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Old 31st October 2010, 00:47   #1
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Default Tyre Upsize: Impact on Odometer / MID reading

Note to Mods: I searched for thread on functioning of Odometer, could not find one so started the new thread. Please merge if similar thread exists

Hello BHPians,

I have been curious to understand the way an odometer works? How would the circumference of wheel will impact the reading?

My understanding is that odometer will work on the principle of wheel rotations to compute distance traveled. Now, if I change the circumference of wheel by up-sizing or downsizing, would this impact accuracy of the odometer reading.

Please comment.
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Old 31st October 2010, 13:56   #2
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The speedo works on the principle of wheel rotations to compute distance travelled.

Take, for example, your speedo is calibrated with regard to company fitted tyres with a rim+tyre radius of 300mm, or a circumference of 300*2*pie = 1884mm. This means that when the wheel completes one rotation, the vehicle would have moved 1884mm or .001884kms. So, to cover 1 km, the trye will make 530.78 rotations.

But then, if you upsize your tyre to say, 17 inchers and the new radius is 335mm and circumference = 2103mm. The car will now cover .002103kms in one rotation or it will make only 475.5 rotations in 1 km.

Now, as the meters are calibrated to show 1km for every 530 rotations, if you upsize your tyre it will still show 1km for every 530.78 rotations, but in practice your car would have covered, 530*0.002103 kms = 1.116kms. ie, the speedo reading is less than the actual reading.

So, if you upsize(dia) your tyre the speedo reading will be less than the actual reading and vice versa.


For more info check THIS out.
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Old 31st October 2010, 19:09   #3
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@dhanushs

Thats the whole point, based on your example we have 11.6% error. This would have it's impact further on:
  • FE computation
  • Service Interval & etc.

Can we calibrate the odometer post changes?
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Old 1st November 2010, 12:35   #4
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Quote:
Originally Posted by drive_or_die View Post
Thats the whole point, based on your example we have 11.6% error.
A difference of 11.6% will not only affect the odometer, but also the way that the car behaves. There is a possibility of the tyre scraping the insides, ground clearance / center of gravity going unduly higher than the manufacturer designed the car for, turning radius increasing and possibly, the ABS / traction control systems receiving incorrect values. The gearing will get taller, hence low end torque delivery will suffer.

My suggestion : Please upsize within a 2% difference from stock, the lesser the better. Refer to the excel sheet on this thread (ARTICLE: Choosing The Right Set Of Tyres for your Car) for recommended sizes.
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Old 28th May 2014, 16:01   #5
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Default Tyre upsize & MID

Dear Enthusiasts,

After covering 60K KMS on Bridgestone B290 I moved to Yoko Earth-1. Upgraded my Fiat Puntos tyres from 175/70 R14 to 195/65 R15 to improve the ground clerance. Now I am getting additional 17 mm more. Ground clearance is now up from 165 to 182 mm.

Downside?
This model comes with MID and it will show errors now. Actual speed will be 105.64 kmph but speedo will show 100 kmph. 5.64% error.

MID Shows:
a) Range (DTE)
b) Trip distance
c) Total distance
d) Avg. consumption
e) Instant consumption
f) Avg. speed

Now, I have 2 questions:
1) Should I multiply everything by 5.64% now?
2) http://www.1010tires.com/Tools/Tire-...0R14/195-65R15 gave a warning: Change beyond 3% of diameter variance, you face the risk of brake failure. Why will this happen?

Btw. I got Yoko Earth-1 from Sai Iyengar for 4750 each. with balancing & alignment free and a complimentary Car vaccum cleaner from Yoko.
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Old 28th May 2014, 23:21   #6
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Default Re: Tyre upsize & MID

Quote:
Originally Posted by NSK123 View Post
Now, I have 2 questions:
1) Should I multiply everything by 5.64% now?
2) http://www.1010tires.com/Tools/Tire-...0R14/195-65R15 gave a warning: Change beyond 3% of diameter variance, you face the risk of brake failure. Why will this happen?
1) multiply by 105.64% or add 5.64% to MID
2) Basically when the tire radius increases, the moment gets increased (force x perpendicular distance) this will require a higher braking force to stop the vehicle. Not sure about the 3% deviation standard
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Old 29th May 2014, 07:15   #7
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Default Re: Tyre upsize & MID

Quote:
Originally Posted by Mr.Boss View Post
1) multiply by 105.64% or add 5.64% to MID
2) Basically when the tire radius increases, the moment gets increased (force x perpendicular distance) this will require a higher braking force to stop the vehicle. Not sure about the 3% deviation standard
Thanks Mr.Boss. Given that top variant of Fiat Punto comes with 15" wheel shouldn't the brakes be designed for it?
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Old 29th May 2014, 09:34   #8
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Default Re: Tyre upsize & MID

Quote:
Originally Posted by NSK123 View Post
Thanks Mr.Boss. Given that top variant of Fiat Punto comes with 15" wheel shouldn't the brakes be designed for it?
To be frank, I'm not a expert when it comes to Brakes.
But IMO these essential systems will be designed considering the maximum possible Tire size available in market.

Forgot to mention, it's not the MID values alone are affected. The wheel end torque will reduce by same ratio (5.64%)
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