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Transmission/Engine RPM question

 
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Old 03 Aug 2017, 10:54 am
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Default Transmission/Engine RPM question

We have some hills here in the foothills of the Blue Ridge. My wife's 07 non-turbo Cruizer, with just her and I in it, winds up to sometimes 4,000+ RPM going uphill. At about 65 on level ground it's already turning 2500 RPM.

Is the 2.4 Chrysler so lame on torque that it really needs to spin that fast, or is something wrong?? No wonder it gets horrible mileage for a small car.

Would appreciate any info.
Thanks.
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Old 03 Aug 2017, 12:15 pm
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Default Re: Transmission/Engine RPM question

Let's do some "gazintas"!



4th gear (overdrive) in 41TE tranny is 0.69. And the differential final drive is 3.91. And the circumference of 195/65r15 base model PT tires is somewhere around 78.4 inches. That means the wheel has to turn about 832 time for every mile traveled.

First take the overdrive ratio and multiply it times the final drive ratio:

0.69 x 3.91 = 2.70 give or take

Now divide 2,500 rpm by 2.70:

2500 / 2.70 = 925.93

To change "per minute" in rpm to "per hour", multiply times 60:

925.93 x 60 = 55,555.56

Now, divide that by the number of wheel revolutions per mile:

55,555.56 / 832 = 66.77 miles per hour

For those with a six-grade Arkansas education, give or take a couple of mph due to the amount air in the tires, this is going to be the speed of most cars using the 40TE or 41TE tranny going 2500 rpm with 195/65r15 tires. Might be a PT, Sebring, Cirrus, Breeze, Voyager, Caravan, etc. The car model doesn't matter. Fuel mileage on the other hand, has to do with how much fuel is required to keep the car going at that speed. The PT is heavy and not very aerodynamic. 22 mpg seems to be the average for me no matter what PT I had at the time.

PS: If my six-grade math is right, at 4000 rpm in 2nd gear, your PT will be running about 47 mph. But I make no guarantees when it comes to "gazintas". Them bein' kind'a magical and stuff.
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Old 03 Aug 2017, 04:04 pm
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Default Re: Transmission/Engine RPM question

Quote:
Originally Posted by Handy_Cruiser View Post
Let's do some "gazintas"!



4th gear (overdrive) in 41TE tranny is 0.69. And the differential final drive is 3.91. And the circumference of 195/65r15 base model PT tires is somewhere around 78.4 inches. That means the wheel has to turn about 832 time for every mile traveled.

First take the overdrive ratio and multiply it times the final drive ratio:

0.69 x 3.91 = 2.70 give or take

Now divide 2,500 rpm by 2.70:

2500 / 2.70 = 925.93

To change "per minute" in rpm to "per hour", multiply times 60:

925.93 x 60 = 55,555.56

Now, divide that by the number of wheel revolutions per mile:

55,555.56 / 832 = 66.77 miles per hour

For those with a six-grade Arkansas education, give or take a couple of mph due to the amount air in the tires, this is going to be the speed of most cars using the 40TE or 41TE tranny going 2500 rpm with 195/65r15 tires. Might be a PT, Sebring, Cirrus, Breeze, Voyager, Caravan, etc. The car model doesn't matter. Fuel mileage on the other hand, has to do with how much fuel is required to keep the car going at that speed. The PT is heavy and not very aerodynamic. 22 mpg seems to be the average for me no matter what PT I had at the time.

PS: If my six-grade math is right, at 4000 rpm in 2nd gear, your PT will be running about 47 mph. But I make no guarantees when it comes to "gazintas". Them bein' kind'a magical and stuff.
Wow thanks. Didn't really need all the math though. The "final drive ratio" explains it. 3.91? Yikes! That's the ratio in my 1971 Boss 351 Mustang.

I don't agree that "heavy and not very aerodynamic", has much to do with it, especially at only 60-65 mph on short trips. My 2014 Roush Mustang has a 615HP supercharged 5.0 V8 and 3.73 rear gear. It weighs 3,700 lbs. and on flat ground at 65mph will get 27 mpg.

I think the motor in the PT is inherently inefficient at burning the fuel, and along with the gearing just makes it a poor package. But let's face it people love the PT for the look (like my wife), and it's not a bad little utility vehicle.
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Old 03 Aug 2017, 04:14 pm
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Default Re: Transmission/Engine RPM question

I am not so sure it's all the engine's fault. I use to have a Sebring convertible with the same engine and tranny. It got around 27 mpg.
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