[alpha1]

Actually in your particular case - if you get a top speed in the 4th gear - and I am assuming that engine RPM is lower than the peak torque RPM- then what you say makes sense.

The operating word here is that the closest you are to the peak torque RPM, the closest you are to achieving all the performance figures.

Otherwise you cannot really predict, it is a play between the torque transferred to ground vs rolling and wind resistance, unless you wish to solve some differential equations.

[padmrajravi]

Smaller wheels will have higher angular velocity;which means the wheel traverses more angle(Angular distance , to be precise) per second for the same linear distance. Angular velocity is different from linear velocity . Linear velocity is what helps us traverse distance. Linear velocity is angular velocity multiplied by circumference.

Since the smaller wheel have a low circumference , your linear velocity will not change by the increased angular velocity. Only thing that will change is the speedo error. Analogy with gears have no relation to this . It is an entirely different phenomenon. Considering the same rotation per second or angular velocity , a larger wheel will have higher linear speed.

[Rshrey22]

One thing to consider is that narrower tyre instead of smaller diameter will create less resistance on road, which will itself make the car go faster.

[sudev]

Originally Posted by Rshrey22

One thing to consider is that narrower tyre instead of smaller diameter will create less resistance on road, which will itself make the car go faster.

Will come to that later.

Have queried many of my rallying friends. The keep using different tyres from standard to smaller Advone's to bigger Tornado's. Let me get their inputs too. Yes part of reason of their choice is terrain but my query is related to top speed experience.

[rangakishen]

Here are my thoughts. The power require to move a vehicle at a certain speed is constant. When you change tire size, what you change is the RPM at which this power is delivered.

Example: Lets not take the top speed as it is power limited. Lets take 50 kmph. if you increase wheel size, it'll decrease your engine RPM at 50 kmph and vice versa.

Now lets consider the top speed. Consider the following figure.

Here, the three lines are the power requirement for speed from 0-200. Please note that in this graph, since we are changing the tire size, same engine speed does not mean the same vehicle speed. Instead, same power is interpreted as same speed. hence all three curves have the same max power ~186kW corresponding to 200kmph for this random vehicle that I have created.

Now, the engine curve is something I have randomly generated. I have also put the gearing and stuff randomly so please do not flame me. Do not consider the actual numbers but focus on the relative change between the different tire size curves.

So, the top speed of the vehicle is basically when the power requirement curve meets your engine power curve. Remember, in this graph higher power = higher speed. We can see that the 19" curve meets the max power curve at a higher power than the 17" one. Hence you will have a higher top speed with 19" tires albeit very slightly and only theoretically as that slight an increase can easily be shaved off by slightly increased head wind and such in the real world.

What I am trying to explain here is that an increase or decrease in top speed will depend entirely on your particular power curve.

I hope I am right.

[GrandTourer]

Originally Posted by vikram_d

To me it is pretty clear that smaller wheel will make the vehicle slower.

I believe you are correct. Here's the real physics behind it:
Theory:



Problem:


Link to page:
http://www.softschools.com/formulas/...t_formula/151/

[PGA]

IMHO, in theory, if everything else remains constant, as Brits say, 'it will not make fig of a difference'.

Torque is capacity of doing work and power is rate of doing work. Thus we see that speed is a function of engine power and not torque and that doesn't get impacted by using smaller wheels.

Wheels are directly coupled to the drive axles which are again directly coupled to the transmission, my apologies again but I don't get it how putting smaller wheels will impact overall gearing.

Yes somethings will change when we put smaller wheels, some of these changes such as weight, lesser road friction will tend to increase speed of car whereas aerodynamic resistance on wheels due to higher rpms will tend to retard.

Something about speedo's, I always thought that these are calibrated with driven shafts in transmission, last I saw speedo coupled with a wheel was in Bajaj 150 scooter of 1975 vintage. So how come we will have speedo errors.

Lastly, will I be able to achieve higher speeds in one gear lesser than overdrive as greater power will be available at the wheels, one will have to take internal friction into account which is directly proportional to rpm.

[Sutripta]

If in top gear at max achievable speed engine rpm is less than max power rpm one will go faster in top gear with a smaller tyre.
BUT cars which are geared that way normally achieve top speed in top gear-1. If this is properly geared for max speed, max speed in this gear, which is the max speed of the vehicle, is going to come down with smaller tyres.

Check engine rpm at max achievable speed for top gear and topgear-1.

Regards
Sutripta

PS. Should mention that the term overdrive has lost all meaning in today's world of all indirect gearboxes.

[ashwinprakas]

Not sure how things are these days but for older cars 4th and 5th gear would be overdrive, the top speed being registered in 3rd gear.

This doesn't mean that I won't be able to do higher speeds than what I can do in 3rd, it just means that the difference would vary greatly on load be it from payload or wind resistance or even gradients.

As for wheel size, since it's similar to gearing on motorcycles my suggestion would be to leave things stock, because though on certain terrains overgearing/undergearing does have its advantages, when you consider a larger area it becomes an issue as the vehicle would perform exceptionally well at one place and pathetically at the other.

As for minor changes, the overall outcome remains the same as far as speed is concerned. Because on the same motorcycle with 36T I'd be doing 130kmph at around 8~9k RPM but with 40T I'd be doing the same speed screaming at 10k+ RPM.

All in all there is no tangible difference.

[arjab]

Had tried this many years back on a Premier Padmini. The Padmini's came with 5-20/14 bias ply, nylon tyres.
Changed the wheels to Maruti 1000 rims which were 13-inch
and fitted 175-70R13 radials.

The effect in real world driving situation was mixed. Overall grip improved as well as braking performance. Because of the reduced overall gearing, the speedo showed an error, for example when we were moving at a speedo indicated 90kmph the actual velocity was somewhere between 82-85kmph.

