DodgeTalk Forum banner

Status
Not open for further replies.
1 - 14 of 14 Posts

·
Registered
Joined
·
133 Posts
Discussion Starter #1 (Edited)
i have a 74 dodge 4x4 it has a 3 speed auto. (727 torqflite) i think. it has a divorce transfer case but my tach says im running about 3500 going down the road at 65 i would like to no if this is right or is my tach off. i would like to somehow bring this down for gas milage if i could. my rearend has no cover on it if thats helps its all solid
 

·
Registered
Joined
·
22 Posts
Swap in a 5 speed. Get a late model bellhousing, tranny, clutch assy., etc. You will need a clutch pedal assy. out of a '93 down truck.
I recall someone else had this idea a while back, don't know who though. lol
 

·
Registered
Joined
·
22 Posts
The OD would give about 25% RPM reduction depending on the unit. I'd guess about the same for gas milege, plus or minus. It would give you two more gears to use while towing too.
 

·
Registered
Joined
·
363 Posts
Those RPM's seem high. I've got 33's, a 3.55 gear ratio and 727 (3spd auto) and I think I'm running around 2200 RPM's at around 60 mph. I can't remember for sure since it is my farm truck and I don't take it on the highway much. Your rearend is probably an 8 3/4 with a third member (no cover). The differential is removed from the u-joint side and comes out as an assembly.
 

·
Registered
Joined
·
133 Posts
Discussion Starter #8
idk what the deal is then because my tac says about 3500 but bout 500 idleing and im perdy sure the idle is bout right
 

·
Registered
Joined
·
353 Posts
500 seems kinda low. I think mine runs around 650rpm or so. I don't have the truck near me as it's in Madison, but there is a sticker on the hood or fender IIRC.

My truck runs pretty high too down the hi-way, but its just 2wd with a 3.73 8 3/4 rear end and stock sized tires.
 

·
Aka Force Fed Mopar
Joined
·
6,120 Posts
I used to have my truck idling at 500. It is a little low though. And yes, you do have the 8 3/4 rear end, if it has no cover.

Sounds like you probably have about a 4.10 gear. I used this calculator http://wahiduddin.net/calc/calc_speed_rpm.htm . I put in 1.2 for the trans ratio (to allow for torque converter slippage).
 

·
Registered
Joined
·
363 Posts
Since 3rd gear is a 1:1 ratio, the formula to determine RPM is

MPH x gear ratio x 336
RPM = ---------------------
tire diameter

or in your case with 4:10 gears and 30 inch tires at 65 mph

RPM = 65 x 4.10 x 336
----------------
30

RPM = 2985

I think idle is usually set around 700 rpm
 

·
Registered
Joined
·
363 Posts
It didn't take my spaces, here is the formula again

RPM = (MPH x gear ratio x 336)/ tire dia
 

·
Aka Force Fed Mopar
Joined
·
6,120 Posts
Right. But, in an automatic, a 1:1 becomes a 1.1:1 or 1.2:1 ratio, because there is always some slippage in the torque converter.
 
1 - 14 of 14 Posts
Status
Not open for further replies.
Top