Lexus IS Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
20 Posts
Discussion Starter #1
After recently putting on my 18"s from my 17"s i had my friend pace me, and i was 10 mph slower than his spedometer. (mine said 70 when his said 80) for example. I knew that this can happen w/ bigger tires/rims and such, but a 10mph change, i was just wondering cuz it seemed like a huge change. I thought it was just a little change? Could that be right?
 

·
Registered
Joined
·
3,155 Posts
stock: 21.5 x.45 = 9.675cm 17"= 43.18cm => 52.855cm
new: 23.5 x .40 = 9.4cm 18"= 44.1cm => 53.5cm

if the tire was 235/30...
23.5 x .35 = 8.225cm 18"= 44.1cm => 52.325cm.... nevermind

of course, the speedo shows slower b/c tires are larger... it wouldn't have made better if you had 35 profile.

plus, speedo gets less accurate as car goes faster.

just dont forget to drive 10mph slower :D
 

·
Manufacturer of Pipes
Joined
·
5,285 Posts
It's a good thing... You will be putting less miles on your car now... :lol:

i have the same thing too... But I don't think i was 10 mph off... i will have to check that out this week.. i have 225/40/18... i guess maybe you will have more of a difference on Speedo.... I will test a same speed you did to get a comparison....

Joe
 

·
Registered
Joined
·
20 Posts
Discussion Starter #6
Well again today i was making note of it, and i'm sure as you said Blue, which i don't know why i wasn't thinking before, that the faster i go, the more speed gap theres goign to be. So its' not as big as I intially thought. thanks for help tho.
 

·
Registered
Joined
·
12,157 Posts
it definitely shouldn't be that big of a gap, you could always change the tire specs to try an get it closer

i made electronics to fix that bullshit

-gte
 

·
Registered
Joined
·
5,941 Posts
blueIS said:
stock: 21.5 x.45 = 9.675cm 17"= 43.18cm => 52.855cm
new: 23.5 x .40 = 9.4cm 18"= 44.1cm => 53.5cm

if the tire was 235/30...
23.5 x .35 = 8.225cm 18"= 44.1cm => 52.325cm.... nevermind

of course, the speedo shows slower b/c tires are larger... it wouldn't have made better if you had 35 profile.

plus, speedo gets less accurate as car goes faster.

just dont forget to drive 10mph slower :D
Math is just a little off in the calculation of the overall wheel diameter. You'll need to multiple the sidewall height by 2 when calculating the diameter. This will factor in the top and the bottom sidewall height. So it should be like so. I'll stick with cm since you did.

Stock: 21.5 x .45 = 9.675 cm 17" = 43.18 cm => (2 * 9.675) + 43.18 => 62.53 cm or 24.618 inches (since we're all used to this measurement)
 
1 - 8 of 8 Posts
Top