The engine also spun higher in each ratio, but due to the ancient mechanicals, the underbonnet racket created when revving to the limits, were so high, that any one with a shred of mechanical sympathy did not bother to redline the ancient Padmini ! Fuel consumption also dropped. And because of the reduced ground clearance the car bottomed out with alarming regularity. And as anyone who has faced bottoming out in a Padmini knows , the first component to "break" was the l-o-o-n-g exhaust muffler!

Of late, we have seen changing the final drive achieves a good balance. We are running an Esteem MPFi fitted with a Zen final drive, (the Zen final drive can easily handle the enhanced power output figures and is a bolt-on fit), and the Esteem has turned very peppy indeed. In gear acceleration is much better and that typical 2nd gear "flat-ness" at low rpm's which Esteem driver's faced is no more. The real fun is when we drive in the hills. The shorter gearing makes the shorter geared Esteem scoot from apex to apex without the driving getting caught out for lack of power. As everyone knows, the old Suzuki G13BB loved to rev and thus this modded gearing is a fun thing to experience.
Top speed has reduced from the Esteem's,(speedo observed),164kmph to 151kmph(again speedo observed). Fuel efficiency has dropped a scant 0.5 to 0.8kmpl.

[sudev]

Interesting arjab.

Would have been so much nicer if GPS speeds could be obtained.

@All never expected so much physics and counter points. Now I am getting shivers buying and trying with smaller tyres. But let me check with my tyre source if - big IF - he can give some test tyres for this.

The experiment would be done on a Maruti Baleno (old one circa 2000~2010). WIll put the results for comments.

Any one up to doing theoretical calculation for expected reduction in speed based on same car same tune same time and same driver? Only change would be tyres from standard to one size down -

As per this site the following are available for old Baleno:
Tire Rim PCD
165/80R13 5Jx13 ET45 4x100
185/65R14 5.5Jx14 ET45 4x100
195/55R15 6.5Jx15 ET45 4x100

https://www.wheel-size.com/size/maruti/baleno/2006/

[Thermodynamics]

Originally Posted by sudev

It's my observation that to achieve max speed in a five speed gear box car like SX4 you need to be in fourth gear rather than fifth. The reason being that power being put out on road through engine, drive train etc. is more in fourth without going into rev limiter. Third gear cannot give max speed as it goes into red lining.

That's an excellent observation Sudev. There is an interesting thread on a similar topic on (Understanding Power, Torque, Gearing and Top Speed).

But I am skeptic about downsizing tires for that. This is because, tires are designed based on several factors and not just the Power. In fact in your case the "Torque at wheels" is going to be much higher than before, downsizing IMO is neither straight forward nor safe. The smaller tires need to dissipate all the energy duty done by bigger rubber. I wouldn't risk on tires because any mistake here could be catastrophic. Instead my question to you is, if we are getting max performance in fourth gear, just log lever in fourth and enjoy. Why risk changing tires. Also the suggestions of 2000rpm makes good sense.

Originally Posted by 2000rpm

Spend the same amount if money for a tuning set-up and enjoy life better with safer tyres.
You might gain more speed with some aerodynamic accessories than this one is my guess.

[Divya Sharan]

Originally Posted by Rshrey22

One thing to consider is that narrower tyre instead of smaller diameter will create less resistance on road, which will itself make the car go faster.

I'd politely disagree a bit. We are confusing between friction and traction here.
Friction doesn't depend on the contact area/patch. Coefficient of friction does.
It's true that we get better traction/braking feel with wider tyres, but that's due to a better (read softer) tyre compound/composition in costlier (wider) tyres.

Soft compound tires are wide so that their sidewalls can support the weight of the car. Softer tires have a larger coefficient of friction (due to a larger contact patch), therefore better traction. A narrow, soft tire would not be strong enough, nor would it last long.

Wear in a tire is related to contact patch and composition. Harder compound tires take longer to wear out and can be narrower. They do, however have a lower coefficient of friction, therefore less traction.
Among various tyres of the same type and composition, there would be no discernable difference in 'traction' with different widths.
Wider tires, assuming all other factors are equal or similar, commonly have stiffer sidewalls and experience less roll. That's why we see them giving somewhat better cornering abilities.

Friction, on the other hand is proportional to the normal force 'N' of the asphalt acting upon the car tires (perpendicular from the road up).

N is equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area.

For a wide tyre, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or narrow.
Even though in real world scenarios, roads would never be even, not even on drag strips, frictional force experienced by different widths would still stay the same, considering all above pointers.

[Tgo]

Originally Posted by vikram_d

Lets assume a wheel has a circumference of 1 meter and it is rotating at 1000RPM. This means it is travelling at 1 x 1000 = 1000 meters/minute.

Now if wheel circumference changes to 1.5 meters, the answer becomes 1.5 x 1000 = 1500 meters/minute.

Now draw your own conclusions.

To me it is pretty clear that smaller wheel will make the vehicle slower.

Crystal clear logic.

Originally Posted by sudev

In fifth the RPM dropped and could not be coaxed further.

What you're experiencing is a mix of gearing in the gearbox and the engine being stuck in a flat spot as a result of the gearing. The total resistance on the vehicle is not able to be opposed by the engine which is in a spot it cannot move out of because of the gearing. This gearing ends at the final half shaft of your driven tyres.

So unless the smaller sized tyres are lighter, thinner or have lesser rolling resistance you should not be going any faster, since the cross section of the vehicle hitting the air is the same.

[Lalvaz]

Smaller wheels will rotate faster however vehicle speed will remain constant in real world. Any speed increase as seen in your testing will likely be the result of difference in driving style and should logically be so minor that it would appear unnoticeable for relatively short distances that you would cover in your test drives.

Hence I think it’s an irrelevant argument.