Speedometer Gear Question - 727

-
1BAD72

1BAD72

Well-Known Member
Joined
Aug 7, 2016
Messages
89
Reaction score
41
Location
Fort Myers, FL
Anyone with 3.73 gears and 275/60R15 rear tires... what 727 speedometer gear did you use? Charts say Red 36 teeth but I rather hear from someone who made a change and ended up with an accurate speedo. 1972 Duster with early 70's 727. Thanks...
 
Understand, if you know what's there now and what your GPS speed is and your speedometer speed is you can get the gear dead nuts on. No guessing.
 
And one more;
If you calculate the exact per-cent error, then the new gear will need that exact percent correction. More teeth will slow it down and fewer will speed it up
Here's an example;
if you went from 3.23s to 3.73 you might find by testing, that the per cent error is 15%, so you need a 15% change in tooth count. If she's reading 15% high, you'll need more teeth.
If you had a 30tooth; now you will need 15% more or 34.5. From 34.5 ,you would round it to whichever way you want, or to whichever gear you can get.
Then retest and make a mental correction forever after.....
 
AJ/FormS said:
And one more;
If you calculate the exact per-cent error, then the new gear will need that exact percent correction. More teeth will slow it down and fewer will speed it up
Here's an example;
if you went from 3.23s to 3.73 you might find by testing, that the per cent error is 15%, so you need a 15% change in tooth count. If she's reading 15% high, you'll need more teeth.
If you had a 30tooth; now you will need 15% more or 34.5. From 34.5 ,you would round it to whichever way you want, or to whichever gear you can get.
Then retest and make a mental correction forever after.....
Click to expand...
Presently the speedometer is off by a factor of about 1.45, so 79.5 mph on my speedometer is 55 mph actual on my GPS.
 
Dana67Dart said:
Understand, if you know what's there now and what your GPS speed is and your speedometer speed is you can get the gear dead nuts on. No guessing.
Click to expand...
You'll never get a 50 year old mechanical speedometer to be "dead nuts" no matter what you do. All you'll ever do is get it "pretty close".
 
I'm fine with that. I just figured if someone already did it and put in a 35 they could tell me if they were a little high or a little low and then I'd know which way to go with my gear selection.
 
That might work, but when you're up in the 30's for gear teeth, each gear tooth change is about a 3% difference. So if you're not "off" by 3% or more, you probably won't get it any closer than that. The charts are really quite good, and they have already rounded everything off to as close as you can get.
 
AJ/FormS said:
wow 45% is a lot
Click to expand...
In answer to the orig. question, I have 3:73 gearing and did run 28" drag radials w/ a 37 tooth gear , seemed it was a little slow. Now 295 60 d15 w/ same gear, is a little faster than what the bouncing bastard says . Bounces about 5-7 mph constantly----------
 
1.45 is a lot because
275/60-15s are about 28 tall compared to E70-14 being about 24.5 tall. right there we have a difference of 14% and the taller tires are slowing the speed-O down.
but the new 3.73 gear are speeding up the speed-O.
The net effect is therefore actually 45% PLUS 14% = 59%
So with 3.73s you would have to add 59% to the tooth count. That means if the correct gear was in fact 35 teeth, then the current gear would have to be 35/1.59=22 teeth, and therefore the original gear mustabin a 2.34
That's a lot.
Plus I've never heard of a 2.34 Mopar rear gear, in an A-body from the 70s.
I have heard of 2.45s and 2.20s, tho, in FMJ cars from the early 80s
Oh wait that's not right.
Since the 1.45 is absolute, then we have to subtract the 14% from the 45% and get 31% on the gears taking us from 3.73s to 2.57s
So it's still a lot, just not as lot as I thhought,lol.

But you can't do any math until the current gear is out and a tooth-count is on the table. Or you can take a chance that the chart will get you in the ball park and if it isn't, then at least you know what's in there, to order another gear.. In either scenario, you are taking it out twice.
If you jack the back of the car up to get the speed-O adapter up higher than or equal to the bottom of the pan; OR jack the car up by the drivers-side rear corner, THEN
very little oil will come outta there when you pop the adapter out; I've done that many many times.
I got a tip for you tho, start the car up and let it idle for a half a minute to prime the convertor. Sometimes they drain back and an extra qt ends up in the trans-pan.
 
Last edited:
oh wait, might want to calculate from a new tire height to a height AFTER burning all the rubber off! lololololol

All jokes aside, I've found the speedo tooth charts close enough and easy peasy.
 
You must log in or register to reply here.
-

Back
Top