When is it appropriate to down shift?

[JackShephard]

I live in a town where there is a 45mph zone and soon turns into a 35mph zone. When going 45 I am usually in 5th gear. I am able to I 35 without down shifting but I wonder if its healthier for the car to downshift.

[Sonic Boom]

It's better to downshift to 4th gear. If you go too slowly in 5th the revs may drop well below 2000 causing the car to vibrate. The vibrations will wear out the gearbox. Also, very low revs trigger a higher fuel intake to stop the car from stalling. So your fuel consumption will be a lot higher.

It depends on the car, but I'd recommend your revs to be between 2000 and 3000. If you're still in 5th and doing 35 mph but the revs are around 2000 then it should be fine (especially if you are free-wheeling down a hill :P).

[Abyssal Echo]

I live in a town where there is a 45mph zone and soon turns into a 35mph zone. When going 45 I am usually in 5th gear. I am able to I 35 without down shifting but I wonder if its healthier for the car to downshift.

using 5th gear in town is prob not a good idea can easily lugg or bogg the engine causing it to stall. better to use 3rd or 4th in town or local driving.
also clutches dont like down shifts or over reving so easier and cheaper to use brakes. unless you are real good at speed shifting. thats up or down shifting with out using the clutch.

[British Royale]

And lugging the engine and then hitting the gas may cause excess pressure in the intake manafold, something I worry about in an airplane! To downshift I like to revmatch when I can (google it) it saves syncro wear and clutch wear. Even when I do this I still try to use the brakes though.

[MartyG]

5th gear in most vehicles is essentially an overdrive....to be used on the highway at speeds over 50 MPH.

Like others have said....using 5th gear in town will probably result in lower fuel mileage; not greater.....and will cause your engine to lug (strain) because it is not operating at an optimum RPM.

[Mireille]

I don't use 5th gear below 40mph, and normally I'll stay in 4th until 50mph unless it's a long 40 zone with nothing in front.