Hey everyone! Just finished a 100% ground-up restoration of 1973 Series III (at least I paid dearly for it). Included in this resto is a new R2.8 Cummins Diesel Engine w/new NV4500 manual 5-speed transmission. Note - engine & transmission install occurred prior to move, with paint and body coming later from other service provider. I have no "real so to speak" 1st gear, so I start in 2nd. When reaching 55 mph speed, the engine runs at 3K RPM - way to high. Any ideas - in terms of easy/cost-effective solution, of what needs to be done to solve this issue? Bottom line, this vehicle will never be used in 4WD (though it all works) and would like to travel cross country. That engine is capable to run comfortably at 80-85 mph...

 

A few misc comments...

That engine is capable of running 80-85... or 150... or whatever. Speed isn't a meaningful measure. Perhaps you mean that you want your Series to run at that speed. If so, that'd be dangerously fast in a giant box stuck on top of buggy springs on an 88" wheelbase. Maybe reset your expectations.

As for the technical aspect of your issue, you're kind of boxed in. A typical solution might be to add an external overdrive unit (like those made by Gear Vendors), but a) you've got a second drive axle that wouldn't be spinning at the same speed = very no, and b) you don't have room for one. The driveshaft and associated angles are already extreme.

You didn't mention the donor for the NV4500 (or ratios), but you're only real option here is to tear it out and rebuild with a more accommodating gear set. First gear in the 4500 was always considered a "granny" ratio, but perhaps someone out there makes a taller overall set (??). If so, there you go. Live with what you can find/afford, and accept that it's at least better than the ratios you have now. Or... yank the 4500 in favor of another model transmission that has more aftermarket support for better ratios.

Finally, you may get a chance to "cause" a solution path when you shatter an axle with the torque from that Cummins. The stock axles are not particularly robust. If you break one, consider going to Defender axles (the whole assemblies). This is a pretty big project, but you'd get better hardware to handle the additional torque of the Cummins, better ring and pinion ratio options, and disc brakes (for when you want to drive your giant metal box 80-85 down the road).

Anyway, best of luck with it. Pics of the Cummins install would be appreciated.