Love these kinds of videos where you teach us a bit every now and again. Thanks David, I enjoyed it.

The conversion factor for metric units is 5305. RPM = (km/h x gearbox ratio x rear axle ratio x 5305) / ((25.4 x rim size) + (2 x tire width x tire profile)) So the RPM for a '97 Camaro Z28 in 5th gear at 250 km/h with stock wheels would be: (250 x 0.74 x 3.42 x 5305) / ((25.4 x 16) + (2 x 245 x 0.50)) = 5153 RPMs

I wish this how I learned math in school

DEBOSS AND ROADKILL need to do an episode together! Canada and the USA come together! Fuckin eh!

that calculation is for cars with a 1:1 fourth gear, or whatever the top gear is. In a 5-speed, just like in the overdrive, the fifth gear ratio has to be multiplied too.

Thanks for the insight. So unless I'm mistaken in the original form this equation assumes the gear selected (in the transmission) is a 1:1 (roughly 4th gear on many cars) creating a gear ratio of 1.0. So if I wanted to account for the gear, to say take a small 4 cyl car at the top of 1st which is probably rocking a 3.25 1st gear and a 4.25 rear (like a civic) and let's just use a 185/60/R14 diameter it'd be something along the lines of: (30mph*4.25*3.25*336)/22.7 = 6,133 RPM That sounds about right. Think this is a good equation to hold onto for sure!

That cougar is cool as hell.

BEST DEFINITION OF RPM AND MPH EQUATION!!!

I'm new to all this, but where does he get "336" from?

I've been mostly bored by classic mustangs, but cougars are awesome. My mom had a 68 carmel colored one when I was a kid. I used to scalp myself getting into it since I was tall from a young age, but it was still a super cool car.

How to find out what gears your car has (if its limited slip)? Rotate drive shaft one turn and see how much the tire goes around. 4 total turns and bit = 4.10's, 3 and a half 3.55's etc.

Roadkill should do episodes with a Reliant Robin & Trabant. A little hard to come by here in the states, but overseas they are cheap and many are made before 1975 so California regulations would be a non issue. I imagine container shipping can't be too bad especially if they put multiples in one container. I would die laughing see two tall americans crammed inside those cars. Put a turbo Honda motor and OMG! The number of views would be insane.

Dave actually made learning math fun, thanks Dave

Thank you David Freiburger!

There's also awesome apps for people with manual transmissions that want to rev match perfectly for every gear. Down shifting to say 2nd or 3rd may wear the clutch a bit if you don't know exactly where the revs should be. The apps can get you into 2nd gear with no slipping the clutch perfectly for a corner. Given once you know a car you don't need to know what rpm to be at, but on a brand new car you may wanna figure those ratios out so the learning curve is a lot faster.

Solving this way gives you what gear ratio you want in order to be crossing the finish line with your engine in its max HP range. But most people don't drive a drag car. So then you need to decide what RPM you can live with at a particular speed, fill out all the other variables and solve for rear end gear ratio. For most people they would solve for whatever gear gives them the max allowable RPM at, say, 55 MPH, or maybe 65 MPH for freeway driving. But an overdrive allows for lower gearing on the street. This calculator is available lots of places on the Interweb.

If USA changed to SI units you could eliminate strange conversion factors like 336. Also why not rearrange the formula, you make gear ratio the answer, you know speed and the max revs you want to hit, it will then pop out ratio, this prevents iterations and hunting for the answer.

Good to know. My daily driver Jeep needs a regear, and with this I can play around with ratios.

Now tell them how the number of gear teeth is in direct correlation to gear ratio; ie take the number of ring gear teeth and divide it by the number of pinion gear teeth, that'll give you your gear ratio. 41 ring gear teeth divided by 11 pinion gear teeth is 3.73, however, add just four more teeth to the pinion gear and you have 2.73. That is a significant difference in RPMs between a street/strip gear ratio and a sole highway cruiser, and the difference between a 2 and 3 series carrier!

First time I learned something on the internet.

Thanks for sharing mate..

