• 1 Vote(s) - 5 Average
  • 1
  • 2
  • 3
  • 4
  • 5
OK, I'm a nerd... But what do you think re: actual RPMs vs expected
#1
So, based on my tire size, transmission and rear-end ratios, I should be in the neighborhood of 2,450 RPMs at 70MPH.

However, I'm actually turning closer to 2,900RPMs.

I have checked the calibration of my tach, and it is dead-nuts on.

So, I have (what I believe are) 3 possible causes of the variation of actual versus expected:

1) a PO put in a steeper rear end. I'm assuming I have a 2.75 based on teh Marti report, but a PO could easily have installed a 3.25, which would match up with the RPMs I'm actually seeing.

2) my torque converter hasn't "locked up" yet, and is still a bit inefficient at 2,900RPMs.

3) I'm using the tire diameter without giving effect to factors that may make the actual diameter a bit smaller, like the weight of the car pushing down on the wheel, and therefore, reducing the diameter... or maybe the air pressure isn't at the top of the scale.

It's really not a big deal... just curios about things like this, and wanted to see if any of you more experienced folks have any other ideas. The only reason I'm even contemplating this is because I'm in the process of swapping transmissions (from a 3-speed auto to a 4-speed auto) and also will be going to a 3.89 rear end... all planned so that I have a targeted final gear ratio (in top gear) similar to what I have now (but my first and second gears will be a LOT steeper). And if my current setup is steeper than I thought, then I ought to see a reduction in RPMs once I'm finished.

I know, I know... My calculations are all theoretical... but are there other factors that would affect RPMs at a given speed other than the three I listed?
  Reply
#2
CZ-75;130687 Wrote:So, based on my tire size, transmission and rear-end ratios, I should be in the neighborhood of 2,450 RPMs at 70MPH.

However, I'm actually turning closer to 2,900RPMs.

I have checked the calibration of my tach, and it is dead-nuts on.

So, I have (what I believe are) 3 possible causes of the variation of actual versus expected:

1) a PO put in a steeper rear end. I'm assuming I have a 2.75 based on teh Marti report, but a PO could easily have installed a 3.25, which would match up with the RPMs I'm actually seeing.

2) my torque converter hasn't "locked up" yet, and is still a bit inefficient at 2,900RPMs.

3) I'm using the tire diameter without giving effect to factors that may make the actual diameter a bit smaller, like the weight of the car pushing down on the wheel, and therefore, reducing the diameter... or maybe the air pressure isn't at the top of the scale.

It's really not a big deal... just curios about things like this, and wanted to see if any of you more experienced folks have any other ideas. The only reason I'm even contemplating this is because I'm in the process of swapping transmissions (from a 3-speed auto to a 4-speed auto) and also will be going to a 3.89 rear end... all planned so that I have a targeted final gear ratio (in top gear) similar to what I have now (but my first and second gears will be a LOT steeper). And if my current setup is steeper than I thought, then I ought to see a reduction in RPMs once I'm finished.

I know, I know... My calculations are all theoretical... but are there other factors that would affect RPMs at a given speed other than the three I listed?

With 2.75s, you'd be looking at 1600-1700 RPMs @ 65. 3.00, around 1800-1900. My guess is you're rear gears are in the 3.89-4.00-4.10 range.

There's an RPM calculator somewhere around here - I can't remember the thread title, though.

Eric

[Image: mach1sig2.gif]
  Reply
#3
I can't think of anything really beyond what you've listed but I do appreciate what you're going after here.

Out of curiosity, have you checked to see if your speedo is accurate? Just thinking out loud here (I don't honestly know exactly how the speedo in these cars works, haven't gotten there yet) but wouldn't it be quite wrong if someone had changed out the rear gears and not re-calibrated it?

Would kind of give you a possible idea as to where your error is coming from.

2013 Ford Focus SE Flex Fuel 5spd - Daily Driver
  Reply
#4
Mister 4x4;130692 Wrote:With 2.75s, you'd be looking at 1600-1700 RPMs @ 65. 3.00, around 1800-1900. My guess is you're rear gears are in the 3.89-4.00-4.10 range.

There's an RPM calculator somewhere around here - I can't remember the thread title, though.

That would be true if I already had an overdrive. But I currently have a FMX tranny with a 1:1 ratio top gear. So according to my calcs, assuming 2.75 rear end, I should be at 2,100 RPM at 60MPH and 2,450RPM at 70MPH.

MechEng;130700 Wrote:Out of curiosity, have you checked to see if your speedo is accurate?

Yes, it is pretty darn accurate according to the GPS readings I get while travelling at highway speeds.

I'm thinking it is most likely either the torque converter still slipping and/or the tire size. Every inch of tire size equares to approximately 100RPMs at 70MPH.
  Reply
#5
CZ-75;130703 Wrote:
Mister 4x4;130692 Wrote:With 2.75s, you'd be looking at 1600-1700 RPMs @ 65. 3.00, around 1800-1900. My guess is you're rear gears are in the 3.89-4.00-4.10 range.

There's an RPM calculator somewhere around here - I can't remember the thread title, though.

That would be true if I already had an overdrive. But I currently have a FMX tranny with a 1:1 ratio top gear. So according to my calcs, assuming 2.75 rear end, I should be at 2,100 RPM at 60MPH and 2,450RPM at 70MPH.

Now, ya see? That's where I'm confused. I was under the impression that you'd put your car in the shop to get the AOD swapped... and I was thinking, "Damn - it's done already?! Good job!"

BTW - FMX and AOD have the same 1-2-D gear ratios (until the AOD goes into OD). Your 'special' AOD might be slightly different.

