Good gear ratios for a Diesel?
 

These are made using 205/75R14 tires, a PT Cruiser transmission, and assuming a 3500 RPM redline, but the redline of Cara's donor engine is more like 3000.

RPM.....MPH
3500...85.07
3000...72.92
2500...60.77
2000...48.61
1500...36.46
1000...24.31

Gear ratios
1st.................3.500
2nd................1.960
3rd.................1.360
4th.................0.971
5th.................0.811
Final drive ratio 3.940

The engine makes peak torque around 2000 RPM (all 75lb ft) and peak HP at like 3000 RPM (screaming 40 HP)

I don't usually go over 60 MPH, and a good bit of my driving is @ 50-55 MPH.

Does this seem like it would work out, or should I use something with longer gears, like a 3.55 final ratio?

With a 3.55 and the same tires, I get the following RPM/Speed:

RPM......MPH
3500.....94.42
3000.....80.93
2500.....67.44
2000.....53.95
1500.....40.46
1000.....26.98

If you'd like to consider aerodynamics before answering, the current numbers for Cara are 3500lbs, .35Cd, and 35FT^2 frontal area. (Yeah, it seems weird, but they're the numbers I've got.)

Either set will put me in 5th gear for my "around town" speeds and keep me about 1250 RPM.

With the longer gear set, I can run the "highway" at about 2000 RPM (close to peak torque), but those gears are harder to find... So would the first gear set be OK? (I can find them all day.)

Is either of them too long? Will there be any other problems with them?

Well, I know it's not exactly the same, but when I geared up my Ninja, (all 300lbs of it), I went from 14/45 to 15/41. First gear felt roughly the same taking off, though it did have more to go before it ran out, but 6th was difficult to use on hills.

I'd say go for the lowest set of gears you can find, you can always shift down.

To put those ratios into perspective, the Merc cars of the time w/ the ~50hp@4k rpm diesels were geared more or less the same, w/ the exception of having only 1-4 and no OD. They were about 700lbs lighter, probably had the same or less CdA, and would do 0-60 in slightly less than 30 seconds, so if you're O.K. with having the slowest vehicle on the road, go for it. Just make sure traffic is at least a mile away before you start to merge. :thumbup:

that is a diesel ratio? it is not utilizing the torque available. Even a 90hp subaru boxer has .780 as the final drive through a ten gear AWD transmission. little diesel Engineers ought to be embarrased. I found not much difference happens once you get into tractor trailers, the weight and other physics is the gears. alot fo steel to take what your digging into (tall gear == deeper dig of course)
for cars and regular trucks however, it is a direct repsonse to the throttle, not much steel to spring to life and drive. Go as tall as you could purchase for your setup. A small diesel has been an incomplete hack job for a long time now...

:)

What diesel are planning to put in Cara.

These RPMs seem awfully high for a diesel.

Dead stock, my diesel ran 2000 RPM@70MPH, and I thought that was awfully wasteful of fuel.

It's an OM636 4 cylinder, only has about 40 HP. I can add a turbo if I'm gonna need more power, but the power per RPM limits kind of keep me from having 60 MPH below a given point, b/c the engine won't have the power to pull Cara's plump rump around.

The engine is a pre-combustion type... I may have mis-stated when I said it was common-rail, as it's referred to as "IDI" or Indirect Diesel Ignition. The fuel doesn't get injected directly into the cylinder, instead, it gets injected into a pre-combustion chamber, which supposedly allows a faster propagation of the initial burn.

Yeah. Your ratios are good for an OM636. One of the weaknesses of these Benz engines (especially the normally aspirated ones) is poor low-RPM torque. You'll need the gears. At least it will drive the accessories without a speed-up pulley.

You can get decent torque from a OM617, but that requires a VNT turbo from a Sprinter and turning the ALDA way up.

How much does a OM636 weigh?