Calculated nonsense?

[ChrisMiami]

I put in a few numbers from things I’ve casually been bookmarking as a sorta shopping list and the speed number that came out is a bit insane and I wonder if I messed up the numbers or is this an actual possibility?!?

With the gear specs from the 6XD, a random rear end ratio I’ve seen, Aluminator’s RPM, and the calculated tire height of 315/35R17’s (so: 3.03 diff, 35” tires (17” rim plus 2x9” sidewall), 7500 RPM, 0.714 trans gear == 337 MPH ?!?

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

Of course there’s wind resistance to think about but really? Even the Chiron doesn’t do that!

[edwardb]

Yeah, nonsense (since you asked). Typical 315/35R17 diameter (or even 18" wheels) is around 25-26". A Toyo Proxes R888R, a common tire on these cars, is 25.7". Tire Rack, among others, gives detailed specs for tires. Also, that 3.03 diff ratio isn't typical. These builds are most frequently 3.55. With 3.27 or 3.73 as the outliers. Depends on the particular combination of parts and what the builder is going for. Typical theoretical top speed is often just over 200mph. But you'll never see it for various reasons.

[Dgc333]

FWIW, a 315/35-17 tire is 25.7" in diameter, a 35" tire is a large off road tire. Using the correct tire diameter the theoretical top speed would be 266 MPH.

BUT, very very few engines produce enough HP at redline in top gear to overcome wind resistance. More often than not vehicles will have a higher top speed in a lower gear because you are in an rpm range that makes more power.

Remember that it takes 8 times more power to double the speed due to wind resistance. For example: if it takes 100 HP to go 100 MPH then it will take 800 HP to go 200 MPH.

[cv2065]

My wife has 35” tires on her Jeep with a 2” lift. Would look wild on a Cobra. ��

[RoadRacer]

The mistake is here - 2 x 9” sidewalls

Sidewall is 315x0.35 = 110mm or 4.3”, so total tire height is

17” + 2 x 4.3” = 25.6” approximately

[jamminj]

no nonsense here this is the real deal
I have r888r 315/30 18 tires they are appropriately 25.50" after 3000 miles and a few burn outs.
if I use the 6xd this will be the gear set I'll be using
and in excess of 2000 hp
the speeds shown are with Hoosier A7 345/35 zr18 26.8 tall

191735

[ChrisMiami]

I have questions! But first, thanks for the tire size correction. New to this math. And yes, 3.03 in the diff does seem to be imaginary, although the Dodge Challenger stock is 3.09.

So - if lower numbers here equate to faster speeds, why are higher numbers (3.73) considered “performance upgrades”? Is it for the torque, sacrificing top speed to be faster off-the-line?

It’s incredible that atmospheres resistance is 8x HP! It’s a wonder we can get anywhere, else!

If I put in 3.09 / 26.5 / 7000 / 0.714 then I get a speed of 241+. I’m assuming that’s in a vacuum. The stock Ford diff of 3.55 gives 210+ MPH.

What would y’all say is the highest reasonable cruising speed to expect from a Coupe? Has someone measures the wind resistance of the FFR body in stock and Coupe-R configs? That huge tail wing on the R must help a bit!

[TBull]

I forgot to post this yesterday and others have already answered. Good Luck.

[JohnK]

So - if lower numbers here equate to faster speeds, why are higher numbers (3.73) considered “performance upgrades”? Is it for the torque, sacrificing top speed to be faster off-the-line?

Yes, typically higher numerical value gear ratios will result in better "off the line" performance." the downside is higher RPM's at cruising speed.


That huge tail wing on the R must help a bit!

Help with what? Downforce and cornering? Yes. Reducing aero drag to help top speed? No - quite the opposite.

Edit: Perhaps you've discussed this in another thread so apologies if I've missed it but can you elaborate a bit on what your goals are for the car? You mention a 6XD transmission which is a sequential dog box. This is not a transmission that would normally be run on the street. The final drive ratio you're talking about (3.09) is ridiculously tall for street use with any transmission that would normally be used. Is this going to be exclusively a track car? Drag car? Street car? It's kind of tough to comment on some of your questions without getting a better understanding of what you're trying to accomplish, but I will say that a coupe with a 6XD and 3.09 gearing would make a pretty miserable street car.

[MPTech]

What is it you're trying to determine? rear-end gearing? HP? trans gearing?
What is your intended use? Cruise, auto-x, track, drag, other?
Theoretical/calculated speeds are not very real-world.

