[athalaga]

Originally Posted by nanasridhar

Hello There,
Well just two days back I have graduated from 2004 esteem lxi to Pre owned 2006 Honda city ZX GXI. I would like to know what is the actual rpm when we do 120 kmph in a Honda city. In my esteem when I used to do 120kmph the rpm was around 3000 rpm. In Honda city at 120kmph the rpm is 3600. Is it because gear ratios are different or could it be the clutch slip. The vehicle has done 31000 kms now. I would appreciate if some one show some light on the RPM of different cars at different speed say at 80,100,120 so that we can cross check if there is a clutch problem. Most of the manuals or the technical data only let us know about the torque at the rpm and not the speed and the rpm sync.

The calculations are in this link
https://www.team-bhp.com/forum/techn...speed-rpm.html (Tata Safari 2.2vtt: Theorectical and observed speed/RPM)

[trrk]

Hi,

Originally Posted by nanasridhar

In Honda city at 120kmph the rpm is 3600. Is it because gear ratios are different or could it be the clutch slip.

I know this is a bit late, but here goes:

For the City ZX with OEM 175/65R14 tyres, in 5th gear, for every 1000RPM the speed is 32.29kms per hour.

I think the RPM at 120kph from the above would be around 3716, so your 3600 appears okay.

In my CVT, that speed would correspond to 3000RPM.

[Shan2nu]

Hello There,
Well just two days back I have graduated from 2004 esteem lxi to Pre owned 2006 Honda city ZX GXI. I would like to know what is the actual rpm when we do 120 kmph in a Honda city. In my esteem when I used to do 120kmph the rpm was around 3000 rpm. In Honda city at 120kmph the rpm is 3600. Is it because gear ratios are different or could it be the clutch slip. The vehicle has done 31000 kms now. I would appreciate if some one show some light on the RPM of different cars at different speed say at 80,100,120 so that we can cross check if there is a clutch problem. Most of the manuals or the technical data only let us know about the torque at the rpm and not the speed and the rpm sync.

Rpm to vehicle-speed will vary from car to car depending on gear ratio, final drive ratio, wheel circumference and speedo error.

The OHC VTEC does 120kmph@3400 in 5th. But you wont find all OHC VTECs doing the same as some might have higher speedo errors.

Shan2nu

[sgiitk]

I am sure you will find the gearing data quite easily. However, remember a slipping clutch shows up only when the problem is very severe. A few percentage points will definitely now show up.

[sanagg1]

The following calculator may be of good help to non-Engg. kind of people
Transmission Gear Ratio Calculator Software Program RPM MPH Speed

cheers

[Shan2nu]

Or you can memorize this formula so that you can cal no matter where you are.

Engine Rpm / Gear Ratio / Final Drive Ratio X Wheel Circumference (Inches) X 2.54 / 100000 X 60 = Vehicle Speed (kmph)

So if i take 3400 for the OHC VTEC, that would be...

3400 / 0.774 / 4.058 X 72.8 X 2.54 / 100000 X 60 = 120kmph

But rem to deduct atleast 2% from the result as a real tyre is never a proper circle.

Shan2nu

[nanasridhar]

well, Thank you guys, and never thought will get these calculations and data. I happened to drive my friends Honda city zx and I found that at 100 kmph the rpm is 3000. I couldnt go beyond that to check at 120 kmph. And in mine also the rpm is the same at 120.

Cheers

[samm]

Originally Posted by Shan2nu

Or you can memorize this formula so that you can cal no matter where you are.

Engine Rpm / Gear Ratio / Final Drive Ratio X Wheel Circumference (Inches) X 2.54 / 100000 X 60 = Vehicle Speed (kmph)

Shan2nu

This would mean that, for a given RPM, the vehicle speed should be constant?
Also, it would mean that if RPM increases at a given gear, the speed should also increase?
Again, the vehicle speed and the RPM will change in a linear manner?

