New Wheels, Old Speedo

[baron25]

Has having larger wheels fitted thrown out your speedo by much? i'm going from the stock 15/16's to 17 soon, will i need my speedo recalibrated?

[tacker]

yep

have noticed my is out by about 25 kms at 200kph with 18" wheels on.

[ebola]

how much different is the total rolling radius (is anyone counting these)...

16in x 55 mm sidewall = 16 * 2.54 + 5.5 + 5.5 = 51.14 Diameter

17in x 40 mm sidewall = 17 * 2.54 + 4 + 4 = 51.18 Diameter

18in x 30mm sidewall = 18 x 2.54 + 7 = 52.72 Diameter

circumference = pi x D... work out the % diff by old circ. / new circ. These should all give you close enough to stock that your speedo will hardly be affected.

does this all sound like high school maths all over again?

and a smaller circumference will give you more acceleration, larger more top speed (if you could do it), same as a diff ratio will.

Keep the same rolling radius... easier all round, no issues with scrubbing, no issues with speedo.

BTW - my speedo is 6% out - as in it reads 94% of actual speed... with stock rims and rubber.

--ebola

[n15m0]

use this to calculate what profile tyres to use to keep ur rolling diameter and u won't need to recalibrate speedo.

http://www.1010tires.com/TireSizeCalculator.asp

[wonkynutz]

how much different is the total rolling radius (is anyone counting these)...

16in x 55 mm sidewall = 16 * 2.54 + 5.5 + 5.5 = 51.14 Diameter

17in x 40 mm sidewall = 17 * 2.54 + 4 + 4 = 51.18 Diameter

18in x 30mm sidewall = 18 x 2.54 + 7 = 52.72 Diameter

circumference = pi x D... work out the % diff by old circ. / new circ. These should all give you close enough to stock that your speedo will hardly be affected.

does this all sound like high school maths all over again?

and a smaller circumference will give you more acceleration, larger more top speed (if you could do it), same as a diff ratio will.

Keep the same rolling radius... easier all round, no issues with scrubbing, no issues with speedo.

BTW - my speedo is 6% out - as in it reads 94% of actual speed... with stock rims and rubber.

--ebola

Too much maths and the wrong maths.You don't need to know the circumference as it's directly proportional to the diameter.Also it's not 55mm sidewalls.It's 55% of 205mm which is 112.75mm.Overall diameter of 205/55/16 is 63.19cm.

I worked out the differences for my own car. 235/40/18=64.52cm diameter.

Difference is 2%,so instead of indicating 100k's it'd show 98k's.

Stuff all.Not worth worrying about.

Oh yeah,235/45/17 is slightly less,about 1.8%.

Edited November 13, 2006 by wun hung lo

[tacker]

Or if you do want to measure the circumference you can try this method.

rpm/gear ratio/diff ratio x wheel circumference in kms x 60 = kph

example is my r 32 GTR

at 7000 in 4th gear ( 1:1 ) with 4.11 diff's and 18" 245/40/18 (2080mm )

7000/1.1/4.11 x .00208kms x 60 = 212Kph while the speedo was happily sitting on 195kph which is a brand new Nismo 320kph jobbie

Edited November 13, 2006 by tacker

[ebola]

Too much maths and the wrong maths.You don't need to know the circumference as it's directly proportional to the diameter.Also it's not 55mm sidewalls.It's 55% of 205mm which is 112.75mm.Overall diameter of 205/55/16 is 63.19cm.

I worked out the differences for my own car. 235/40/18=64.52cm diameter.

Difference is 2%,so instead of indicating 100k's it'd show 98k's.

Stuff all.Not worth worrying about.

Oh yeah,235/45/17 is slightly less,about 1.8%.

and oops - pi x D, pi is constant.

and 55% means that my 205/55R16's are smaller than 235/55R16 wheels.... !?!?!?

then again, you are right that its *not* 55mm, so, umm, yeah.

[salad]

205/55 is a fair bit smaller than 235/55.

The proper correct size which is only a TINY bit (</=1%) bigger than stock is 225/45R17, 255/40R17, 225/40R18, 255/35R18

Sometimes these sizes are more expensive than the usual 235 or 265 but then again, sometimes they are cheaper.