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Brakes and gas mileage

when the mechanic is showing the car’s rotors to me how can I tell if the rotor actually needs replacement?

Also my gas mileage has gone down recently, I have had the air filter replaced, tires inflated, spark plugs checked for gap and oil changed. Any ideas?

Visually you really can’t tell unless they are very deeply gouged. The thickness has to be measured to see if they are less than the minimum set by the manufacturer. That dimension is usually on the radius of the rotor.

Gas milage my vary from time to time depending on the temperature of the day to the driving changes that you may be having. I would not worry about it too much unless it is something really drastic that is happening.

Helpful to know year, make, model, type of engine in car.

A rotor can be measured and compared to manufacturer’s minimum specs, to determine if it can be re-used after being machined. Many rotors cannot, these days and are considered throw-aways. I rely on my mechanic to make that judgement, but I also trust my mechanic 100%.

Gas mileage can go down simply because the weather is colder and you use more gas. Can you provide a bit more info about how much your gas mileage has gone down? How long have you kept track, before and after you noticed the problem?

I used to own a Mazda 626 and seemed whenever I did the various “xxx thousand” milestone services the MPG increased a lot. It always got good MPG (close to 30 hwy) but I remember when I did a “servicing” at 150,000 miles (they flushed a bunch of systems - transmission, feul pump I think, etc.) and the MPG jumped to as high as 34 on the hwy. Sorry I don’t know more details, but hopefully this helps!

“whenever I did the various “xxx thousand” milestone services the MPG increased a lot”

I’ve seen this before. The mechanic was over-inflating the tires 10 lbs.

This is called the “placebo” effect, you went and spent the money and you want your money’s worth,whether the improvement is real or imagined.

Well, math is math so I’m not sure how an actual calculated increase of 3-4 MPG can be a ‘placebo’ effect (if I’m missing something, please do advise). But that said, does over inflation of tires really have that kind of impact on MPG (positive impact at that)?

Also - are you saying that these kinds of preventative maintence items are a waste of money? The “car talk guys” seem to support this kind of preventative maintence so now I’m confused. I drove this particular car for 13 years-nearly 200K miles- problem free, sold to a friend who drove it another 40K miles problem free, who sold it to another individual after he got a job w/i walking distance - so I don’t know what happened after that. So seems like taking care of it helped, or was I just lucky? Looking for your furter thoughts on prev. maintence. Thanks!

The only thing flushes accomplish are a lightening of your wallet, Seriously, unless a condition was corrected by the action no mpg increase will be realised.

Overinflation of your tires will give a mpg increase (reduced rolling resistance)

The Forum has spoken extensivly about the value of flushes and the only one of value in my opinion is the brake fluid flush, auto trans should be a “drop and fill” with filter replacement,no flush, fuel injection system very rarely benifit from a flush (if you don’t have a F.I. problem don’t flush) there is no manufacture requirement on power steering flush, coolant should also be a “drop and fill” no flush, yes I am saying flushes are a waste of money. Look back at previous posts about the “flush” question,there is not 100% agreement, but it leans very heavy to a waste of money.

Also - are you saying that these kinds of preventative maintence items are a waste of money?

Chances are if “these kinds of preventative maintence items” are not listed in the owner’s manual they are a waste of money. On the other hand if they are listed in the owner’s manual, then not having them done is a very foolish thing to do.

As for the rotors, if you are replacing the brake pads, then it is likely time to replace the rotors. Most modern cars have gone to light weight rotors (less mass, better ride and control). The light weight rotors are cheaper and don’t have the material to allow much turning, so most of the time it is best to not even try and just replace them as part of every brake job.

I’ve never seen an owners manual that says to flush any fluid. They always say something like “replace”. By replace they generally mean to drain the fluid out of whatever pan or reservoir is relevent, then pour in new fluid. Flushes often involve specialized machines and shops promote flushes in an attempt to have those machines earn their keep.

Replacing fluids more often than the manual calls for may be a good thing, like if the vehicle is used in some severe duty. I think motor oil and automatic transmission fall in that category, since they protect such expensive parts. I agree with Oldschool that brake fluid is the one fluid that can benefit from periodic flushes, but draining the fluid from the reservoir with a turkey baster and replacing with new fluid is better than nothing at all and requires no tools. I flush the brake fluid in my cars every 3 years.

This is helpful - thanks for the added comments.