.56 gears and 35" tires. When I drove to VA for the Round up last summer I constantly got 20MPG on the hi-way if I kept the tach below 2500RPM (70MPH) the worst I got was 13MPG and that was traveling at 80MPH. I too thought about going with the 4.88 gears but after driving both on road and off. I'll keep the 4.56 gears and maybe run some 33" street tires for all the around town drive I do.  Last fall I drove about 75 miles from home (150 round trip) to run with some J**p guys and spend 6 hours in low range and only used 7 gal.s of gas. I've never really checked the mileage doing around town driving but I'll have to some day.

Ken

 68 EB 5.0L / ZF 
35" BFGs / 3" Lift
4.56 ARB Front & Rear

----- Original Message ----
From: "NHieger at aol.com" <NHieger at aol.com>
To: q.dusenberry at comcast.net; bfix at broncofix.com
Sent: Saturday, May 3, 2008 5:23:35 PM
Subject: Re: [BFix] Gearing for Best Mileage

FWIW, I have 4:56's in my 77 5.0 sefi/AOD, 35" BFG's. Everything was pretty good when I was running 33's, but now that I am running 35's I kind of wish I had gone with 4:88's. I just seem to be a bit over geared when on long extended climbs at freeway speeds & heading into strong winds. What I need is gear right between 3 & 4th. Having said that if I keep the speeds down (55 or less) I can get around 17 mph.

Norm

In a message dated 5/3/2008 11:50:04 A.M. Pacific Daylight Time, q.dusenberry at comcast.net writes:
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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