275/40/17 same as 275/40/18?

[x10-wakerider]

Iam getting new wheels and I currently have 275/40/17" tires on all 4, the new wheels are 18x9. Is the 275/40/18" tire the same size as the 17" the only difference being the size of the hole for the wheel?

[Eugenio_SS]

you need 275/35-18.

stock size = 275/40-17
means 275mm width
sidewall = 40% of 275mm = 110mm
overall diameter = 2x sidewall + rim-size = 220mm (8.66") + 17" = 25.66"

now, 275/35-18
means 275mm width
sidewall = 35% of 275mm = 96.25mm
overall diameter = 2x sidewall + rim-size = 192.5mm (7.58") + 18" = 25.58" (very close)

if you were to use 275/40-18
means 275mm width
sidewall = 40% of 275mm = 110mm
overall diameter = 2x sidewall + rim-size = 220mm (8.66") + 18" = 26.66" (would be quite off)

Hope this helps.

[91Z28]

Hey Eugenio, since you seem to be the wheel/tire man, I have a question. I have 275/40/18 on my rear tires and want to know if these are bad for traction. I know dedicated drag racers use small wheels with lots of sidewall, but if this setup works for corvettes it can' be that bad?

[Eugenio_SS]

Hey Eugenio, since you seem to be the wheel/tire man, I have a question. I have 275/40/18 on my rear tires and want to know if these are bad for traction. I know dedicated drag racers use small wheels with lots of sidewall, but if this setup works for corvettes it can' be that bad?

1st, i'm not "the man"... just a guy that knows some ...
as for traction, it's not really a matter of a given size.
it depends a lot more on the compound and suspension setup than size itself.
another thing is sidewall helps for deflection, so having a 275/40 either on 17 or 18s would allow for roughly the same lee-way.
18s do offer more rotational mass, and on top of that further away from the center of rotation, increasing rotational inertia... which won't help... but you'll be fine.
i use BIG skinnies 315/35-17 even when @ drag... and they are over 55lbs on each corner.

[x10-wakerider]

Thanks EUGENIO!!!!!!!!