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Discussion Starter · #1 ·
I was wondering at what RPM the engine should be turning at 70MPH? I have an 03 Zetec with an automatic.

My mileage has dropped and I thought maybe this could be the biggest part of the problem.

Any insight is well appreciated! [twothumbs]
 

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Discussion Starter · #2 ·
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For manual transmission, the theoretical RPM can be derived by using the formula -

(mph X 12 (inches per foot) X 5280 (feet per mile) X axle gear ratio X transmission gear ratio) / (pi (3.1416) X tyre diameter X 60 (minutes per hour))

I don't know how to calculate the RPM for auto trans but I suppose you will need to factor in the torque converter.

Going back to a manual gearbox, one would be looking at in the region of 2822 RPM to achieve 70MPH under the following conditions which are relevant to the 2.0L 2001 Focus ZX3 -

i) axle gear ratio = 3.82:1
ii) transmission gear ratio = 0.76:1 (5th gear)
iii) tyre diameter = 24.2" (195/60R15)

Hope this helps
 

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Discussion Starter · #4 ·
That seems kinda high for a manual transmission, especially if it's a 5 speed. I seem to remember back a while ago (I have 509,000 on this car) I would be turning around 2300/2400 @ 70mph and that included lock up on the converter. I'm only getting about 22mpg on the highway now and it was close to 31 before.

This is why I'm looking for conformation on my thought as to why it has dropped considerably.
 

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My 2003 zetec focus is at about 2900-3000 rpm at around 70mph, that is with the auto trans. My duratec 2007 is at about 2800 at 70.
 

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If you suspect an issue with the trans, I think it is possible to use a scantool and get a reading of the gear status at any given speed and RPM.
 

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ELVIS LIVES !
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My 05 5 speed is 2850 at 70 mph. The automatic should be about the same in over drive.
 

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Discussion Starter · #8 ·
I appreciate all the input guys but this just blows my mind! I honestly thought it would be no more than 2500 @ 70mph. I do know that I have a bad shift solenoid from 1st to 2nd but I didn't think that would affect it.
 

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If it's an issue of fuel economy and seeing that the engine has over 500K miles, maybe it's a good time to take a look at the compression. Reduced mileage and torque could be symptoms of compression loss.
 

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Vince your Moderator
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I am at just shy of 2900 RPM @ 70 MPH in 5th gear.

If you are worried about mileage check,

The air filter, the plug gap(.055), plug wire resistance(under 5k ohms is okay)and when was the fuel filter last changed?
 

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Vince your Moderator
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just read about 509,000 MILES????

Even in KM's that's 316,000 miles.

So like the other poster said, a compression test and a leak down test is in order.
 

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just read about 509,000 MILES????

Even in KM's that's 316,000 miles.

So like the other poster said, a compression test and a leak down test is in order.
HAHA I was wondering why everyone else seemed to gloss over that fact!

There are a number of bad gas mileage threads on here. Both me and my wife's focuses get milage in the low 20s. A buddy of mine who had an 02 brand new thought that was odd because he remembered his getting 30+. It seems to be pretty par for the course for these things once they get some miles and age on them. 509K is AMAZING!! Even most Honda's can't brag about THAT! A leak down test and compression test are not expensive or hard and would be a nice snapshot of what's going on in the engine, and as long as those results are good, I personally wouldn't worry about the MPG much!
 

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Discussion Starter · #13 ·
HAHA I was wondering why everyone else seemed to gloss over that fact!

There are a number of bad gas mileage threads on here. Both me and my wife's focuses get milage in the low 20s. A buddy of mine who had an 02 brand new thought that was odd because he remembered his getting 30+. It seems to be pretty par for the course for these things once they get some miles and age on them. 509K is AMAZING!! Even most Honda's can't brag about THAT! A leak down test and compression test are not expensive or hard and would be a nice snapshot of what's going on in the engine, and as long as those results are good, I personally wouldn't worry about the MPG much!
I should mention that the engine was changed out around 110,000 due to an unforeseen incident....I hydrostatic locked it and shot #3 rod out the side of the block. The new engine had 56,000 on it when installed and now is around 440,000 (this was 3 years ago) but the transmission is original with 509,000 and a few other things as well...fuel pump, rack, front calipers just to name a few. I drive for a living and this is my primary car.
 

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440K (and I think you're saying miles) is still an amazing no. for a Focus still on the road. Do the compression tests and take it from there. With the kind of mileage that you do, you'd probably pay for an engine replacement in a year on the fuel savings alone !
 
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