[jamminj]

If you have the power to pull the gear the Theoretical speeds are very important
in the case of my car I'll be choking @ 8 mph 740 rpms in city traffic it would be a pain to drive in a slow cruise
i will have to be on and off the clutch constantly.

[ChrisMiami]

Well, I’m mostly just trying to understand all the variables and see what is possible, what is feasible, and what is achievable — and how to do it.

I’m not an automotive engineer, I’m a software architect. So while I’m at home with technical stuff, I have no real experience juggling the various elements that “make it go”.

In my daydreams? I want to cruise at 215 MPH down the Autobahn or on the salt flats or on a track day at Le Mans. I wanna drive in first gear from 0-80 mph. I want a barely streetable track monster with a loud, loping idle that tries to kill me every time I get into it.

But realistically, I’ll probably go for a car a lot like Dave’s: https://youtu.be/PYXbT9Vf5C8

[Dgc333]

The final drive ratio you're talking about (3.09) is ridiculously tall for street use with any transmission that would normally be used.

These cars were originally designed around using a Fox Body 8.8 rear end. The stock gear in this transmission was 2.73. Latter in Fox Body production they offered a 3.08 as the stock gear with an automatic. Even the current S550 Mustangs come with 3.08 gears.

A 3.09 gear most certainly is not "ridiculously tall" for a street driven car.

FWIW, I picked up an 88 Mustang rear end for my 33 Hot Rod that has 2.73 gears. My original plan was to rebuild it with 3.55 gears but when I saw how good a shape the 2.73 gears were I figured I would drive it for awhile before deciding. So I did the 5 lug conversion and put it back together.

With 305/35-20 tire (28.3" tall) and T5 I was expecting the car to be a dog off the line. Quite to the contrary I have to be very careful with the gas in first, second and third for fear of it breaking loose and coming around.

[Windsor]

Even the current S550 Mustangs come with 3.08 gears.

What's the axle code for 3.08 on a 6th-gen (S550) Mustang?

[JohnK]

These cars were originally designed around using a Fox Body 8.8 rear end. The stock gear in this transmission was 2.73. Latter in Fox Body production they offered a 3.08 as the stock gear with an automatic. Even the current S550 Mustangs come with 3.08 gears.

A 3.09 gear most certainly is not "ridiculously tall" for a street driven car.

FWIW, I picked up an 88 Mustang rear end for my 33 Hot Rod that has 2.73 gears. My original plan was to rebuild it with 3.55 gears but when I saw how good a shape the 2.73 gears were I figured I would drive it for awhile before deciding. So I did the 5 lug conversion and put it back together.

With 305/35-20 tire (28.3" tall) and T5 I was expecting the car to be a dog off the line. Quite to the contrary I have to be very careful with the gas in first, second and third for fear of it breaking loose and coming around.

I've never heard of a 3.08 rear end being offered in the S550 Mustang, but I am happy to be proven wrong. Would love to see a source for this. Here's what I've found:

https://www.americanmuscle.com/everything-about-s550-mustang-rear-gears.html#:~:text=3.31%20%E2%80%93%20Mostly%20in% 20automatic%20V6,and%20the%20Shelby%20GT350%2FGT35 0R

Stock 2015+ Mustang Rear Gear Ratios2015 Mustang came offered with the following rear gear ratios from the factory:


3.31 – Mostly in automatic V6 Mustangs
3.55 – Common for manual Mustangs
3.73 – Standard for Performance Package Mustangs and the Shelby GT350/GT350R

These gearing ratios are fairly ideal for demographic. 3.31 gears are great for fuel economy and automatic Mustangs. 3.55 gears are a good in between gear, offering decent acceleration and overall drivability. 3.73 gears are the more performance oriented, making them perfect for the performance pack S550s as well as the track focused Shelby GT350.



This aligns pretty closely with the recommendations for our cars (roadsters and coupes) when using a 5 or 6 speed Tremec. The 3.55 is probably the most commonly chosen gear ratio as it offers a good balance between good acceleration and good cruising RPM's, with some going as low or high as 3.27 or 3,73 depending on their desired use. So, "ridiculously high" is perhaps an exaggeration but a 3.08 rear end is certainly out of the norm in terms of what's typically chosen with the transmissions normally used in these cars. The main reason you see rear ends geared that low in production cars is to meet certain fuel economy numbers.

[Jeff Kleiner]

It always amazes me how often transmission ratios are ignored when talking about gearing and many times a blanket statement about rear axle ratios is thrown about. You have to look at the entire combination—a 3.55 (or 3.08 or whatever) rear will perform quite differently when combined with a 3.35 first gear than it will with a 2.87 first. Similarly a .82 fifth will be entirely different than a .62 with the same rear ratio.

