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-   -   About Gear Ratios & their Relation to Tyre OD, Torque, Speed etc (https://www.team-bhp.com/forum/technical-stuff/1760-about-gear-ratios-their-relation-tyre-od-torque-speed-etc.html)

Guys,

Does changing the tire affect the gear ratio?
If yes,then change in the tire pressure will also affect it.

Can anyone pls explain this?

Speedsatya.

changing tyre to a different circumference? = the actual speed will change. but not exactly gear ratios..... in a way the final drive ratio will change (correct me if i am wrong)

tyre pressure? = No (unless its flat)

the circumference of a tire will be more if the air pressure is more as compared to one with lower pressures.
so does this affect the overall gearing

True.
Lower overall diameter (OD) will give you shorter "gearing".
Higher OD will give you longer/taller "gearing".

However tire pressure...thats a tricky one...its not as obvious as it seems if you think about it...

cya
R

I don't think the circumference will be more when pressure is high. b'coz the thing is tyre does not act like a balloon (they don't expand and get bigger)

Think it this way.... 2 tyres..... (ignore the load at once, i.e. think they are not put on any car, just lying around)
1st is not set into the rim (say a fresh tyre)
2nd is setup in a rim properly inflated.

Now can u say these tyres have different circumference?

Tyre size wont afect the final drive ratio. Shouldn't change the gear ratios too. Only your speed in each gear is effected depending on tyre diameter.

Shan2nu

so wat wud happen if u opt for a tire having 2mm lesser OD.than the stock one.
wud shorter geaqring mean lower top speeds .

Technically, i think your top speed would come down a bit.

The tyre rotates at a specific rpm ay any given engine rpm. *So lets say the vtecs wheels rotate at 3000 rpm when the the first gear hits 7100 rpm. When you change the tyre diameter, this wont be affected. The wheel will still rotate at 3000 rpm at 7100 rpm (engine speed) in first gear at aroud 56kmph.

But what happens is that since your tyre is 2mm smaller in diameter, the max speed in each gear will reduce by a certain margin. Thus reducing your top speed too. I don't think it should affect acceleration times by much. Lesser max speed in each gear but, the engine will redline quicker, compensating for the loss of max speed. Provided the tyre weight is reduced.

Shan2nu

hey shan,
dont u think the change in OD will change the speedo rdg also.
so it cud be because of this also u might be getting lesser or more speed rdg.

satya

Smaller wheels will show + ve error. ie, if you speedo is showing 100kmph, tru speed will be less than that. Bigger wheels will have a tru speed of more than 100 when speedo reads 100kmph.

Shan2nu

hmmmmmmmmmmm

satya

Bcoz see, speedo is caliberated to wheel rpm. It has noting to do with how big the wheels are.

So, lets say that at 100kmph (speedo) the wheels are turning at 3000 rpm. Now a wheel with smaller diameter (for eg let us say the wheel is 0.5 meters in circumference) should technically cover 1500 meters every min at the rate of 3000 revvs per min.

Now lets replace this tyre with another having a circumference of 1 meter. So at 3000 rpm this tyre will cover 3000 meters every min. Double the speed and dist.

Shan2nu

Of course in technical terms.. it won't change!... after all that damn thing is inside the gear-box,how can it change when you change the tyres?
But putting different size tyres would have a similar effect as change in final drive. As the speeds in all the gears will increase or decrease in the same proportion.

Not exactly, it won't have any effect on the top speed of most cars as they don't hit redline in the final gear.
Rather on second thought the car will be able to do more RPM in the final gear hence generating more power and then more top speed!

Think this... if you *put bigger tyres in a car (larger diameter)... would the top speed increase?

True, and the point to note is .. the speedo will always show the same speed at a given RPM in a given gear, whether you put a tyre at all or not!

Llike i said, it'll depend on the weight of the car. Cars not redlining in top gear, is more dependent on frictional resistance and on the aerodynamic wall the resistant air creates. The car just isn't powerful enuf to put up a fight against resistance, beyond that point. I think if a car is lifted with the wheels suspended in air, it should redline in top gear.

But i know one car that can redline in top drive, Accent CRDI. hehe

But yeah, as long as the gear is not redlined, top speed wont change. Aerodynamical wall is dependent on a cars ability to cut through air. So if a vtec hits the AW at 195kmph (true speed) at 6000rpm in 5th gear. Using a smaller wheel will increase your top speed according to the speedo, but the car will still hit the AW at 195kmph (true speed). Maybe with a smaller wheel, the car will reach 6500rpm in top gear, but the true speed will still be 195kmph.

Shan2nu

well... AW is not only dependent on the car but also the engine. The BHP it produces @ 6000 RPM *is say 100..... so it can't cross 195km/h taking into consideration the shape. but if somehow.. its running at 6500rpm its producing 103bhp.... so it might be able to move forward say till 105bhp and 200km/h.. i.e. it hits its new AW equilibrium.

Actually.. i can be wrong.. just.. being a bit imaginative!