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Somewhere in cyberspace we found an old scan of a chart that purported to show efficiency of a Volkswagen TDI engine, and if we believe this chart (What choice do we have? It came from the Internet) the TDI is most efficient at 1,800 rpm. Our target, then, is to achieve a tolerable cruising speed at 1,800 rpm.
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This sounds to me like the classic misinterpretation: "best BSFC RPM = most efficient cruising RPM". But I could be wrong, not knowing the TDI well.
Tas, care to comment on that bit?
Regardless, it sounds like they've also sourced the tallest top gear (top gear, get it?) they could fit in their tranny (from a Euro Transporter), so it's kinda moot. They're going as low as they can, RPM wise.
We also get a tip off about the context they're working with for their 70 mpg target:
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we should be able to cruise at 61 mph right on our 1,800-rpm sweet spot.
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Sounds to me like they're saying 70 mpg @ 61 mpg = success.
Somehow I don't think that's too much of a stretch for a TDI with tall gearing. They probably aren't going to have to
Aerocivic the crap out of the thing to meet that goal.