[sudev]

Most of the modern cars have overdrive gear ratio in fourth and fifth gears. This is great for economy and general driving.

As you go faster the wind resistance increases in geometric proportion. That is going twice as fast increases drag or wind resistance times. To achieve top speed you need power to overcome this drag.

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.

So extending the same logic of we change into smaller sized tyre would we achieve higher max speed?

Expert's please advise before I spend money only to discover this theory is flawed.

It is also counter to what most people do by going in for tyre size upgrade.

[vikram_d]

Originally Posted by sudev

So extending the same logic of we change into smaller sized tyre would we achieve higher max speed?

I'm no expert but logically it makes sense. With smaller wheels you would go faster but cover a smaller distance per revolution of the wheel where as with a larger wheel you would go slower but cover a larger distance.

For any fixed RPM an imaginary fixed point on the smaller wheel's circumference will be faster as it has to cover a smaller distance.

For the same RPM a fixed point on a larger wheel needs to cover more distance and hence will be slower.

Well I maybe wrong but this is what my logic says.

Edit:

But I also think this logic is wrong as irrespective of diameter both wheels will have the same RPM. So the fixed imaginary points on the diameter of both wheels will travel at the same speed.

On any wheel make a white mark on the diameter of the wheel and one exactly below on the rim. They both travel at the same speed for any given RPM. Only the distance travelled by both points will change.

I'm sure I have confused you even more.

[aargee]

Originally Posted by sudev

So extending the same logic of we change into smaller sized tyre would we achieve higher max speed?

What you're saying is right. Add another task - calibrate the speedo after changing the tire & observe the top speed.

I've done a reverse of this experiment (without the actual intention) in my old WagonR. Switched the OE tire size from 155/65R13 to 145/70R13 & observed that speedo was slightly pessimistic, but negligible though.

[am1m]

Won't vehicle stability and safety go for a toss though?

I guess each vehicle is tested with a certain tyre size in mind, with a +/- x% tolerance either way. Won't going smaller below a certain size cause stability issues?

[audioholic]

I am no expert either but I think there will be no direct way to put it. In the higher gear as the speeds increases the engine wont be able to provide the torque needed to pull the car against the wind and other forces acting against the vehicle. The fourth gear being shorter puts a lesser torque demand and hence you will be able to spin till red line thus achieving a higher top speed. If you switch to a smaller tire, the top speed in fourth will decrease due to change in wheel diameter. However, what happens in fifth gear might remain the same, except that the car might reach the same speed as before but with the engine RPM going slightly higher.

To go faster in such a scenario I believe we should work at getting better torque output at higher RPMs. Switching to smaller tyres is similar to getting the gearing shorter, but end of the day its the engine fighting against the external forces, so the net result might remain the same. Also regarding tyre upgrade, I believe people dont upsize the tyres to achieve a better top speed. It would only be true if the engine has massive torque higher up the revv range. Else I feel the top speed will reduce since the bigger wheel and higher rolling resistance will bog down the engine further. An important thing to consider here is that the true speed of the vehicle matters and not what is shown on the speedo, since the speedo calibration will go for a toss when you change tire circumference.

[Shreyans_Jain]

Absolutely. A tire with a smaller total diameter will make the effective overall gearing taller. You'll be able to achieve higher max speeds in each individual gears

[sudev]

Guys I am mentioning true speed observation based on GPS with external antenna.
Smaller diameter will have smaller circumference but also better gearing. That's the toss up.

Not going with drastic change but one size smaller.

@aagree would have been great if you could have measured actual GPS speed in both cases.

[Altocumulus]

The old riddle. A scooter and a truck going at a speed of 50 km/hr from Point A to B. Which will reach the destination faster ?

When you have the answer for that, you will get the answer for the opening question as well.

In the same car, smaller wheels will require more rotations to cover the same distance. So your speedo may show you are going faster, but actually you are not covering the same distance. And Speed is equal to distance over time.

[aravind.anand]

Originally Posted by Altocumulus

The old riddle. A scooter and a truck going at a speed of 50 km/hr from Point A to B. Which will reach the destination faster ?

When you have the answer for that, you will get the answer for the opening question as well.

In the same car, smaller wheels will require more rotations to cover the same distance. So your speedo may show you are going faster, but actually you are not covering the same distance. And Speed is equal to distance over time.

Not necessarily. What is happening when we go for a tyre with smaller diameter is we are effectively going for a 'shorter' final reduction so your car's acceleration would improve a bit. But then the top speed will also come down a tad bit.

Say a car with larger wheels can cruise at 120 kmph at 3000 rpm, the same car with a smaller wheel would be at 3100 or 3200 rpm at 120 kmph. So the top speed would come down, but the rate of acceleration would improve.

[Divya Sharan]

Post downsizing wheels, speedo would need recalibration otherwise the smaller wheel would obviously indicate higher speeds.

Here are 2 comparo charts from Tacoma world.
What happens if I downsize the wheels on my Punto from 195/60/15 to 185/70 and 175/65/14 respectively. See how enthusiastic the speedo gets with a 175 section 14 incher!

[vikram_d]

A lot of people here are talking about speedo error. After giving it some thought I have come to the conclusion that putting smaller wheels will not make the car go any faster. It will only increase the speedo error.

The vehicle will not go any faster because the gear ratios in the gear box are fixed. So irrespective of wheel size the wheel hub is still spinning at the same RPM. So if you want to make a car go faster you need to change the gear ratios.

Edit:

To confuse everybody here even further.

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.

Larger wheels need more engine power/torque to get moving and smaller wheels need less engine power/torque. So you might hit the top speed of your vehicle more easily with smaller wheels but you need to account for odo error.

[Guna]

Smaller wheels will offer greater torque and theoretically should spin more freely against the resistance of air and the road friction. They will spin faster but this is negated by the lesser circumference and hence vehicle speed will remain the same (could be slightly lesser in fact)

[sudev]

Guys guys guys


Read my post. All speed observation were on GPS and independent of wheel circumference. And in any case my observation were on same wheel size. We did go faster on GPS as well as vehicle speedometer in fourth gear vis fifth gear.

In third gear engine RPM were redlining so going faster was not possible.

In fourth and fifth we were not redlining as total power output was being balanced against wind resistance. The gearing in fourth was able to work with higher engine RPM while still not redlining. In fifth the RPM dropped and could not be coaxed further.

All this on a straight as arrow road with no traffic.

After this my question. Will it go faster with one size smaller wheel? Observation will be based on true GPS speed.

[2000rpm]

Theoretically Yes, a smaller tyre (by smaller I mean thinner) should be able to achieve higher speed, but then you are chasing the 5% in the S-shaped curve. Spend the same amount if money for a tuning set-up and enjoy life better with safer tyres.

With a smaller radius tyre, I dont think so.

The wind resistance will be the same, bearing friction will be the same. You might gain more speed with some aerodynamic accessories than this one is my guess.

[Guna]

Originally Posted by sudev

After this my question. Will it go faster with one size smaller wheel? Observation will be based on true GPS speed.

Very unlikely. Even if you are using GPS to track the speed, smaller wheel dia means it covers lesser distance in a given amount of time. Yes, the smaller dia allows the engine and wheels to spin somewhat faster but it is negated in terms of the distance covered.
This is different from driving in lower gears (like 4th instead of 4th) since the lower gears always provide greater torque and thus higher pulling power.
In other words, you may be able to push the engine further to higher revs but it will result in slower vehicle speed (compared to the original setup).