Jeff

[GoDadGo]

It always amazes me how often transmission ratios are ignored when talking about gearing and many times a blanket statement about rear axle ratios is thrown about. You have to look at the entire combination—a 3.55 (or 3.08 or whatever) rear will perform quite differently when combined with a 3.35 first gear than it will with a 2.87 first. Similarly a .82 fifth will be entirely different than a .62 with the same rear ratio.

Jeff

Ditto Sir Jeffski!

When I built REDBONE I set up a spreadsheet that calculates the MPH at for every gear at any RPM desired.
I did that so that I could make the best compromise for my engine and transmission combination.
With that said, I only use 6th gear when I'm on the Interstate.
1,700 RPM equates to 70 MPH with 3.73.

Steve

C4 Corvette ZF S6-40 General Specifications:
> Gear ratios - 1st-2.68, 2nd-1.80, 3rd-1.29, 4th-1.0, 5th-0.75, 6th-0.50, Reverse-2.50
> Description - 6 Speed 95 mm Fully Synchronized
> Weight (w/shifter) 145 lbs
> Lube Capacity ~ 2.2 Quarts (engine type oil P/N 1052931 or BMW P/N 07510009420)
> Reverse Lock Out Mechanism - ('89-'94) On-shift handle pull-up to release, ('95-'96) Crash-thru (force required ~15 lb.)

[Dgc333]

I stand corrected, the S550 came with 3.15 gears not 3.08. Here is a listing of available gear ratios for the S550.

191834

[RoadRacer]

It always amazes me how often transmission ratios are ignored when talking about gearing and many times a blanket statement about rear axle ratios is thrown about. You have to look at the entire combination—a 3.55 (or 3.08 or whatever) rear will perform quite differently when combined with a 3.35 first gear than it will with a 2.87 first. Similarly a .82 fifth will be entirely different than a .62 with the same rear ratio.

Jeff

And if anyone wants a calculator to play with these variations, this is a common one:

https://www.tremec.com/calculadora.php

[Al_C]

Well, I’m mostly just trying to understand all the variables and see what is possible, what is feasible, and what is achievable — and how to do it.

I’m not an automotive engineer, I’m a software architect. So while I’m at home with technical stuff, I have no real experience juggling the various elements that “make it go”.

In my daydreams? I want to cruise at 215 MPH down the Autobahn or on the salt flats or on a track day at Le Mans. I wanna drive in first gear from 0-80 mph. I want a barely streetable track monster with a loud, loping idle that tries to kill me every time I get into it.

But realistically, I’ll probably go for a car a lot like Dave’s: https://youtu.be/PYXbT9Vf5C8

I appreciate what you're analyzing as well your daydreams. I hope you've heeded the opinions of many of the responses. You make one comment above that really stands out - "that tries to kill me every time I get into it". Have you read Freddiehebert's post in the "sticky" section? If not, please do. He notes that every time you turn the key the car says "take me for granted and I will kill you" or something like that. Have you ridden in a coupe or a roadster? If not, find someone near you who will take you for a ride. It might help shape your vision. You might find that something that is more "streetable" is actually more fun. Then again, maybe not.

[ChrisMiami]

Have you read Freddiehebert's post in the "sticky" section? If not, please do. He notes that every time you turn the key the car says "take me for granted and I will kill you" or something like that.

Thanks for the pointer! I'll definitely take a look through the stickies, and search for that one in particular. I actually didn't come up with that phrase on my own - I think it's something Shelby said at some point. I heard it in a YouTube video where a girl bought a Hawaiian built Coupe. IDK, but it pretty well describes the thrill of racing.


Have you ridden in a coupe or a roadster? If not, find someone near you who will take you for a ride. It might help shape your vision. You might find that something that is more "streetable" is actually more fun. Then again, maybe not.

There might be one or two around here in South Florida but I honestly don't think I'd have the chutzpah to bug someone just for my own edification like that. When it comes time to start plunking bills down, I plan to visit FFR and do the transaction in person. Have you seen all them order options? I feel like the "guy becoming his parents" at the salad bar...

[ChrisMiami]

Not being a transmission engineer, but having the 2 ends of performance on my mind (take-off and top speed), I toyed around with gear sizes for my 6XD sequential transmission and came up with these. The bottom three are torque-friendly, the top three are intended to be HP friendly. In reverse order (from 6th gear to 1st): 0.714 0.952 1.25 1.95 2.0 2.578.

