Acceleration VS Wheel Diameter, has someone got an answer!

This question is a bit out of the box but is relevant, I’m not a Physics Professor but this has been driving me crazy trying to work out why this happened.

I have owned and restored over 150 cars and bikes and i have never had this problem come up before now.

I have a small hatch which my daughter drives, it’s an 100k automatic 1.3 DOHC VVTI F/I 2006 model, it went like a cut cat before i changed the wheels over (well as fast as a 1.3lt auto can go anyway), since i changed the wheels over its lost its acceleration and performance.

The standard wheels are 14” x 5.5 steel/plastic hubcaps running a 175/65 x 14 tire which has an overall diameter of 583mm with sidewall height of 4.48 inches

The factory mags i fitted up are 15” and run a 195/50 x 15inch tire, overall diameter of 576mm and sidewall height of 3.84 inches.

I would have thought that the mags i just fitted with a smaller overall diameter would accelerate faster because of less torque required causing less inertia being required to moving car forward during acceleration, EG: I believe that as you increase the diameter of the wheel you increase the inertia required (Inertia = 1/2 mass * radius ^2). Hence the same amount of torque applied to the two different size wheels should cause the wheel with the smaller overall diameter to rotate quicker give that the 15’wheel has a smaller diameter overall than the 14” wheel (aprox 7mm) difference.

I can’t believe how much difference that made in acceleration from standing start, even hills that we travel on all the time are now almost requiring a downshift to keep up with traffic flow.

I thought it may have something to do with the weight of the 14” steel wheel and tire VS the weight of the 15” mag and tire (15” is 9.7KG heavier) , but if weight were an issue then when we had 2 extra people in the car (say an extra 130 kgs combined) the car running the standard 14” wheels still maintained its performance etc, even with the extra passengers, so what changed.

When i used to ride 2-3 day enduro’s back in the 70’s in the NT and outback QLD, if ever we wanted to change our gearing (lower it) we would always fit a smaller sprocket to the front shaft and if we still needed a bit more we would change the rear sprocket to a larger size. So why does this seem to be defying all logic that i have been using all my life given that the cars drive is from the front wheels so dropping overall diameter (similar to dropping 1 x tooth on the front sprocket of a enduro bike) should cause increase in acceleration???

Just to prove to myself i wasn't dreaming i refitted the standard 14 inch wheels and everything is back to normal.

I don’t know if i am going crazy or what, but can someone explain why this is so, is it something so simple that i have missed it?

Sounds like a really curious dilemma... I would have thought the same as you, with a smaller driven wheel should follow increased toque and therefore acceleration. Perhaps a weigh is required of wheels and tyres? As the wheel is unsprung mass, changing the sprung mass is not an overly relevant experiment. I'd be thinking along the lines of the whole sprung vs unsprung mass and the inertial properties of each wheel. Ie the resistance to a change of speed.

Not sure how to test or prove any of this, but perhaps slightly worse braking properties are present with the 15" wheels too? Hard to compare with so many variables like tyres, condition & tread ect.

as i understand it, rotating mass is a pretty substantial factor. that being the case, its at least plausible that the extra weight in the wheels is more than enough to cancel out the gear advantage from the reduced diameter and hurt your acceleration

going by just the diameter of the wheel you should have a bit better torque. but the weight of the wheel adds inertia plus going from 14 to 15 will move the weight further away from the centre of the wheel adding to the inertia problem.
I = k m r2
so the further your radius is you have to square that so the 10kg packs a bigger punch.

then you have to spin it up and that takes energy.
Ef = 1/2 I ω2
there is another square in that one too so your in more trouble.

so people generally simplify it and just use some multiple factor for wheel weight. ie 1kg added to wheel = 2.5kg added to chassis sometimes even up to 10x


also if you like do a search for "dyno and wheel weights" you'll see some interesting graphs.

More rubber on the road and marginal extra wind disturbance could create drag and be a factor. Its only a small motor.

pelican said:

as i understand it, rotating mass is a pretty substantial factor. that being the case, its at least plausible that the extra weight in the wheels is more than enough to cancel out the gear advantage from the reduced diameter and hurt your acceleration

Click to expand...

Thanks Pelican, its starting to look that way, extra mass of wheel and tire against reduced diameter, initial drive force required as opposed to extra weight of passengers.

DrB said:

going by just the diameter of the wheel you should have a bit better torque. but the weight of the wheel adds inertia plus going from 14 to 15 will move the weight further away from the centre of the wheel adding to the inertia problem.
I = k m r2
so the further your radius is you have to square that so the 10kg packs a bigger punch.

then you have to spin it up and that takes energy.
Ef = 1/2 I ω2
there is another square in that one too so your in more trouble.

so people generally simplify it and just use some multiple factor for wheel weight. ie 1kg added to wheel = 2.5kg added to chassis sometimes even up to 10x


also if you like do a search for "dyno and wheel weights" you'll see some interesting graphs.

