Tire Diameter Affect on HP TQ?

[beauwalnut]

I gotta start of by saying I DID search and found almost nothing...

Has anyone tested the performance attributes of various tire diameters?

Gears, cams etc. are designed with stock wheels/tire diameter in mind. In a 240 that is roughly 24 - 25 inches
Stock S13 195/60-15 have a diameter of 24.2"
Stock S14 205/55-16 have a diameter of 24.9"
(Per: http://www.federaltire.com/download/specFD1.pdf )

My theory:
If over sized wheels/tires hurt 0-30, under sized wheels/tires should help 0-30. This would make all gears "lower" so the engine is revving higher at the same gear. Making a car better at time attack... or zipping around town. I am thinking 195/45R15 or 215/35R16 both @ 21.9"

Any Zilvians tried this?

[59bhp]

it will affect gearing obviously but a lot of thetime attack cars etc will run specific gear ratios (and final drive) to suit the needs of the track, which will take into account the diameter of the wheels

The diameter of the wheels is usually dictated by suspension design, cooling, braking etc etc

as for road cars, a more effected way of changing the gearing is to change the final drive (which will be more dramatic than a wheel and tyre change) which then allows you to select wheel size based on neccersary tire size and width, brake size, aesthetics etc etc

in short yes you can, but there are better ways to do it

[Rich260z]

Stock diamter size is 24.7" which is what most all nissans from this era run. Just fyi.

In regards to that I run at 245/40/17 which is a fraction of a percent bigger than 24.7" and I still don't get accurate speedo readings (compared to my gps speed and phone gps speed). At 60mph I'm at 60mph, but at 80mph speedo I'm at 77mph gps.

As per the original statement of if it will help your 0-30, smaller tires will. Miata guys run undersized tires when they show up to autox events all the time. Will it work in a road race track? maybe if you don't mind running out of gearing on a long straight. Undersized tires for a autocross is the easiest way of making your car "faster" around the cones since its more 1st and 2nd gear stuff

[dawagarage]

tire diameter affects both acceleration and braking times.
bigger tires hurt acceleration and helping braking,
and vice versa. ill look for that tech article and try to post it up.

[beauwalnut]

sweet. can't wait to see it.

[mikerbike]

Originally Posted by beauwalnut

Making a car better at time attack...

Originally Posted by beauwalnut

...195...

Originally Posted by beauwalnut

...215...

I don't think tire diameter will be your issue.

JK, I use sissy tires too.

I usually use 235/45/17s on the rear for silly drifting activities. I'd like to think I feel the difference when I can use 235/40/17s. Not so much when I use 225/45 though.

For grip driving, I'd be willing to sacrifice a bit of acceleration in return for better braking and more speed through the turns. At the same time, It's already been mentioned that 245/40/17 is pretty close to stock rolling diameter. No reason to use tires meant for early 80s Hondas.

[beauwalnut]

Originally Posted by Rich260z

Stock diamter size is 24.7" which is what most all nissans from this era run. Just fyi.

Well... You maybe correct about the "diamter"

When I look up 240sx on tirerack.com, I see S13's came with 195/60-15. When I look those tires up from the link above it shows a diameter of 24.2"
S14 SE wheels are listed as 205/55-16 with a diameter of 24.9

I don't know where you are getting 24.7 but I do appreciate your input.

I just put 235/40-17 on my 8 wide wheels in the front (a big change from the 225/50-17's I had) going from 25.8" to 24.3" and so far, I can say there is a noticeable improvement in steering. Maybe it's my imagination but this KA N/A seems faster. The tires are on back order so I will report again when I have the rears on.

If I find some 5 lug, 8 wide 16's or 15's at a good price I'll try those out and see what happens.