According to my calculations, 60km/h requires me to shift to the 1st gear? What?
Posted by netwizzie@reddit | askcarguys | View on Reddit | 43 comments
Hello, I've recently started to learn manual gearboxes and dug this piece of info that typically the first gear has around 1:3 gear ratio (varies, 1:3 or 1:4)(which means that wheels turn 3 times slower that the engine).
2nd gear and 3rd have slightly higher gear ratios, 4th is direct drive, which is 1:1, and 5th and 6th gears are overdrive (of course this will be different for different gearboxes).
Now check this out. If we travel at 60km/h, which is around 17m/s, average wheel is 65 cm in diameter, which is around 2m in circumference, which means that it spins at 17/2 = 8.5 rev/sec.
8.5 times 60 = wheels spin at around 500 rev/min.
Engine typically spins at 1500 rpm, which means that we need to use 1st gear at 60km/h? I'm very confused, can someone explain please?
9BALL22@reddit
You're overthinking it. I don't even look at the tach unless I intend to approach redline. When you can shift by feel and/or sound, your getting closer to being proficient.
Pimp_Daddy_Patty@reddit
Your calculations are based on inaccurate ratios, assumptions, and you're missing a final drive as well.
netwizzie@reddit (OP)
Thank you for the tool. It shows that for 650 mm wheel 1st gear would match to 15-20 km/h. Still, it does not show how exactly it made those calculations and I can't understand where is the flaw in my logic.
rolikogreen@reddit
You're calculations and assumptions are decent enough. The logic was fine, just missing a bit of info about the rest of the drivetrain.
Car guys aren't known for their maths and proper technical knowledge. Better to ask any of the engineering subreddits instead.
No-Habit-7079@reddit
Have to consider rear end ratio which generally average 3.55 to 4.10
ontheleftcoast@reddit
I've seen gear ratios down to 2.73, and up to 5.10. Small modern cars are probably around 3
ClickKlockTickTock@reddit
Final gear is critical. Diffs can be anywhere from like 3-5:1 normally. Thats a 3-5x reduction in your output.
AppropriateDeal1034@reddit
Why are you even overthinking this so much, nobody ever calculated great ratios when deciding if they need to change gear or not, but you've ignored final drive in your calculations anyway.
turbocones@reddit
Diff ratio / final drive
netwizzie@reddit (OP)
This. Final drive further reduces RPM in 1:4 ratio. Thanks
Pimp_Daddy_Patty@reddit
You can find the math here
Pimp_Daddy_Patty@reddit
The flaw in your logic is the final drive aka the diff ratio.
riennempeche@reddit
Here are the numbers for my car:
Tire size: 26.6 inches
Final Drive Ratio: 3.46
1st Gear 2.97 53.91<---speed in MPH
2nd Gear 2.07 77.34
3rd Gear 1.43 111.96
4th Gear 1 160.1
5th Gear 0.84 190.59
6th Gear 0.57 280.88
If my car had the power to do it, the top speed allowed by the gearing would be 280 mph or 450 kmh! The sixth gear ratio is only for fuel economy to keep the V8 engine to around 1,700 RPM at highway speeds. Maximum speed would happen in fifth gear and is purported to be around 180 MPH.
aquatone61@reddit
Calculations? What does your car actually do is what matters. Put the calculator down and drive your car.
GeriatricSquid@reddit
Do NOT shift into first gear at 60km/h. That may be the redline of the engine but you’ll be destroying the car by doing it, esp since you’re obviously new to a manual transmission. Generally, first gear is only used to get the car moving. Second is usually your slow speed working gear. Unless you’re planning to drop below about 7-8km/h you’ll normally use second gear for low speeds. Keep the engine speed at a reasonable level where you’re not “lugging” the engine by running it too slow and shift up when you get to 3-4k rpms as you’re accelerating or when you’ve reached a steady speed. Do not equate the full operating range of a specific gear as your shift point- where the car CAN be shifted and where it SHOULD be shifted are radically different.
crypticcamelion@reddit
I have been driving manuel my whole life and never even consideret doing any calculation. You can hear it when to shift. There is no at this speed you need this gear in general, that depend on design. When the engine is screaming like a pig it might be an idea to shift up and when it grubbles and doesn't really pull its more that time to shift down.... How hard can it be?
lbl_ye@reddit
right ;)
lbl_ye@reddit
where you dug your piece of info look for this extra piece of info which is the most common available and includes all necessary calculations
car speed at 1000 rpm for each gear
eg. for most cars 1st gear gets you at 12-14 kmh at 1000 rpm
so at 60kmh your engine would be running at 5000rpm, too high
realcanadianguy21@reddit
Wait until V-Tech kicks in, yo.
lbl_ye@reddit
I gave numbers for 2nd gear, updated now with more correct numbers
funny, but I have VTEC engine too, and I once forgot and pushed 2nd until almost 90 km/h 😂
AggravatingSpread837@reddit
You really are overthinking this.
