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
About this Discussion
7 Replies
5 Participants
lambchop11078
Lexus IS Forum
Community dedicated to Lexus IS Enthusiasts. Come in and enjoy our articles, galleries and information on aftermarket parts for the IS300, IS250, IS350.
Full Forum Listing
Top