Those dummy lights tell you to shift at ridiculous times as far as I’m concerned. You can " feel " when it’s time to shift, and I’ve always felt it’s at far higher RPMs than that light would indicate.
lilah and I recently bought a 2001 Ford Focus ZX3. To my dismay, it has an upshift indicator. The nice thing about the more recent processor is that it considers acceleration, i.e., it does not signal while accelerating whereas older models do. For example, if I’m tooling along at 40 MPH in third gear the indicator will illuminate until the engine speed reaches approximately 1500 RPM; however, if I’m in third gear pushing 7000 RPM the indicator won’t illuminate provided engine speed is increasing.
Coldfire made an excellent point. If your friend is in fifth gear at 35 MPH and attempts to accelerate, (s)he will have to fully open the throttle, thereby consuming a lot of fuel for a long period of time. Conversely, if one optimizes the powerband, accelerating requires much less time regardless of how much throttle is utilized. Our ZX3 has a fairly broad power band. Torque peaks at 4250 RPM. In second gear, I can accelerate from 20 MPH to 40 MPH in a few seconds while utilizing a range of roughly 3000 to 5500 RPM. Attempting the same maneuver in fifth gear would take eons.
One small note, Coldfire, utilizing the maximum torque output only yields better control in specific conditions. One can use the low end of the torque curve to provide better traction in other conditions, e.g., snow or recovering from slides.
Another thing to consider is gear ratios. Fifth gear in manual transmissions (for five speeds) is similar to an overdrive in automatics. First through fourth gears have ratios designed for specific ranges of speed. Your friend is not utilizing them appropriately. Using a tall gear where a short should be used or vice versa is not desirable.
Pertaining to the mechanical ramifications your friend has experienced, the wear on the clutch has probably doubled due to superfluous shifting. Additional strain may have been induced on the timing belt due to erratic cam (and crank) revolutions–that nifty little phenomenon of “lugging”–although other possibilities may have played a significant factor in its demise.
To sum up, the upshift indicator or idiot light is precisely that: idiotic.
All things being equal (which they often aren’t), normally aspirated, internally combusting, reciprocating engines act very much the same. The OP describes a situation that can potentially greatly increase the side forces of a piston to its cylinder. The bearing in the connecting rod that surrounds the journal on the crankshaft will likely see a similar increase in load. Lubrication on both old and new engines can vary in effectiveness with engine rpm’s. In some instances, up to a point, a higher rpm will provide better lubrication, particularly on the cylinder walls.
Low vehicle speed coupled with a higher gear and lower engine rpm, combined with the fuel/air feeding system trying to provide more potential food than the engine can readily eat (lugging) will decrease the life of an engine - how much depends on a variety of circumstances.
Even if the car isn’t lurching, and there is no audible pinging, knocking, pre-ignition or whatever your local specialist chooses to believe it should be called, regular lugging will quite possibly lead to an earlier than normal service charge from your friendly neighborhood, and totally above reproach, service technician.
Ps: Shift lights have and do vary in the methods they use to decide when to inform you to shift. Electronic rpm sensing coupled with manifold vacuum are fairly standard. Different driving styles that are still well within some perceived norm can cause those cute little lights to be completely useless.
I ignored those Idiot lights happily on the late 80’s GMC s-10 I used to drive. (4.3 liter v6, 5spd manual. Fun.)
the engine is lugging if he’s doing 30 in 5th gear. Stupid, if I may be blunt. Tell him to downshift to 3rd, or 2nd if the motor likes it and everyone will be happy. Or exile him to a slushbox (automatic) and be done with it. 
Absolutely. I was assuming dry tarmac, where plenty of grip is available.
Heck, I might as well add I was assuming a decent set of tires and well-functioning shock absorbers as well
You get the idea - but thanks for clarifying, you’re 100% correct. Don’t attempt a ninety degree curve at 30 MPH on ice, kids 
Coldfire, you don’t want us to have any fun, do you?
A few nights ago, I attempted a ninety-degree turn at roughly 30-mph on a clean patch of black ice in my BMW. The results were exactly what one would expect. I was not expecting the ice at all.
The tow arrived quickly, the damage was minor and no one was hurt.
Just doing my part to fight ignorance by proving by trial what would ordinarily be perfectly obvious to a chimpanzee.
On a Mustang, dude? You take out like a 9 month loan or something?
Way off topic, but I was asked…
Nah, 36 month loan at 0.9% APR.
I paid about $100 under invoice for the car - I think it was $24,500, give or take. Being the sucker I am, I went for the 5-year extended warranty, and lump on tax and title, and it was nearly $29,000, and I put $3,400 down.
My payments are $735 a month, but I pay $170 a month in auto insurance leading to the $900 a month I stated.
As Handy suggested, I shift at the recommended speeds in my manual. For a Honda Civic, 3d gear comes in at 28 mph, 4th gear at around 40, and 5th at 45. But I don’t shift into fifth until I reach cruising speed (which is - any cops around? - around 60-65).
One thing that has puzzled me is that the manual states I don’t have to downshift until the mph is a bit less than they would be to upshift. I mean I could stay in 3d until 25 mph. Is this due to some effect of inertia?
barbitu8:
The reason that I downshift at lower speeds than I would upshift is that otherwise my car jerks as my engine attempts to jump from 2500 rpm to 4000 rpm (e.g. trying to switch into 2nd gear at 35, which is about when I upshift). Thus, I let it drop a bit in speed before I downshift, which means the car doesn’t complain as much.
Speaking of which, is there a good way to get around this problem, to all of you people who undoubtedly have more experience driving manuals than I? I’ve heard about double clutching and the like, but I’m not particularly good at it.
Just disengage the clutch gradually, and you’ll be fine - provided you don’t redline the engine. And don’t take 10 seconds to declutch - you’ll wear the damn thing off in 3 weeks
The Upshift light is a dodge. It’s put in the car so that the engineers can tell the EPA that the car is shifted according to such-and-such a schedule, and when shifted according to the upshift light, the emissions are X amount, and the mileage is Y.
The one in my '85 Dodge Omni is completely unrelated to real-world situations, and I ignore it totally, except to cast a bemused eye at it every now and then when it comes on at some totally inappropriate time.
When I first bought my 88 S10 I was showing it to my father in law and decided to try to shift using the shift light. Not only did it confuse me,I’d been driving for 25 years, My father in law looked at me like “well do you want me to get out and show you how to shift a stick?”.I never used it again.
As far as when to shift I always considered 5th gear as overdrive. So you only have 4 gears. I shift by sound,right or wrong thats how I learned and it works for me.Even if the car/truck has a tach I still find myself shifting by ear. Very simply you shift, listening to the accelerating smooth running engine, when it becomes a racing engine .If the engine is not smooth running after the shift you have shifted at the wrong speed. In the case of a truck the load makes a difference as does the road conditions, hills etc…