Carry on! Wink

Eric

[Image: mach1sig2.gif]
  Reply
#6
So everyone is in the loop here are the formulas...

RPM = (rear gear ratio * (mph * 88)) / ((tire Circumference/12)) * trans gear ratio

MPH = (rpm * tire diameter) / (rear gear ratio * trans gear ratio * 336)

Tire Height = (width / 25.4) * aspect Ratio / 100

Tire Diameter = (2 * tire height) + rim diameter

Tire Circumference = tire diameter * PI

Driven Gear teeth = (drive gear teeth * rear gear ratio * revs per mile)/1001

Revs per mile = 20168 / tire diameter

CZ-75;130687 Wrote:I know, I know... My calculations are all theoretical... but are there other factors that would affect RPMs at a given speed other than the three I listed?


Is your tach correct?

What are your rpm readings at say 50, 60, 80 mph? (to determine TC lock up point)

[Image: 386_07_10_13_5_58_42.jpeg]
My Mustangs:
71 M-code Mach 1, Medium Blue/White Sport, 4R70W, 3L50, Factory Ram Air.
72 Q-code Mach 1, Pewter/Black Sport, 4-spd, 3L25.
65 Convertible, Britney Blue/White/White, more modified than original.
05 Convertible, Legend Lime/Tan/Tan, future classic??
  Reply
#7
You can find a RPM/tire size/axle ratio calculator ( Excel spreadsheet) in Wicki under Tire Sizes.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
  Reply
#8
Mister 4x4;130729 Wrote:Now, ya see? That's where I'm confused. I was under the impression that you'd put your car in the shop to get the AOD swapped... and I was thinking, "Damn - it's done already?! Good job!"

BTW - FMX and AOD have the same 1-2-D gear ratios (until the AOD goes into OD). Your 'special' AOD might be slightly different.

Carry on! Wink

Ha! Yes, it is at the shop... hopefully swapped out later this week.

My AOD will have the gear-set from a 4R70W, so you are correct:

First Second Drive OD
Normal AOD = 2.40:1 1.47:1 1:1 0.67:1
Wide ratio AOD= 2.84:1 1.55:1 1:1 0.70:1

Don65Stang;130733 Wrote:Is your tach correct?

What are your rpm readings at say 50, 60, 80 mph? (to determine TC lock up point)

Yes, the tach is correct. I tested it with an external tach at idle and at 3,000 RPMs.

Good idea about taking RPM readings...Unfortunately, by the time I have the chance to test read RPMs at various speeds, the car will have its new wide ratio AOD installed.

But now that I think of it... the new tranny will have a lock-up converter... and I won't be swapping out the rear-end gears until later. So I should be able to remove one variable once the new transmission is installed.
  Reply
#9
Why don't you pull the speedometer drive gear out of the FMX (it is just one bolt) and count the teeth. Using the number of teeth (since your speedometer is correct) you can back into the calculation for your rear gear.

More complicated is to jack up the rear end and put it on stands, have someone rotate a rear tire 1 full turn while you carefully count drive shaft rotations. This will get you very close. just a chalk mark and a reference mark and you can figure out what you have.


Not sure of your tire size, but I'm thinking you are already sporting some fairly low gearing. 3.70-4.10

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
  Reply
#10
CZ-75;130738 Wrote:My AOD will have the gear-set from a 4R70W, so you are correct:

First Second Drive OD
Normal AOD = 2.40:1 1.47:1 1:1 0.67:1
Wide ratio AOD= 2.84:1 1.55:1 1:1 0.70:1

Don65Stang;130733 Wrote:Is your tach correct?

What are your rpm readings at say 50, 60, 80 mph? (to determine TC lock up point)

Yes, the tach is correct. I tested it with an external tach at idle and at 3,000 RPMs.

Good idea about taking RPM readings...Unfortunately, by the time I have the chance to test read RPMs at various speeds, the car will have its new wide ratio AOD installed.

But now that I think of it... the new tranny will have a lock-up converter... and I won't be swapping out the rear-end gears until later. So I should be able to remove one variable once the new transmission is installed.

If you have 26" tires then you currently have 3.25 rear gears. With a wide ratio AOD and that rear, if it is a 3.25, will be almost perfect for a street driven car. A slightly better ratio would be a 3.50 rear. My reasoning for either of these two rear ratios is that your first gear ratio is 9.23 for the 3.25 rear. For a first gear ratio less than 8 a car will hit 60mph in first and if the first gear ratio is greater than 12 it will launch like a dump truck with a huge amount of noise and little movement down the road (aka granny gear). A first gear ratio of 9.5 - 10 is just about perfect in my book and is what I try to hit when picking out trans/rear gear/tire height combos.

For comparison, your FMX with presumed 3.25 rear has a first gear ratio of 7.8. How do you like the way your car launches with the FMX from a stop? The 9.23 first gear ratio will feel a whole lot faster. Wink

On the back side your overdrive rpm will be 2050 with 26" tires. That's a tad low for what I'd like to see for 351C engines. However you'll get really good mpgs.

Once the new trans is installed report back with your impressions of how it launches and how it does accelerating from 60 or 70 to 90 mph.

[Image: 386_07_10_13_5_58_42.jpeg]
My Mustangs:
71 M-code Mach 1, Medium Blue/White Sport, 4R70W, 3L50, Factory Ram Air.
72 Q-code Mach 1, Pewter/Black Sport, 4-spd, 3L25.
65 Convertible, Britney Blue/White/White, more modified than original.
05 Convertible, Legend Lime/Tan/Tan, future classic??
  Reply
Share Thread:  




Users browsing this thread: 1 Guest(s)