This is what I saw on my Innova, all data on 5th gear. Sorry I dont know the gear ratios, so didnt do any calculations.
a) 1500RPM-> 60KMPH; 2000RPM->80KMPH; 2500RPM->90KMPH. This gives me a overall ratio of 1500/60 = 25. I expected, that the car will reach 100KMPH at 2500RPM (=100*25)...
b) sometimes, the rpm goes to 1750 or more, with the car speed at around 60KMPH. Seems like it is not linear?

Any help is appreciated! Thanks!

[dhanushs]

Originally Posted by samm

This would mean that, for a given RPM, the vehicle speed should be constant?

Yes, if rpm and gear ratio (gear) is constant, speed should be constant.

Originally Posted by samm

Also, it would mean that if RPM increases at a given gear, the speed should also increase?

Umm.. isn't that obvious?.

Originally Posted by samm

Again, the vehicle speed and the RPM will change in a linear manner?

For every change in rpm, the corresponding change in speed is constant for a given gear.

Originally Posted by samm

Seems like it is not linear?

Depends on which gear you are. You can reach 100kmph in 3rd, 4th and 5th gear. With different rpms.

[samm]

Originally Posted by dhanushs

Yes, if rpm and gear ratio (gear) is constant, speed should be constant.

Umm.. isn't that obvious?.

For every change in rpm, the corresponding change in speed is constant for a given gear.
Depends on which gear you are. You can reach 100kmph in 3rd, 4th and 5th gear. With different rpms.

dhanushs,
I'm talking about 5th gear only. Sorry if I wasnt clear. My problem is in understanding why I wont reach 100KMPH at 2500RPM, and why I should reach almost 1750RPM to reach 60KMPH? Is the speedo and rpm meters not accurate enough? I might attribute reading error for 1750RPM -> 60kmph issue, but not for the 100KMPH problem. the RPM was 3000 by the time speedo showed 100KMPH.

[cranky]

I would be linear if you use 0rpm as the base, try recalculating with the idling rpm as the base, how does it look then?

[ecenandu]

@Samm
Did you change the OE tyres? If you have gone for a different profile tyre, there is a chance for speedo error.BTW How many kilometers have you clocked?

[DWind]

I some how feel the speedo speed cannot be linear with respect to engine speed. It all depends on power-torque characteristics of the engine. It can be linear for certain rpm range (more so for Petrol engine) but definitely not at all range.

Samm! In case of Innova, it has a Turbo charger which probably has a sweet spot at around 2000 RPM (not sure) and beyond that torque turns flat and the engine needs to work more to keep up.

[samm]

Originally Posted by DWind

I some how feel the speedo speed cannot be linear with respect to engine speed. It all depends on power-torque characteristics of the engine. It can be linear for certain rpm range (more so for Petrol engine) but definitely not at all range.

Samm! In case of Innova, it has a Turbo charger which probably has a sweet spot at around 2000 RPM (not sure) and beyond that torque turns flat and the engine needs to work more to keep up.

Thanks, but if it is not linear, what does the gear ratio mean? The drive from engine to the transmission is positive drive (ie., there cannot be any slipping). So the ratio of RPM to car speed has to hold, right?

Innova diesel has a flat torque curve from 1300 rpm to 3600 rpm. All (most of them!) diesel cars will have a flat torque in a similar range. My questions are from 1500 rpm to 3000 rpm

ecenandu & cranky -
Yes, I changed the OEM tyres to 225/60R15, but the ratio I gave is based on the changed configuration. Also, the error will be less than 2% with this upsize.

I will try recalculating with 0rpm as base, but I need to think how to do it. But on the face of it, it doesnt seem to be any different.

[cranky]

^^What I meant was that 60km/h should be counted as 1000rpm (above idling) and 100km/h as 2000rpm (above idling). The higher the speed the more the engine has to work, so you can expect the relationship to not hold true after a point but for speeds up to 100-110 the relationship is mostly linear (sort of, not always, and depends on the car and gearing - lest I get into trouble with the more technically exact members here).