AggravatingSpread837@reddit
If your car has a Manuel gearbox then chances are it has a tachometer. That’s the tool to use to know when to shift.
gettin-hot-in-here@reddit
most people just use the sound once they're reasonably familiar with the car. until then, the tachometer is a reasonable way to judge what's happening with the engine (combined with what you feel/hear)
DarthElevator@reddit
It goes from the crank -> transmission gearing -> final drive -> transfer case -> differential(s) -> tires
Some of the above are not included if the car doesn't have that component.
All of these factors multiply to arrive at the overall torque advantages and top speeds.
Fun fact: the manual transmission Ford bronco has a "crawl gear" that in combination with the transfer case low gear and diffs can create a torque ration of 95:1 which is the highest of any production passenger vehicle. Top speed of 7mph!
gettin-hot-in-here@reddit
higher than passenger vehicles ... but not higher than some types of farming equipment :-)
TheNerdE30@reddit
Giving this comment some play so other people can apply this logic to their problems because it is “generally” correct and applicable to various drivetrain configurations.
bfjt4yt877rjrh4yry@reddit
Don't think, just feel
FitAgency8925@reddit
Learn to listen to engine and let it tell you when to change. ..
Dismal-Rip-1222@reddit
You may get to 60km/h in first gear in some cars
FREDTUC@reddit
Is it that difficult for people to learn how to drive a manual? My 1st vehicle was a manual ( self taught ), it didn't even have a tachometer. You just listen to the engine. Put tape over that tach, & drive that thing
djsimp123@reddit
Yea bro just drive the car lmaooo
FeelTall@reddit
Glad to know the rule of the internet where to find an answer, post the wrong the answer, and you'll get a flood of people telling you you're wrong along with the correct answer.
Congrats, OP. You did it!
Seriously though, thanks for asking/posting. You asked an interesting technical question because you couldn't math it out, you got your answers, and thanked/humbled when told why. Stay curious!
FLOHTX@reddit
Shift around 2000-2500 rpm in general, adjust up or down for how fast/aggressive you are trying to accelerate, whether you're going uphill, whether you have an underpowered vehicle, or diesel engine. You'll want to shift by feel, not by theoretical math.
lakimens@reddit
You don't need this math, even if it was correct. There are better ways to go about this. Like feeling the car, checking the ideal rims for that engine, etc..
Dexford211@reddit
Read your car's owner manual. Most of the time, you'll find the suggestion speed to shift your gears.
Prudent_Situation_29@reddit
There is a differential between the transmission and the wheel. It has it's own drive ratio, which multiplies the ratio of the current gear. If we use a common example of 3.73:1, your 1500 RPM becomes 5600 RPM.
actual-hooman@reddit
No need for any math, just shift where the power’s at in your car
Original_Bicycle5696@reddit
You forgot the final drive reduction.
Also stop making up numbers, its not helping you understand. Find a car, look up the ratios installed in that model transmission, oe tire size, and final drive ratios. These numbers will make sense.
Math doesn't work on vibes.
netwizzie@reddit (OP)
This. Final drive further reduces RPM in 1:4 ratio. Thanks, now my math checks out.
Own_Shallot7926@reddit
Those calculations are pretty clearly wrong, but either way that's not info you want to be thinking about behind the wheel.
If you like doing head math while you drive, keep it simple. What's your ideal RPMs for cruising around efficiently? Let's say just under 2,000. How fast can you go in each gear to stay near that engine speed? Probably 10mph in first, 20 in second, 30 in third... It will depend on your car and is simple to remember once you figure it out.
That obviously helps you pick the right gear but also make the correct shifts to maintain speed and efficiency. You're doing 50mph in fifth gear and are about to take a corner into a 20mph street? You know in advance you're going to drop down to second gear and maybe move back up to third. Now you can focus on the mechanics of making those shifts instead of figuring it out on the fly.
Willing_Sink_3623@reddit
You don't math so well
LAM678@reddit
almost all differentials also have a gear reduction, usually somewhere between 2-4x, that's what you're missing
GiraffeFellator@reddit
Are you building a car from scratch? If not, put your calculator away, for two reasons - (1) the car will tell you if you're in the wrong gear - if the engine is svreaming you need to shift up, if the engine is chugging you need to shift down, and every car is different. (B) I believe your math is fucked - it looks like you left out the final drive ratio in the calculation.