first, thanks to gsxr for the great gear ratio and rear axle/subframe table.

ok, this much i know about gear ratios: the number represents how many turns the driveshaft needs to make to turn the wheels 1 time.

my 87 300DT is geared at 2.62. i have a donor rear axle/subframe which i believe came from a 91 260E (i need to double check this weekend) which has a gear ratio of 3.27.

check my understanding here: that means that if I swap the rear axle in WITH the 3.27, i will get better pickup from stop, but i will suffer in the mpg department. if my guess is yes, by how much will mpg suffer? this is exactly what i dont want. so onto my next question:

can i swap in my current diff into the donor axle/subframe? here's whats identical:

axle housing number (124-351-06-05)
Housing (124-351-06-05)
Rear cover (124-351-25-08)
axle flange (208-350-01-45)
axle shaft (129-350-40-10)

But here's what's different:
Carrier (124-350-63-23)
Prop flange (211-350-00-45)
Flex disc (202-410-13-15)

Can you guys provide me with some ideas/input?

Thanks,
Bob

IMO you don't want a numerically higher diff in your 300D.

Yes your mileage will suffer. More important (to me anyway) than fuel mileage is the engine RPM at freeway speeds, it will increase the rpm: 3.27/2.65xrpm will give you the new rpm at any speed, or 1.23xrpm. So, cruising at 75mph will go from 3000rpm to 3700rpm.

What's wrong with your old diff/subframe? Rust?

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Gone to the dark side

- Jeff

jeff,

thanks for the simple and clear explanation. sounds like its not a good option to pursue. i didn't realize the net effect esp at highways speeds.

yes, the rear subframe is rusted, but the differential is in fine shape. if putting in my existing diff is an option and it physically can fit with the 89 260E subframe, ill do it. given what i wrote re differences, is it possible?

bob

I don't know, but would be surprised if it isn't a bolt-in swap.

I'm in SW Michigan, have a spare 300D subframe sitting on the shop floor, worst case (if you're in the area) you can strip the subframe and take it. I only kept it for the suspension and diff.

To your differences question: The prop-shaft flange and flex-disk will stay with your 300D differential, so ignore that difference.

The carrier, I'm used to the carrier being the casting inside the housing that holds the gears together, and if so that is again not a problem as you're keeping your old diff. If it's a different piece, a bracket or something, swap it with the diff.

__________________

Gone to the dark side

- Jeff

Jeff,

Thanks for the offer, but I'm in So Cal And thanks for the suggestions re parts differences. I'll definitely need to go to the donor subframe and give it a good inspection again. Hopefully the bolt pattern for the diff is identical.

bob

what's your majority of driving ??

swaping a 3.27 into mine because of the lack of really driving 80+ very often,
rather have a little more off the line because of city driving

That will probably increase your fuel usage by twenty percent more or less. So if you are getting twenty four it would drop to about twenty mpg.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Have you already verified switchover valve operation, wastegate (adequate boost) proper actuation, and modified the ALDA teezer? While it won't spin the tires on dry pavement, these things will get you to a decent launch. Turning up the rack will give you more (albeit with a little black smoke) but only when you put your foot in it.

__________________

Gone to the dark side

- Jeff

T: thanks for the estimate. i figured this evening that if the gear ratio is approx 23% taller, mileage would probably be equally affected.

babymog: i have not done any kind of engine tuning at all. i am still repairing and welding in patch panels: www.biodieselbob.blogspot.com. the only work that i've done to the engine is changing a cracked fuel line, stuck a bb in the hose leading to the egr...i think thats it. engine stuff will come later after the rust repair and then the suspension R&R.

teezer: the majority of driving i will do once i get it safe and operational will be a 65 r/t commute to work and back home. thus, i'm going to stick w/ the current differential. if you want this diff, i'll be more than happy to sell it to your for cheap.

bob