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Old 1st July 2004, 02:32   #1
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guys maybe this wud help u to go for an appropriate upsize.
a +-5mm is said to be acceptable .as anything differnt cud affect the handling,the ground clearance,the speedo reading

consider 145/70r13,of the carb zen
to calculate the tire overall diameter(OD),we multiply the tire width by the aspect ratio to get the height of the tire.
tire height =145*70%=101.5mm=3.9961inches

then we add this tire height to the rim diameter.
2(3.9961)inches+13inches=20.9962inches=533.2018mm.
this is the unloaded tire diameter.as soona s any weight is added this will reduce.

for 145/80r12 ,this is 536.8mm(difference of 3mm)in the mpfi zen,so ground clearance increases by 1.5mmand speedo rdg is same.
for upsize of 155/70r13,this is 547.2mm(a massive increase of 14mm over the original tire),so that is 7mm increase in grnd clrnce.
for 155/65r13,this is 531.7mm ,so that is -2mm and so 1mm decrease in grnd clearance .
so to get an almost similar peedo reading the ideal upsize for a zen shud be 155/65r13 that is provided the 155mm does not foul with any body part.
simi;arly u can calculate the required upsize for ur cars
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Old 1st July 2004, 09:24   #2
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http://www.car-videos.com/info/tirespecs.asp

Well if you happen to be bad at math or don't like to work those equations out.

Check out the link above, it's got a online calculator that will do the summing up for you.

Regards....
Shan2nu
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Old 1st July 2004, 10:14   #3
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this is a very good info. i was just thinking that the upgrade of tyre is all abt width and length. never tought of these calculations.
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Old 26th April 2010, 15:57   #4
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i think this is a fantastic pierce of information; hope the mods would make it a sticky so enthusiast would not over do the up sizing exercise.

while on the same topic; how much harder would it be to park your vehicle (with bigger tyres) if it was not power assisted? The mahindra invader 4WD does not come with P/S.
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Old 10th August 2011, 07:00   #5
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Default Re: how to calculate tire diameter

Quote:
Originally Posted by speedsatya View Post
guys maybe this wud help u to go for an appropriate upsize.
a +-5mm is said to be acceptable .as anything differnt cud affect the handling,the ground clearance,the speedo reading

consider 145/70r13,of the carb zen
to calculate the tire overall diameter(OD),we multiply the tire width by the aspect ratio to get the height of the tire.
tire height =145*70%=101.5mm=3.9961inches

then we add this tire height to the rim diameter.
2(3.9961)inches+13inches=20.9962inches=533.2018mm.
this is the unloaded tire diameter.as soona s any weight is added this will reduce.
Hello Speedstaya, I have a question regarding tire size and diameter.

Suppose I stay on my current rims on my Endeavour and stock tires of 245/70/16, and then choose to upgrade tire size to say 275/60/16, the formula shows that tire diameter should be 28.9 inches compared to stock tire diameter of 29.5. However what effect does putting 30mm wider rims on the same rim size have on the tire diameter? Will the tire diameter be greater than 28.9 inches once the tire has been mounted on the rim?

I'm looking out for a tire and am finding it really hard to find the size that I want! Thank you for the help.
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