I bought a 1991 Jetta GL Diesel last week with 72,300 miles on it and I love it. Problem is it has no tachometer, so I'm shifting by ear and I'm worried I might be shifting too early or late. I know about the little shift marks on the speedometer, but those seem to be set for the gas engine. Can anyone give me approximate shift up/down speeds for my new/old car?
There is really no fixed shift points that you MUST follow when shifting a manual car. With a diesel you have a much smaller rev range, but that doesnt mean you can't "shift by ear". If you are looking to minimize fuel consumption, you will want to hold gears just enough to keep up with the flow of traffic. Holding lower gears and letting the engine rev to high will burn more fuel than necessary. Conversely, being in too HIGH a gear and trying to drive slowly in a very high gear can be very hard on the engine as well. (Imagine trying to pedal a bicycle in a high gear at low speeds, its hard isnt it?)
I've never driven a Jetta without a tach, but in Vanagons without tachs there are little orange dots on the speedometer to indicate the maximum recommended speed in each gear. Single dot for first gear, double dot for second etc. Is it possible that your car has feature too? If so as long as you're shifting somewhere in the middle of the range between dots you should be fine.
By all means, go by the feel and sound. If anything, err on the side of higher revs when shifting up, but within reason, of course. It is very hard on an engine when you lug it (under rev it.) In the long run this means you will wear out the engine much sooner. That is a fact. Some people claim that running the engine periodically (say, once a month) at higher revs is actually good for the engine. I'm inclined to agree.
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