OK, I'm a nerd... But what do you think re: actual RPMs vs expected

[CZ-75]

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?

 

[Mister 4x4]

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.

 

[MechEng]

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.

 

[CZ-75]

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.

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.

 

[Mister 4x4]

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!

 

[Don65Stang]

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

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)

 

[Don C]

You can find a RPM/tire size/axle ratio calculator ( Excel spreadsheet) in Wicki under Tire Sizes.

 

[CZ-75]

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!

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

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.

 

[Jeff73Mach1]

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

 

[Don65Stang]

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

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.

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.

 

[CZ-75]

See? I knew there would be smarter people here than me! But the car is aleady at the shop for the swap.

Tire height is 26.4" FMX tranny =1:1 in top gear. 2.75:1 rear end.... should be ~2,450 RPM at 70MPH

With the same combination, a 3.25 rear end would be ~2,900 RPM at 70MPH, which is what I'm reading.

But... if the torque converter isn't locking up, then my top gear isn't acting like it's 1:1

It will be interesting to see what reading I get once I know the torque converter is locked in top gear.

 

[Tubo]

Yeah, just cos Marti sez, don't necessairly make it so. It's only had 40 odd years for the PO's to have swapped out that cruiser gear, fer a "better" one. My guess iz, "Ya have a 3.25"

IMHO you are going to regret that 3.89 choice to go with the Wide Ratio gear set. I mean we are Brothers in CZ arms, and our Mustang choices, So I can tell Ya my opinion, & no harm done. I'd have gone with a 3.55 that is unless you plan to live at the track.

Butt Wait! There's still hope. Cos IF you do indeed have a 3.25, You can swap out fer those extended long hauls (We take our Stang's on annual Multi-state trips) and a have a "stump puller" 1st gear, fer around the farm,...er, around town. Try the 3.25(if thats Ya have) with the new tranny, Then you can install the 3.89, and have a good compaison to go by. Swapping a 9" chunk iz no big deal at all, and check out the axle bearings while your at it.

Keep us informed,

Tubo

 

[CZ-75]

Yeah, just cos Marti sez, don't necessairly make it so. It's only had 40 odd years for the PO's to have swapped out that cruiser gear, fer a "better" one. My guess iz, "Ya have a 3.25"

IMHO you are going to regret that 3.89 choice to go with the Wide Ratio gear set. I mean we are Brothers in CZ arms, and our Mustang choices, So I can tell Ya my opinion, & no harm done. I'd have gone with a 3.55 that is unless you plan to live at the track.

Butt Wait! There's still hope. Cos IF you do indeed have a 3.25, You can swap out fer those extended long hauls (We take our Stang's on annual Multi-state trips) and a have a "stump puller" 1st gear, fer around the farm,...er, around town. Try the 3.25(if thats Ya have) with the new tranny, Then you can install the 3.89, and have a good compaison to go by. Swapping a 9" chunk iz no big deal at all, and check out the axle bearings while your at it.

Keep us informed,

Tubo

Dammit!!!! I had a hard enough time deciding on gears...

Is your concern too agressive 1st & second, or not a tall enough top gear?

Because I figure the FMX I have now with a 2.75 rear-end (final gear ratio of 2.75) is about the same as my new over-drive (0.70:1) coupled with a 3.89 to achieve a final gear ratio of 2.72... fairly comparable. And if it turns out I actually have a 3.25 rear-end, then its final gear ratio of 3.25 is a lot worse at highway speeds than the 2.72 I should get from my new setup.

I was originally going to go with a 3.5 rear end, but decided on the 3.89 in order to get as agressive as I could in 1st/2nd, yet maintain about where I was (at least where I thought I was) in top gear.

It's not too late to switch back to a slightly taller rear-end, and I value veryone's opinion. So thanks for chiming in!


I plan to rebuild the entire rear... trac-loc, all bearings, seals what have you... also plan to refurbish the brakes, seals and bearings at the hub...

Good advice to compare the existing to the 3.89 I have waiting.

It will be interesting to see what I really have in the back. I'm hoping it is a 2.75 because I was also hoping that going to a most agressive 1st/2nd gears would help me get from light to light with a little less tread than before.

 

[Don65Stang]

Now that I see you are shooting for an aggressive 1st and 2nd I'm personally ok with recommending the 3.89. BTW, do you have an 8 in or 9 in rear? The smaller 3.89 gears will raise your 1st gear ratio toward 11 which is the upper end of ideal for everyday street but is perfect for an aggressive street setup. It will be quite snappy.

The wide ratio AOD gear set is well suited for 3.25 to 3.89 rear gear ratios depending on what the end goal is. A 4.11 might be stretching it a bit though so don't get too crazy.

 

[MotoArts]

Yep CZ, your variation in calculated cruise RPM is because the FMX does not "lock up".

Basically, the non lockup, old style 3 speeds always "slip" a little, even in high gear. Not really a slip, but easiest to explain that way. The converters have no lockup provision.

That's the advantage to an AOD... lockup; (theoretically speaking) fuel economy, reduced wear and heat, etc.

I know that on my 5.0 with its stock 3.27 gears, it sings pretty good @65/70mph in third gear (when my OD was stuck and non functional). I can't remember exactly what RPM was, but yeah, around 3 grand or so.

If I end up with a small block in my '71, the AOD will be a no brainer for me. I have 3.25, 3.50, 3.82 and 4.00 gears for it...

 

[CZ-75]

Thanks for the reassurance, guys. It is a 9" rear.

I'm thinkink about returning the 3.89 gears to Jeggs and picking up the 3.7 instead.

 

[Tubo]

Dammit!!!! I had a hard enough time deciding on gears...

Is your concern too agressive 1st & second, or not a tall enough top gear?

It's not too late to ........

Determine exactly what gears you have

Raise rear wheels off ground...shifter in neutral....rotate DS, 1 revolution...count exact number of times rear tire rotates.

Or pull the chunk and count pinion & ring teeth and do the math.

I'm still betting 3.25ish. However, it's better to Know than to guess,..Right?

Do continue with, and complete the trans swap. Then drive the car, get a feel for what you have. Seat of the pants happy?? Then you can make a better informed gear selection descision.

Besides which on the 3.89 thing,.....IIRC your engine is a essentially stockish 302?? Unless your adding Intake, Cam, valve springs, better heads & such, that engine is going to have too narrow an RPM range, to take full advantage of the 3.89 gears.

 

[Don65Stang]

Corky, It's a 351 2V.

I agree get the new trans in and drive it for a while to see how well you like the existing setup. Focus on how it feels accelerating off the line and how well it accelerates from a rolling 60 to 70 mph, and from a rolling 70 to 90 mph without downshifting. Your looking for how much accelerating power it has in overdrive.

Also see how it feels/how much throttle it takes the car from that rolling 70 to downshift to 3rd and accelerate up to 90 mph or so.

Report back your findings on these three areas. First and top gear (overdrive) are the two areas where you'll have smiles or frowns.

 

[CZ-75]

GAh!!!!

I'm like a kid on Christmas Eve... It's all I can think about