Is there an easy and accuate way to determine the differential ratio without pulling the cover and counting the teeth? My 62 Signet has a manual trans and a 225 which is as originally equipped according to the recent exercise in reading the broadcast sheet and the fender tag. According to the FSM the standard ratio is either a 3.23 or a 3.31, depending on where you look in the manual, and a 3.55 was optional. I jacked up one side and gave the wheel one turn while counting the turns at the drive shaft, it looked like about 1 5/8 turns, multiplying by 2 i get about 3 1/4 or more likely 3.23 but I'm not particularly confident of that method. Can anyone help me pin it down a little better, or am I going to have to crack it open?

Related to that, if it is indeed a 3.23, how will that work behind an engine with about a 270 camshaft and a high stall converter? Most of the cam grinders are recommending 3.55 or better gears; will it make that much difference if they are a little taller than that?

OK, I see the confused look... I'm transplanting a 904 into the car as well.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

3.31 was not available in the 7-1/4" rear axle used in the A-bodies; think you're seeing 3.31 in the full-size car specs. Your procedure's almost right, but to get the precision you need, do it this way:

Chalk the inboard sidewall of the tire that'll turn and block the other tire (or don't lift it up) so that it can't turn even a little bit. Turn the marked tire through two full revolutions and count quarter-turns of the driveshaft by watching the rear U-joint caps as the come to face the pavement. Divide the number of quarter-turns by four to get your ratio.

16/4 = 4 = 3.91
14/4 = 3-1/2 = 3.54 or 3.55
13/4 = 3-1/4 = 3.21 or 3.23
12/4 = 3 = 2.93 or 2.94
11/4 = 2-3/4 = 2.71 or 2.76
10/4 = 2-1/2 = 2.45
9/4 = 2-1/4 = 2.26

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks. Yeah, that method should yield better results. I knew that I had never heard of a 3.31 but in chapter 3 of the FSM it clearly lists that ratio but at the very front of the book it list 3.23. I'm fairly sure that's what I have but I'll recheck using your method.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Another idea for you read it directly off the ring gear. Use the oppurtunity to change you rear axle lube and while you are there find out what gear it has. If you don't know for certain it has been changed before there is no telling how old and contaminated it is. Mopar axle rtv works very well instead of a gasket for the cover.

I will definitely do that later but I didn't want to open up the case just yet. I might be looking for a 3.55 anyway although no one has answered that part of my question yet.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

3.55 is too steep for a 904 if you ever plan on driving above 45 mph for extended periods of time.

A car as light as yours with small tires will be fine with that cam and 3.23's. Any lower gears and you wont be able to freeway. What size are your tires?

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Yeah....Im the one who destroyed this rare, vintage automobile.....

I don't have them yet but I'm thinking of 205-65R14, or 195-70R14. I still have to get the wheels as well but I've got a line on some 14x6 slotted aluminum wheels at a decent price. I have a complete K-H 4 piston disc brake setup that I need to install so I will have to have at least 14" wheels for clearance.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Depends on how fast you want to drive and how much "high" revs bother you. I drove my car with 205 70R 14's, a 904 transmission, and 3.55 rear regularly on the highway, keeping the engine at 3500 to 4000 RPMs. Of course, it'd only top out at 85...with the wind pushing. And then there's the danger of throwing a rod while trying to pass a minivan.

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'68 Dodge Dart 4dr Sedan

If you clean the bottom of the passenger side flange you can usually find the ratio number stamped into the housing.
DD

OK! Now there's an easy method I can handle! Thanks, I'll check it out.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.