It seems to me that'll give me a nice pop off the line and a smooth transition up to maximum wind resistance. With a 3.08 rear end, I'll be holding on for dear life the whole way! (I'll have to hope the lock on my removable steering wheel holds fast!)

[Jeff Kleiner]

A 2.578 first gear coupled with a 3.08 rear results in a 7.94:1 overall reduction which is going to be pretty soft off the line (even the Fox Mustangs with a T-5 and 2.73 rear end have a 9.1:1 overall first gear). For first gear I usually like to see something >9.5:1 for snappy performance but <11:1 so that it's not so short that you're having to grab second gear before even getting across an intersection. You're proposing a 2.0 ratio second gear and 1.95 third---that's virtually the same. Maybe you made a typo??? Also, somewhere in there, either 4th or 5th gear I'd expect to see a direct drive 1:1.

Just one guys opinion...

Jeff

[RoadRacer]

A 2.578 first gear coupled with a 3.08 rear results in a 7.94:1 overall reduction which is going to be pretty soft off the line (even the Fox Mustangs with a T-5 and 2.73 rear end have a 9.1:1 overall first gear). For first gear I usually like to see something >9.5:1 for snappy performance but <11:1 so that it's not so short that you're having to grab second gear before even getting across an intersection. You're proposing a 2.0 ratio second gear and 1.95 third---that's virtually the same. Maybe you made a typo??? Also, somewhere in there, either 4th or 5th gear I'd expect to see a direct drive 1:1.

Just one guys opinion...

Jeff

Ooh, fun, I've never heard that 9.5-11 range before, thanks Jeff.

I'm at 9.38 (2.87 & 3.27), so that's pretty close.

[ChrisMiami]

Oh man! This is great info. I hadn’t gone the step of combining the ratios. I’ll get back to the drawing board and see what I come up with! 👍

[Jeff Kleiner]

Oh man! This is great info. I hadn’t gone the step of combining the ratios. I’ll get back to the drawing board and see what I come up with! ��

What transmission do you intend to use? And what are the available ratios? You realize that they are not infinitely adjustable, right? Unless you have the ability to manufacture your own gear sets ;)

Jeff

[ChrisMiami]

Here’s the gear chart for the 6XD Sequential transmission I like. I’m pretty sure, if it becomes super important to me, he’ll stamp out any cogs I want to pay for.

https://6xdgearbox.com/wp-content/uploads/6XD-Ratios-WEB-VERSION-UPDATED-10.30.19-V3.pdf

[Jeff Kleiner]

$20,000 transmission.

Jeff

[egchewy79]

$20,000 transmission.

Jeff

Yikes!!

[ChrisMiami]

Heh It’s only money. 🤣

[jamminj]

A 2.578 first gear coupled with a 3.08 rear results in a 7.94:1 overall reduction which is going to be pretty soft off the line (even the Fox Mustangs with a T-5 and 2.73 rear end have a 9.1:1 overall first gear). For first gear I usually like to see something >9.5:1 for snappy performance but <11:1 so that it's not so short that you're having to grab second gear before even getting across an intersection. You're proposing a 2.0 ratio second gear and 1.95 third---that's virtually the same. Maybe you made a typo??? Also, somewhere in there, either 4th or 5th gear I'd expect to see a direct drive 1:1.

Just one guys opinion...

Jeff

they are quick change headset not every gear set has a 1:1 ratio.
this is the only set available for me because I'm using the wide 1" gears.
the only thing I can change is the headsets which is kind of like changing the rear gear only in the front of the transmission.

headset - 23 - 21

rear gear 3.55
final ratio with 3.55
1st - 2.070 = 7.348 yes this gear is 102 mph with Hoosier A7 345 35zr 18 tread 13.2 sect 13.9 height 26.8 at 9400 rpms

2cd - 1.421 = 5.043

3rd - 1.096 = 3.889

4th - 0.878 = 3.118

5th - 0.744 = 2.641

6th - 0.652 = 2.314

[jamminj]

currant list of 6xd ratios available
Rick Lamber emailed me this list.
the list online is not currant.
g-force makes the gear sets for them in batches.
6xd make most of the other components.

191930

[mikeinatlanta]

What transmission do you intend to use? And what are the available ratios? You realize that they are not infinitely adjustable, right? Unless you have the ability to manufacture your own gear sets ;)

Jeff

That's basically what I did. Full custom GForce T-5 with straight cut gears and dog rings. :) Not one of my smartest financial choices. In reality, I'm betting maybe one in a hundred FFR owners knows all of the gear ratios in their tranny.