How to Calculate Engine RPM vs. Gear Ratio - Roadkill Extra
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- Опубликовано: 18 авг 2017
- It’s time for more math class with David Freiburger on this episode of Roadkill Extra. You’ll learn how to calculate the change in engine rpm at any given speed after you’ve changed rear-axle ratio, tire diameter, or transmission ratio.
We’ve provided this free episode on RUclips to give you a taste of Roadkill Extra so you can decide if you'd like to subscribe for more episodes.
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Love these kinds of videos where you teach us a bit every now and again. Thanks David, I enjoyed it.
I wish this how I learned math in school
Moose Chocolate the main problem for me is trying to remember all of the complications at once for on demand use
I truly believe that if it was applied more towards kids interest they would learn and excel in subjects I know I would have I’ve learned more on my own from really enjoying mechanics etc then I did in school I retain info better when I truly love what I’m learning and I hate math but when applied to mechanics I enjoy it
DEBOSS AND ROADKILL need to do an episode together! Canada and the USA come together! Fuckin eh!
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
can you expalin well
not clear to me
Its about 700rpm higher than real
5,148 rpm for me. But what is 5 rpm.
Thank you David Freiburger!
I'm new to all this, but where does he get "336" from?
Just a constant I would say, math works that way
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!
Thanks for sharing mate..
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.
That cougar is cool as hell.
Yup!. My dad had a 68 XR7 GT
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.
Thought my calculation seemed Ludacris so decided to check the comments.. Thanks for pointing this out, would have taken me a little bit to work this out myself. When there's the stress of a whole build it's hard to think of the little things.
@@bradhh1234 thanks, found out i would have 9300rpm in 125mph in an M52B28 xD
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
I use this formula almost daily because i drive off the rpm of my car now that i changed my rear gear
@@johnwolf1113 can you explain the math here if you can? i didnt know where he got most the numbers
@@variegatus4674 overdrive or non over trans and what transmission?
@@johnwolf1113 hey you there? hope that what i said made you go away.
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.
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
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.
I want more roadkill extra with out having to pay for it hahaha
Thank you proved me correct.
This was helpful that k you
First time I learned something on the internet.
Very interesting. Does the same formulae work for metric?
Milwaukee inkzall markers are awesome
OMG want to see that episode with the cougar engine going in.... more FORD stuff plzzz
Ian Fosbrook It's coming up on Roadkill Garage
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!
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.
Dammit, you should of been my math teacher. For some reason that was easy to make sense of
Is the gearing of the transmission, how many gears it has, what gear you're in at the given speed irrelevant?
Is this in 4th gear or the 1:1 gear in your transmission?
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
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.
or check vin/option sitcker
Can't always rely on that though. It's always possible that a previous owner changed it.
Limited slip diff or not, that's irrelevant.
renek755 - Don't comment when you know nothing. Lee L is spot on. For open diff, it's a different approach - only lift one wheel. The other must stay planted on the floor. Then do as the Lee L says for LSD/locker.
impalaSS65 If you do it one wheel on the ground, you are going to get double numbers due to open differential. 4.10 rotates 8.2 times etc.. Yes you can do it that way too, but it's more accurate to lift both tires and then it does not matter if its open diff or something else. "Dont comment when you know nothing".
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
Good to know. My daily driver Jeep needs a regear, and with this I can play around with ratios.
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!
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
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 :)
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 your tire diameter is 29, and assuming you have an auto car becuase 3rd is 1:1. If all you care about is the best time ever in the quester and nothing else. Yes. But then do you wanna cruise around at 4k on the highway? Plus like he said add 3-12%
This calculation is considering a direct 1:1 transmission ratio right?
from where you get that 5305 unit
Hey motor trend, I've been wondering. Is 7.17 gear ratio good for 35 or 37" tires?
Or is it to much? Could it be bad for my 94 xj
Math class just kidding keep up the good work.
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
does this formula work in big rig ?
is it the same for km/hr ?
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
How do you know what your top speed will be, before the car is complete, to plug the speed into this formula, thank you?
That sharpie and cardboard sound 🤯🤯🤯🤯🤯🤯
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
Will that Cougar ever be on an full roadkill episode?
Where its the main focus of the episode that is
It does.. In RKG episodes..
yes but actual ROADKILL episodes that cmere on to youtube and Demand (obv), i know it comes to Demand thanks to the pilot episode and this, excluding the few Roadkill Garage episodes that come to youtube
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
a drunk driver would be useful
In this episode of Roadkill Extra, Freiburger takes 5 minutes to explain basic algebra.
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 ?
idk ye why
It's just a number that you have to use.
Doesn't make sense! I can put any number into this equation... But why 336?
Its called a constant
Actually i calculate the real number. 336.1352398 that is come from 63360/pi/60 this is conversion number between the parameters which are rpm parameter rpm, tire diameter inches, mph and the actual formula is lack. Axle ratio must be on formula which is generally equal to 3.5
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
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?
