Wheel Size Question...

[texasz]

Ok, now I've been hanging out listening to people talk about going from the stock 14" up to 15", 16", 17" and even 18" wheels. What I want to know is how does this change the way the car drives? I have heard that changing the wheel size changes your gear ratio. How? Does this provide greater high end speed or what?

Can someone please explain the effects of going to a larger wheel to me?

[JEFF]

If you want to keep your speedometer reading correctly ....

the larger the wheel the lower the profile on the tire,trying to keep the total diameter the same as the stock combination. This works up to a point .... then you need to replace the speedo gear in the transmition ( thats easy to do ).

The larger tire over all diameter the smoother the ride over bumps/pot holes. Example: Roll a penny (small dia. tire) down the road into a pot hole about the size of your fist (sat 1/2" deep) The penny falls in the hole (hard clunk on small tire) Now roll a large frizbee over the same hole ... see what i mean?

Remember: the lower the profile on your tire the less cusion there is for the bumps.(harder on suspention parts). But the trade off is that you get less sidewall flex --- improving your handling.

- Hope that helps.

- Jeff

[Brett240]

first off when changing up wheel sizes in most cases the aim is to keep your overall wheel tyre diameter combo the same to keep your gearing the same.

by increasing the diameter the cars theoretical top speed will increase and acceleration will decrease, vice versa for decreasing tyre diameter.

if you change from a 14 inch wheel to a 15 inch wheel, to keep the overall diameter the same you now need a 15 inch tyre with a lower aspect ratio (distance from outside of wheel rim to the start of your tread ). this has 2 effects, with a smaller aspect ratio there is less rubber to flex, effectivly stiffening your ride and improving handling. the lower the ratio the better you grip and braking... to a point. if you put heavy 20 inch wheels on your car it will increase unsprung weight to a point where you will stuff your cars handling characteristics.

generally a 17 inch wheel is considered the best compomise.

hope this helps

[Mudge]

http://www.geocities.com/z_design_studio/transmission.html

[Brett240]

mudge, thats a pretty useful tool. only thing is it doesnt have the 240 five speed the came into Aus and the UK. is there a box on that graph that uses the same ratios?

[Mudge]

Actually the early US 5 speed should match the 240Z 5 speed, I believe this is correct according to the Haynes I have, this would be a .864 5th gear.

I would personally shoot for a 15-17" wheel and no more, I have 15x7" and they are pretty nice overall, but would eventually like a 16x8-10" or possibly a 17x10" if I could get one at a reasonable price point (insert laughter here).

Take into consideration going this big requires fender flares and coil overs, not exactly cheap, but for road racing I want the ultimate in grip. Right now with 195/60 15" I dont have nearly enough tire to hold the brakes, otherwise the car handles very very nice.

[texasz]

So how big can you go before you have to do other mods (coil-overs, flares, etc.) 15", 16", 17", etc.? Also what is the offset?

[Victor Laury]

I have a excell spreadsheet, called TransCalc with your aussie/euro/jdm market 5 speed ratios as "std. Roadster trans"

Plug your tire size and rear ratio in and it charts speed/RPM in each ratio.

If Mike will instruct me, I'll Upload it to our Downloads section. I don't want to email to each person that asks for it, last time, it was too much work to keep up with the requests.

[zed240au]

its not the diameter that will really bring in new costs its the width that will make the need for flares or coil overs

its not till you go over 7" wide that you need to look at flares or coil overs

I actually used the jag that run idea and raise rear spring perches to get my 15 x 8 under the back instead of coil overs

I am running 15 x 7 all round on my car with 225 50 tyres orginally was running 15 x 8 on back with 245 50 tyres but cant get the 245 50 tyres anymore

Mick

[That Ozzy Guy]

Originally posted by texasz

So how big can you go before you have to do other mods (coil-overs, flares, etc.) 15", 16", 17", etc.? Also what is the offset?

Like Mick said it's the width that requires mods. I remember asking one of the local tyre specialists what is the widest he could fit on my bog stock 240. He quoted me a comfortable 215 which is about 8.5in.

[escapedan]

I am still only 2 weeks into z ownership....pls forgive my early ignorance!!

Is it safe to assume that 16x8 rims will fit nicely under stock wheelwells at all corners without requiring fender flares? If so, what tire size would be recommended for this rim?

I have just purchased a 73 240z with the intent of autocrossing it. Any suggestions for shock/spring and rim/tire setup would be greatly and sincerely appreciated!

[zed240au]

my experience my car is 70 model but you wont get 16 X 8 under a 240 with out modifing struts or flares i would say 16 x 7 with 225 wide tyres probaly even 235 if they make them in 16 i know my 245 on 15 just rubbed with the inner rear lips cut off

Mick