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Better MPG with gear change - WHY ? [message #357965]

Sun, 23 August 2020 09:14

hertfordnc

Messages: 1164
Registered: September 2009
Location: East NC

Karma: 0

Senior Member

I've seen it said here many times that folks claimed better MPG after switching to a higher final drive ratio.

Also in my Ford Excursion groups, many people claim better MPG after going from 3.73 to 4.10

BUt i know a few car engineers and they are pretty smart guys so when something beats OEM design I am always suspicious.

In the case of the Excursion, it's often related to compensating for larger tires.

Are there other explanations?

Dave & Ellen Silva Hertford, NC 76 Birchaven, 1-ton and other stuff Currently planning the Great american Road Trip Summer 2021 It's gonna take a lot of Adderall to get this thing right.

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Re: Better MPG with gear change - WHY ? [message #357966 is a reply to message #357965]

Sun, 23 August 2020 09:43

6cuda6

Messages: 975
Registered: June 2019

Karma: -6

Senior Member

Basically, in a nutshell, puts the engine rpm verses power in a more useable spot in relation to speed.

Rich Mondor, Brockville, ON 77 Hughes 2600

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Re: [GMCnet] Better MPG with gear change - WHY ? [message #357969 is a reply to message #357966]

Sun, 23 August 2020 09:59

powwerjon

Messages: 849
Registered: March 2013

Karma: -2

Senior Member

Dave,
The GMC motorhome final drive was a compromise at it was the best that MG had on hand.

The 403 and 455 are really not in a useful power band until the motors are up in rpm say in 2700 to 2800 rpm. As hot rodder say it gets up on the cam!

403 = Big bore and short stroke The motor likes rpm. Our first coach was a 77 with a 403 and it was a real dog in any type of hills and on the flats it never really ran well until over 70 MPH when the rpm was up towards 2700 RPM. Put a Jim K 3.55 final and the coach really woke up and would put the hills with any 455.
Because the engine is now “up on the cam” it is happy and more fuel efficient and not lugging. The 455 is similar but it is a small bore long stroke motor. Mine is a basic explanation and I am sure that Matt C can give you a technical explanation.

Presently run a 3.50 chain drive with a 3.21 final for a 3.66 final ratio and it pulls my 29 1/2’ Buskirk stretch all over the western US. 3.55 and 3.70 gears provide the best service and Jim K thinks that the 3.70 gear is best.

JR Wright
Michigan

> On Aug 23, 2020, at 10:43 AM, 6cuda6--- via Gmclist wrote:
>
> Basically, in a nutshell, puts the engine rpm verses power in a more useable spot in relation to speed.
> --
> Rich Mondor,
>
> Brockville, ON
>
> 77 Hughes 2600
>
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Re: Better MPG with gear change - WHY ? [message #357992 is a reply to message #357965]

Sun, 23 August 2020 19:40

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

More vacuum with less load (higher numerical gears) means the primary rods stay pulled in more of the time, metering less fuel. That gives better MPG even with more combustion events per mile.

John Lebetski
Woodstock, IL
77 Eleganza II

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Re: Better MPG with gear change - WHY ? [message #358010 is a reply to message #357965]

Mon, 24 August 2020 11:16

Matt Colie

Messages: 8547
Registered: March 2007
Location: S.E. Michigan

Karma: 7

Senior Member

I'm sorry.
I did write a page long explanation, but the forum dumped it when I tried to post it. I don't have time to redo now.
Matt

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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