RepoDraghon then your formula is (70*3.55*[sixth gear ratio]*336)/([tire diameter])=1500 then you plug in tire diameter and solve for sixth gear ratio.
An easier way is to plug in the sixth gear ratio of your transmission and solve for the final drive ratio.
Daniel Senti cool, tire diam stock uhh well its a 235/50R18 but eventually I'm thinking 305/30R18
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
There´s a formula for KPH and centimeters?
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
My only question is why would you use a points distributor in that engine?
what about metric system?
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.
I found a whole spreadsheet calculator that I reverse engineered and I use it in excel all the time for everything.
why is math fun !!! what sorcery is this ??
I like my 2.73 that gives 28 mpg with my v8
How do you work it out in metric for those outside of the USA?
1 mile per hour equals 1.609 kph
Use all metric. For tire circumference and speed.
So, where does the 336 as a constant come from?
Zach Coldwell I was wondering too. Hopefully someone sheds some light on it soon.
It's from baking in all the stuff that is the same in to one constant.
/(*)
63360/(60*3.1415...)=336-ish
1mph=0,44704m/s=26,8224m/min
1inch=0,0254m
circumference=pi*diameter
1mph/(1inch*pi)=26,8224(m/min)/(0,0254m*pi)=336,135 (1/min)~336rpm
Because of imperial funny units:P.
The Metric system has yet to put a human on another planetary body.
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
For a mod like that you just multiply 6618 with the overdrive ratio 2:58
long time ago i was this early
is 125 mph the topspeed?
125 mph is the estimated speed at the end of a quarter mile drag strip. The Crusher Camaro was set up as a quarter mile drag car.
Does this formula work for automatic as well? (Torque converter..)
Around 2:20 he speaks on that. Vaguely, but he covered it somewhat.
Timmy Turner find out if your converter has lockup.... if not just subtract like 6% or so.... if it locks up just run it as normal
2:12. He explains that an automatic out of lockup will rev slightly more at speed. ie 1/4 mile, or accelerating (3% up to 12%'slippage'). On the hiway in lockup it's the same as a manual trans.
Timmy Turner watch the video and this time...LISTEN...
moyadapne i
for all of the metric people, just convert your damn kmph to mph it's that fuckin simple.
There's a big myth going around the internet saying if you add too much engine oil you lose power, is it true?
Where does 336 come from???
cool Cougar
The formula works only if your last gear ratio is 1:1, right?
0:50
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
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
So basically the original formula assumes a 1:1 final transmission ratio, right?
andrewt248 Yes
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
Easier just to use the ratio calculator from several sources. That math makes your head hurt!
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?
Charge your phone Freiburger!!
It's simple really. Put a turbo on it and hope it doesn't blow up.
I have an app for this on my phone, I clicked for the cougar
I was lost the moment numbers were being written down.
Freiburger charge your damn phone!
What's with the cutting Freiburger mid-sentence in the intro?
whats the 336 for?
It's a constant derived from many other variables multiplied and divided.
ahhh that makes sense
*points at empty engine bay while talking about an engine*
Must get old saying "We're really behind on this one"
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.
Must be completely gutless.
Talk about torque reduction to the wheels!
2.18 gears? Lol My Monte Carlo has the factory 2.29 gears. At least you have a bunch of power to move it too. Try moving a 3500 pound car with 2.29 gears with 110 hp.
Am I the only one that's never heard of 4.57s?
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.
There's no magic ONE rear end ratio that's fastest... gotta take engine power, tranny gear ratios, tire size, rear end ratio, intended vehicle usage all into consideration... and I don't know if there is any actual 3.83:1 rear end ratios available...
BuzzLOLOL. I know I've looked. I have tried it on i6. V6. V8. V10. V12. And v10 electric boost. On multiple games and it has a good rate of excelaration.
Destin - only in games... not the real world... although it would be an OK street/strip compromise ratio... if it even existed...
A lower gear (higher number) is ALWAYS faster within one gear - it will ALWAYS deliver more torque to the rear wheels (given the same engine output). Though you need to take into account shift times. Even if you have 1000hp, 0-60 will be slow as fuck if you have to shift 5 times during that time.
This is why most gear ratios are designed so 2nd winds out to about 62mph, it's marketing for the magazines who will do 0-100kph test, so there's only 1 shift in the 0-60 time.
Though in terms of 1 gear (ie. no shifting), higher number is always faster. You will top out at a lower number, though.
You see, the problem here is that you're playing a freakin video game. You're not "tuning" crap. Get over it.
How about just go online and use a gear ratio calculator. Lmao 😂
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 RUclips 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.
You lost me at rpm 😂
Legend has it that if you binge watch roadkill episodes you gonna grow a mustache on your ballz .
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.
Dumbest question of the year. But could you do this with a CVT transmission or an electric car? 🤔
Electric, no. They work completely differently.
CVT, uh...I don't think the space magic behind those is understood enough for anyone to figure that out.