Click to expand...

Thanks Dr.B, i think it all starts making sense, i spoke to another member on a different forum and this is what he had to say.

Quote: "Every manufacturer in the world spend countless hrs finding the “ideal” size wheel and tire combo with yields the best combination of use for maximum acceleration/handling and comfort for that particular model for a mass market vehicle.


He calls it the “optimum point" reached on a performance graph” So by changing the wheel/tire combo you change the sprung weight measured from the wheel hub center to outer diameter, the extra weight combined this with a slightly larger footprint from the tire on the road which in turn results in noticeable lesser acceleration on a smaller engine vehicle.


He said this problem is relevant to the size of the engine vs wheel/tire size dia. increases, but on a smaller motor (1.3lt) this becomes far more noticeable as you pass this "optimum performance point". You would notice this transition less on a larger motor (2.0lt or 2.4lt+) as the increase in available torque will compensate, so that you would not notice some/if any change in vehicle characteristics even though there is one. This particular engine 1.3 with VVTI only produces max power higher up in the rev range, coupled to an automatic than any change to larger heavier wheel even though its external diameter is reduced, would not be enough in this case to not have an effect on acceleration."

So, going by DrB's and Pelican's reply's above i think that this all makes sense now, all previous car's i have owned have always had engines ranging from 2lt's up to 6.7lt V8's, which is why i have never noticed minor changes to mag wheel/tire combo changes before this. I guess the "optimum point" on the 1.3 lt VVTI was more critical this time around because of smaller engine size, as the wheels came from it's bigger brother's engine the 1.5lt VVTI, i didn't think that would make too much difference, but i was wrong this time.

Changed my MY15 CJ ES 2ltr tyres from 205 60 16 to 215 60 16 ( notice MY16 CF ES uses 215 60 16) Tyre retailer was honest enough to tell me, I would get a drop in economy.
Told him I couldn't stand the noise of the original Yokohama's and was looking for a more comfortable ride, economy wasn't a concern as I only did about 2000 to 3000ks per year.

All three items proved to be as expected. Huge drop in noise and now a more comfortable ride Although economy I didn't really check just accepted it. and surprisingly speedo is now spot on.
Performance, Subjective, haven't noticed any difference.

Had some old clown claim that the reason why cars are coming out with bigger wheels are for better fuel economy, i said BS it has nothing to do with that at all.

If a wheel tyre combination is the same over all, for get what it says 175 65 x14 to say 175 55x15, if the tyre sitting on the ground is the same width and patch and overall diameter all will be equal.
But one type of tyre vs another even being the same 175 65x15 does not make them the same thing as one may roll better, it's just like having low tyre pressures, but in this case it's the tyre design.
If she is pushing tyres with more contact on the road it will take more power to drive the car.

peregrine said:

Changed my MY15 CJ ES 2ltr tyres from 205 60 16 to 215 60 16 ( notice MY16 CF ES uses 215 60 16) Tyre retailer was honest enough to tell me, I would get a drop in economy.
Told him I couldn't stand the noise of the original Yokohama's and was looking for a more comfortable ride, economy wasn't a concern as I only did about 2000 to 3000ks per year.

All three items proved to be as expected. Huge drop in noise and now a more comfortable ride Although economy I didn't really check just accepted it. and surprisingly speedo is now spot on.
Performance, Subjective, haven't noticed any difference.

Click to expand...

I think that my GSR is truly only doing 96KM/H at my speedo reading of 100KM/H and my Toyota Aurion is the same, I believe that all newer Asian cars are set like this.
I first come across this in the Aurion, thinking what the hell is everyone passing me on the highway at about 4KM/H more and I don't have cars passing me in my Holden like that at all, not to mention cars sitting right up my cars rear all the time.
So I set the cruse-con up to 104KM/H at 100KM/H now and I hate such a stupid setup, I think they have set this up that way it for idiots.
The KM/H should be correct at all times.

I would say that the trucks that are setup speed limited 100KM/H start sitting up the rear of cars on the highway and the car owner thinks he is truly doing 100KM/H but he is not truly doing it because the speedo is reading 4KM/H out and nowadays with the Nazi 1 km/h over Laws, people freak out and go mental about thinking the truck drivers are the problem, but it's their speedo and the Nazi type laws like every k over is a killer nonsense, that's truly a revenue driven scam that Political Correct fools swallow hook line and sinker and start stomping about like little Hitler's.