As a reverse engineer who does gaming. Forza horizon 4. The equation to find your gear ratio final would be Max RPM/ Top speed/ 287.5 “based on game physics algorithms x tire diameter. Divide 3 and multiply tire diameter for forza horizon 4. If anyone showed up here wondering how to find a blind gear ratio for the game :)

Dammit, you should of been my math teacher. For some reason that was easy to make sense of

Thanks Friburger I am starting a build I have a 2002 ford E350 boxvan with a POS 5.4 triton motor ( when is ford going to learn how to build a motor ??? I will be putting a whole new drive train in it It will be a cummins 4bt with a ATS built 47 RH and a atlas transfer case it will be a frame off build and my question would have been at 65 MPH what would be the RPM so 65x4.10x .69 x336 /33.5 = 1844 I will be running BFG 35 12.50 16 which is right in the power band for the cummins thanks for the info and keep the videos coming more info it has already been converted to 4wd by Salem Kroger but I will put in unjointed off road parts it has a Dana 60 high pinion front axel and a Dana 70 HD rear end with 4.10 gears I will also be installing ARB lockers front and rear this will be my retirement expidition van goal is to drive to the tip on South America turn around and drive to the North Pole to see Santa

I want more roadkill extra with out having to pay for it hahaha

I'm sure I'm not the first here to say but just in case, you fudged a bit there, Dave. You said the higher gear ratio (lower gearing) would lower the RPMs and conversely. Otherwise, interesting lesson and thanks!

Why 336 ?

Some would complain this American didn't use metrics in the making of his video. If this video was made on foreign soil your complaint would be a valid one. He's giving you free, and I believe really cool information. You people are so damn spoiled! Stop always trying to find errors in others and what they do. Point your finger at this man now look at your hand. That's right you have three fingers pointing back at yourself!

for all of the metric people, just convert your damn kmph to mph it's that fuckin simple.

Milwaukee inkzall markers are awesome

So how do you get the magic number of 336 I would love to know the brainiac that sat down and did that math to find the 336 was the number. I will bet that if you use the equation in reverse you can find what gear you need to be at desired MPH

So, where does the 336 as a constant come from?

This calculation is considering a direct 1:1 transmission ratio right?

OMG want to see that episode with the cougar engine going in.... more FORD stuff plzzz

That sharpie and cardboard sound 🤯🤯🤯🤯🤯🤯

Thank you proved me correct.

You can find your top speed of each gear by dividing your max rpm by rear axle ratio by gear ratio by 336 times tire diameter like with my car I know top speed in first gear is 55 and by doing the math of 5500/3.5/2.47/336x29=54.9mph

My only question is why would you use a points distributor in that engine?

How do you know what your top speed will be, before the car is complete, to plug the speed into this formula, thank you?

Let's say I want to hit 200 miles per hour and I'm using an LS 5.3. How much power does it need? What transmission do I need? And what rear end do you suggest? With a car weighing 3000 lb

In this episode of Roadkill Extra, Freiburger takes 5 minutes to explain basic algebra.

The formula is RPM = MPH X Gear Ratio / tire DIA X 3 or MPH = RPM X DIA X 3 / Gear Ratio or DIA = MPH X Gear Ratio / RPM X 3 or Gear Ratio = RPM X DIA X 3 / MPH

Is the gearing of the transmission, how many gears it has, what gear you're in at the given speed irrelevant?

This was helpful that k you

There's a big myth going around the internet saying if you add too much engine oil you lose power, is it true?

Hey roadkill needs to go do a big block swap in a bronco in Texas for Matt from demolition ranch/vet ranch/off the ranch

I was lost the moment numbers were being written down.

Will that Cougar ever be on an full roadkill episode?

why is math fun !!! what sorcery is this ??

Legend has it that if you binge watch roadkill episodes you gonna grow a mustache on your ballz .

That's pretty good I did a little bit of different math to find out how many RPM on a tachometer would I be turning at 60 miles per hour with no speedometer I found out how many rotations per mile my particular Tire was rated per manufacturer and I did something with the number 60 because 60 is a mile per minute and I use the gear ratio so you have a 350 ratio you know your engine turns three and a half times for one tire rotation and you go from there leave Steve alone he don't want to work on that Ford

Very interesting. Does the same formulae work for metric?

Stupid question it's possible for a automatic transmission car to stall or buck like a manual transmission if the brakes are dragging and it's carbureted with a distributor??? had to work on my brakes and I think the first time I tried driving it they got stuck on that or I have a new problem but it was driving ok before I did the work on it. Ps. Was in a hurry to get to work so I think I fixed or in the process of the sticky brakes ( residual valve ) but haven't gotten to drive it yet the Engine idles ok and revs ok

I like my 2.73 that gives 28 mpg with my v8

I found a whole spreadsheet calculator that I reverse engineered and I use it in excel all the time for everything.

cool Cougar

Is this in 4th gear or the 1:1 gear in your transmission?

I bought a caprice 1978 with a 305 en th400 transmission. En my transmission is ticking so the oil has to be changed (flushing is very expensive) how can you change transmission (and moter) oil at home? Greets from the netherlands

Where does 336 come from???

Hi , can you tell me if I can use this same formula to find out my actual gear ratio . Because I know the speed , rpm ect ? Thanks Glen

There´s a formula for KPH and centimeters?

