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PostPosted: Sat Aug 19, 2017 2:33 am 
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4 BBL ''Hyper-Pak''
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Joined: Mon Jun 19, 2017 7:25 pm
Posts: 48
Location: Waco, TX
Car Model: '69 D100
Another one of these threads... I don't quite understand the math here so bear with me. Motor is stock, 3-on-the-tree manual and I'm running 215/75/r15 tires. Planning to do about 50/50 highway/city driving with very occasional towing (most of the time the long bed has more than enough room).

It seems whatever gearing I've currently got can't be too ideal for my usage because it sounds like she's going to explode over 55, ie anytime on the highway. Suggestions? The manufacturer's plate looks like it says 3.5 but it feels like it's lower than that. Maybe it's because my tires are so small.

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PostPosted: Sat Aug 19, 2017 6:51 am 
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Supercharged
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Joined: Sun Nov 03, 2002 9:20 pm
Posts: 13008
Location: Fircrest, WA
Car Model: 76 D100
I would say a 3.5 or even a 3.2 gear (if you don't do much hauling) would be a good choice for you. You might need to crawl under there, pull the cover off the rear differential, and count the teeth.

For what it is worth, my 76 D100 short bed has factory 3.9 rear gears and it does spin up high on the highway. I am running 235/75/15 tires and it has the factory 727 trans. The truck can still get up over 80 and keep up with traffic, but fuel economy is terrible and the poor little slant is turning way more RPMs than I want to see.


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 Post subject: I would...
PostPosted: Sat Aug 19, 2017 10:04 am 
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Joined: Tue Oct 29, 2002 8:27 pm
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Location: Salem, OR
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Looking at the heavy brick the truck is, and the tire size... the slant will need a better diff gearing to help the little motor... Just looking at the calculator for rpm at highway speeds... 3.55 with that tire size gets you about 24-2500 rpm at 65 mph, which would be good for the 225 and as best mileage as the rig can get....3.73's are a bit better if offered... another thing to consider is whether you are comfortable with the sound at that rpm... I'm going to change out the tail pipes on the Aspen because 65 mph at 3000 tends to drone more and sound like I'm at the top of the clock on I-5 compared to when I would drive the Duster in the 22-2500 range....

As always, you will want to degree the cam and get the distributor curve set to make peak torque around this rpm band for best grunt and mpg....

If it's a '69 you probably have an 8 3/4" rear so there is no diff cover... just pulling the pumpkin....


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PostPosted: Sat Aug 19, 2017 12:44 pm 
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Joined: Wed Oct 23, 2002 7:57 pm
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Location: Waynesboro, Pa.
Car Model: 65 Valiant 2Dr Post
Quote:
it sounds like she's going to explode over 55,


If it is really that bad then you probably have a 3.91 or even a 4.10 rear gears. At this point there is no way to know what may have been put in. Just jack it up and check the ratio by spinning the driveshaft and see how many turns you get for one revolution of the rear wheel.

FWIW- my truck has 3.55's and 255/70 tires(close the same height as the 235/75) and 55 is just about the perfect cruise speed. 60 - 65 is Ok too but once you get above that then I would not want to go on any 500 mile trips! :lol:

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PostPosted: Mon Aug 21, 2017 6:40 am 
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Turbo EFI
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Joined: Mon May 12, 2008 1:11 am
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Location: North Georgia
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Like Reed said, 3.55 would be a good compromise, or if you live in a flat area, even something in the 3.2 range. I live in the mountains so a 3.55 is a good overall gear in my D100. I have an A833OD but seldom use overdrive.

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PostPosted: Mon Aug 21, 2017 1:48 pm 
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4 BBL ''Hyper-Pak''
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Joined: Mon Jun 19, 2017 7:25 pm
Posts: 48
Location: Waco, TX
Car Model: '69 D100
Thanks all for the recs. I live in a fairly flat area so I'm leaning toward 3.2-3.5. Would just like to have fun on the occasional highway cruise... Might need front disc brakes and AC for that as well, anyway

@Dadtruck, I got a PM about your gear and pinion.. I tried to respond but I think it's stuck in the outbox. Email me if you like, zackery@willoughby.net

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'69 D100, 225 SL6, A745→NP435


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PostPosted: Mon Aug 21, 2017 4:50 pm 
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TBI Slant 6
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Joined: Mon Aug 03, 2015 2:39 pm
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Location: Dallas Texas
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With your tire size, you will be turning 3000 rpm's at 70 mph with 3.55 gears. 3.21's will turn 2700 rpm's at 70 mph. If you want to keep up with the 80-85 mph traffic on Texas interstates, I would recommend 3.21's.

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PostPosted: Sun Jul 15, 2018 4:36 pm 
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TBI Slant 6

Joined: Thu Dec 23, 2004 9:35 pm
Posts: 248
Location: South Eatern Pa
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check the drivers door tag, it should say the axle ratio

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PostPosted: Sun Jul 15, 2018 9:37 pm 
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Joined: Tue Oct 29, 2002 8:27 pm
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Location: Salem, OR
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Quote:
check the drivers door tag, it should say the axle ratio


Assuming that no one changed the rear end pumpkin in the meantime... (I pulled a 225 and pumpkin out of a '63 that the door tag said was a 3.91 8 3/4" diff... turned out to
be a 1962 3.58...one year only ratio...)


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PostPosted: Mon Jul 16, 2018 6:11 am 
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Supercharged
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Location: Downeast Maine
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I run my Dart at 3000 rpm all the time on I-95 100 mile trips at a time; = 60 mph. 3500 rpm = 70mph. It tends to use oil over 3000 rpm, but no smoke.

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