I see your question is 12 years old but in any case, I have a 1990 Range Rover Classic with the 3.9. It sounds like you're going up to Arrowhead, whereas my place is in Big Bear. With the gas pedal on the floor on either of the three different routes up the hill my truck barely goes 40 MPH. In 3rd gear it just makes more noise and the temperature starts climbing.

As a matter of fact the temperature rises uncomfortably high anyway when going up that hill on a hot day. On the flats everything is good (easily cruise at 70-75 MPH) and on decent sized uphill grades the temperature is fine but still low MPH, but once I start up the 330 or 38 it's really slow and the temp gauge creeps up over half. I've replaced the head gaskets, water pump, both electric fans and the fan clutch. Several people and a trusted mechanic who owns a truck like mine have all said it's normal but it's still nerve racking.

Anybody have any of the same experiences?