Slant *        6        Forum
Home Home Home
The Place to Go for Slant Six Info!
Click here to help support the Slant Six Forum upgrade!
It is currently Sun Dec 10, 2017 11:48 pm

All times are UTC-08:00




Post new topic  Reply to topic  [ 7 posts ] 
Author Message
PostPosted: Sat Aug 19, 2017 2:33 am 
Offline
4 BBL ''Hyper-Pak''
User avatar

Joined: Mon Jun 19, 2017 7:25 pm
Posts: 31
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.

_________________
'69 D100 3-spd 225


Top
   
 Post subject:
PostPosted: Sat Aug 19, 2017 6:51 am 
Offline
Supercharged
User avatar

Joined: Sun Nov 03, 2002 9:20 pm
Posts: 11718
Location: Fircrest, WA
Car Model*:
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.


Top
   
 Post subject: I would...
PostPosted: Sat Aug 19, 2017 10:04 am 
Offline
Board Sponsor
User avatar

Joined: Tue Oct 29, 2002 8:27 pm
Posts: 9212
Location: Salem, OR
Car Model*:
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....


Top
   
 Post subject:
PostPosted: Sat Aug 19, 2017 12:44 pm 
Offline
Board Sponsor

Joined: Wed Oct 23, 2002 7:57 pm
Posts: 4830
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:

_________________
2 Mopars come with Spark plug tubes. One is a highly refined, record setting, world class, racing machine. The other is a 426 CI. boat anchor!
]Image


Top
   
 Post subject:
PostPosted: Mon Aug 21, 2017 6:40 am 
Offline
Turbo EFI
User avatar

Joined: Mon May 12, 2008 1:11 am
Posts: 1164
Location: North Georgia
Car Model*:
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.

_________________
If it was easy, everybody would be doing it.
Image


Top
   
 Post subject:
PostPosted: Mon Aug 21, 2017 1:48 pm 
Offline
4 BBL ''Hyper-Pak''
User avatar

Joined: Mon Jun 19, 2017 7:25 pm
Posts: 31
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

_________________
'69 D100 3-spd 225


Top
   
 Post subject:
PostPosted: Mon Aug 21, 2017 4:50 pm 
Offline
TBI Slant 6
User avatar

Joined: Mon Aug 03, 2015 2:39 pm
Posts: 135
Location: Dallas Texas
Car Model*:
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.

_________________
1976 Feather Duster /6 4sp
1984 W100 /6 np435 np208
1972 'Cuda 340 4sp
1985 D250 360 46RH


Top
   
Display posts from previous:  Sort by  
Post new topic  Reply to topic  [ 7 posts ] 

All times are UTC-08:00


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Limited