Hello,
A friend just got me a good deal on a used set of 321 gears in good condition and i found someone who can instal them for a reasonable price of about $215 so i think i am going to have them put in. The friend, who has bought and sold over 100 step vans just like the one i have (from the post office) has been telling me for a while that the 321's will greatly reduce wear on the engine since it will run at a lower rpm and he also insists that it will boost the fuel economy of this thing from 10 mph to at least 15 mph, but i am a bit sceptical about that. I am interested mainly in converting this from a local delivery type van to a highway cruiser so i am going to do it anyway. Does anyone have experience with this kind of thing that could say if it really could increase fuel economy by that much? thanks,
Paul

How much does this thing weigh? 321's seems to me would be harder on the engine because you are requiring more torque at a lower speed.

I agree. Plus, don't those step vans have some pretty tall tires already? Taller tires offset low gears. Do you do a lot of in town driving, or mostly highway? What are you carrying for weight most of the time?

I think that your biggest down fall is that your van has the aerodynamics of a brick. You are burning lots of fuel to overcome wind resistance. Your mileage could very well be worse with that high of a gear. A better choice would be 3.73. If your van has the gm 14 bolt 10.5 inch ring gear rear end, they are one of the easiest around to set up. I used to set them up all day long for 100.00, but with inflation and the price of fuel, maybe 215.00 isn't too bad. The only way I would recommend 3.21's is if you need very little throttle to maintain speed once there, but I doubt that this is the case, you probably need a lot of throttle to overcome the wind resistance so I think you might be stuck with the mileage you are getting. To prove the point, I will take my travel trailer, when towing with my 18:1 6.5, I could get 11mpg while towing 60 mph, 8 mpg while towing 75, with my cummins towing the same trailer, 12 mpg towing 60, 8.5 towing 75, with my wifes duramax, 10.3 towing 70 mph. I think it all boils down to wind resistance.

Well i haven't bought anything yet and i am glad now that i hear some other opinions.
To clarify a bit, this is a 5,000 lb vehicle with 19.5" tires. I don't tow anything yet but often have close to 1000 lbs cargo in it, sometimes closer to 2,000 lbs.
So the point was made that tires this size would offset the need for 321 gears? How is that, not sure i understand.
As for driving, a fair amount of local driving but a lot more highway miles.
I was told that at 60 mph, the engine is a 2750 rpm with the 410 gears which will cause it to wear out faster than at with the 321 gears, which would bring it down to 2050 rpm at 60 mph. Is this correct?
It does have the gm 14 bolt, maybe the 3.73 gears would be better. What is a good price on them and where to get?
thanks,
Paul

Specs are:
4.10=2250 at 65
3.73=2050 at 65

I guess, those numbers would be different with the size tires you have. 3.73 would be as far up the scale as you want to go. My bus has 4.10's and pushes a lot of air, thus the lower milage than what the trucks post they are at. The most that I've seen is 13 mpg really, really going easy on it. It's usually about 10 or sometimes less depending upon idling.

I was told that, based on 19.5" tires and the turbo 400 transmission, that the numbers were actually 2750 rpm at 60 mph with 410 gears and 2050 mph at 60 mph with 321 gears. My friend got this info from a place which i think is called Andy's ring and pinion, i think they are in oregon. I may have to call them myself to confirm this since the numbers are quite different than the ones knkreb provided.

Would the 321's present a problem if i do mostly highway driving with no towing and rarely a heavy load? I really don't do much in the city and even when i do it is a 45 mile drive to get there.

thanks,
Paul

I've never heard of " 3.21's ", I tought 3.42 was the lowest.

Daniel.

3.21 were found only in vans and I once had a set that went into a dumpster years ago. What is the overall diameter of your tires? 235-85-r16 are about 32 inches and they will tach about 2700+- at 65. That is actually a good rpm for your engine, but 2400 would be better. If you drop to 2000, you will be very low on power and mileage probably won't improve. Try doing a search on ebay for GM 14 bolt and you could probably come up with a set of gears for less than 100.00.

turbo 400 transmission..............no overdrive = 2700+- rpms at 65. 4L80E ............. overdrive = 2200+- rpms at 65.

Would that be why there seems to be a difference in rpm's vs. speed ???