I have tuned on 4 games and they always tell me a 3.83 rear end will out run every year between 2.10 or a 5.56. But I didn't calculate them yet. So thanks for the video.

So I have a question. If I want to find out what RPM I will finish a drag race in at the end of third, which is my 1:1, would I estimate my MPH ((117)(4.56)(336)/(29)) this comes up to about 6200. So I would need 4.56 gears to be at the end of third in the quarter?

If I have a 5.5 hp engine and I want to have enough torque to move a atv frame with good speed what gear ratio do I need plz help

I have a question a good towing gear for hauling junk cars f250 f350s I'm building a 1986 Chevy c20 Cheyenne I want to put a gear ratio where I can do 80 mph down the highway hauling three cars what the 262 straight six

Math class just kidding keep up the good work.

Does this formula work for automatic as well? (Torque converter..)

does this formula work in big rig ?

For a mod like that you just multiply 6618 with the overdrive ratio 2:58

is it the same for km/hr ?

what about metric system?

Hey motor trend, I've been wondering. Is 7.17 gear ratio good for 35 or 37" tires?

The formula works only if your last gear ratio is 1:1, right?

It's simple really. Put a turbo on it and hope it doesn't blow up.

from where you get that 5305 unit

So basically the original formula assumes a 1:1 final transmission ratio, right?

So, if I wanted a new transmission, how would the equation be for the 6th gear if say I had 3.55 rear diff and wanted to cruise the highway at 70mph at around 1500rpm?

What about changing the actual gearbox gears when you build a motor for more horsepower? Wouldn't you get more out of an engine if you tuned the gearing with that?

I remember hotrod or car craft Doing this, but they included the weight of the car and that gave required horsepower, then they figured gear ratio to keep it in a certain speed bracket. Anyone know what I'm tslki about? I need that article in my life.

but if you predict higher MPH and less gear it comes up with Lower rpm with less gear and more mph, so this is only part of the way

*points at empty engine bay while talking about an engine*

I have 3.42 with 3500 stall th400 calculation shows rpm at 65 to be 2600 but I see closer to 3000 rpm at 65. So if I went to 3.73 I would be at 2900 in calculation and actual around 3400. With 4.10s at 65 actual will be 3600. I think I would brake alot more stuff with 4.10 will prolly just do 3.73

my brothers old mustang had a 302 with 2.70 rear end with a crappy 4 speed stick. it would bog on the freeway in 3rd gear.

Easier just to use the ratio calculator from several sources. That math makes your head hurt!

Actually I've already calculated that so it's tough before because my dad says 10 is really old and it's a V6 I'm wanting to get a small block Chevy because it's a very old truck hits all beat up and I calculated what a new motor gears in a translator and it would get boost turbo in it 139 miles an hour

long time ago i was this early

I have an app for this on my phone, I clicked for the cougar

Charge your phone Freiburger!!

we know what motor trend on demand is, a lot of us even got it at one time or another. However the Android app for it sucks so bad that it's easier to just wait and watch them here on KZfaq rather than pay for an app that freezes, doesn't screen rotate half the time, doesn't remember where you left off, or only opens after I kill the program in settings. By the time I'd get the stupid app running my lunch break would be over. Now you know. Other than that I am a fan, just not paying for that app. It may be fixed by now, but it's like a cable company, you don't go back to what didn't work before. Maybe it works great on iPhones, but more than half of us don't want to drink that kool-aid.... I've had both so I'm not starting that argument. Beta test on iPhones AND Android platforms. lol sorry, I know it's no your fault Freyburger (I know I spelled that wrong). The constant advertising for the on-demand app finally got to me.

How do you work it out in metric for those outside of the USA?

What's with the cutting Freiburger mid-sentence in the intro?

Must get old saying "We're really behind on this one"

whats the 336 for?

is 125 mph the topspeed?

336 came out of nowhere 🤦🏿‍♂️🤦🏿‍♂️

If I see one more tire calculation equation that says "plug in your old tire to determine what size tire you should have", I will scream. PLEASE, Let's say you have changed the engine in your car, transmission and rear end. You would NOT use the same size tire that was originally on the car as all parameters have changed. Too small a tire and you'll be doing burnouts when you take off at "normal acceleration." Conversely, if the tire is too large, your engine will be screaming at let's say 20mph. There is a chart, which you do certain math using variables like vehicle weight, torque, rear gear ratio, and transmission ratio, to determine what range of tire you should have for various performance levels. +Motor Trend Channel, please tell me you know of this chart??? I used to have the reference chart, and have lost it and can't find it anywhere on the 'net.

Freiburger charge your damn phone!

You lost me at rpm 😂

Shouldn't that Number be 330 and Not 336 ??🤔

Dumbest question of the year. But could you do this with a CVT transmission or an electric car? 🤔