I'm going to need traction, and I'm going to need it bad.

What would be the optimal offset/wheel width if I plan on running 18" diameter and 315mm width in the rear? How about the front? And I do plan on rolling the fenders. Thanks in advance.

 

sweet this will help out a lot

 

You won't be able to fit 315's in the rear without ridiculous camber and modifications to the sway bar mounts and inner fenders. Even with my rear flares it's a tight fit. I think a 285 (though a 295 might squeeze in by a hair margin) is about is wide as you can go in stock wheel wells before you start running into clearance issues.

 

This is my first post here, so don't take this as me being a d*^& and pissing all over something that is in fact cool. However, I have lived and breathed wheel and tire fits for a living.

There are two critical effects of tire sizing being left out by Jimlab's original spreadsheet. One is overall tire diameter, the other is the actual width of the tire, installed on the wheel.

If a tire has a shorter overall diameter, it will have more clearance to the fender at all points. For example, a 245/40-17 will have approximately 0.5-in more clearance to the fender in all directions than a 245/45-17 (looking at mfr. tire data more than calculations based on nominal). The same thing can apply to variations from nominal. Hoosier has historically cheated the OD of certain tires downward. For example, a 275/40-17 typically measured 25.3 or 25.4 OD, whereas the nominal works out to 25.7ish. This let those tires go places that others of the same nominal size would have had problems.

The inner and outer clearances are all based on wheel width. As far as I can tell, the spreadsheet assumes that the sidewall of the tire will be 1/2-in. out from the bead seat all the way up to the tread. Discussion in this thread says that is assumed to be the plane of the outer face of the wheel. As I played with the numbers, changing tire width had no effect on clearance. I have a set of 18x10.5 wheels. The spreadsheet points to a 295 width, but I would more likely use a 285/30 or 265/35. These would give me more clearance, but the spreadsheet doesn't show it.

There are also variations from nominal in width. According to data sheets, in 265/35-18, the BFG R1 and Yoko Neova both measure 10.7-in. in section width on a 9.5-in. wheel, while the Michelin PS Cup measures 10.9-in. The 255/40-17 PSC is almost as wide as a 275/40-17 Hoosier. Some tires also have more rounded profiles, some have more square shoulders. Also, as a rule of thumb, the actual section width of a tire increases approximately 0.2-in. for every 1/2-in. increase in wheel width. (I didn't make that up, I just got it from a Michelin motorsports engineer.) That lets you work your way from measuring rim width on tire data sheets to whatever wheel you are planning. All these details matter.

Random detail worth mentioning: I've never found a 265/30-18 tire. I have found evidence that there was once a 265/35-17 Yokohama of some sort, but if so, it has been out of production so long I can't verify it.

I haven't invested to understand the QE3K sheet as thoroughly.

For sure, this is a good tool, but ultimately, if you're pushing the envelope, you'll have to either follow exactly what someone else did, or understand that you may spend some money on "research".

Still, this has helped me understand the FD fit a lot better, so thanks!

 

Hi Guys! I'm running on stock 16'' wheels and i need to change my tyres. Currently, the size is 225/50/16. How much wide can i go with the same wheel size? I read all the thread but it seems that i'm getting nowhere (stupid me). Can anyone help please? (P.s The car has been lowered but unfortunately i do not know by how much, as i bought the car already like this from Japan) I've also included a couple of pics of the car with the current tyres (if it helps). Thanks!

 

245-45/16. I have toyo proxes ra-1 on there and the car is lowered quite a bit with no problems. the tires are unbeatable.
t

 

Thanks Tom.Jelly! Can you post some pics of your wheels please or send me an e-mail? Many thanks again.

 

I don't have many good pics, but these might work. The RA-1 is used a lot in vintage racing on 914/6 porsche and such. excellent track day tire too, even in the rain. they are pretty cheap, so even if the car is driven on the street they could be replaced fairly often if need be- Not sure of what kind of street driven tread life you would get, but this is the only tire I'll use on stock wheels on this car. Toyo has a new dot legal race tire, but I don't think they have 245-45-16 size yet.

t

 

Many thanks again Tom. Very helpful !! I am tempted to go for Toyo R888 with the same size that you suggested.

 

toyos website has all the dimensional data..

t

 

Because it is assumed that most people will try to maintain comparable diameters when upgrading so that speedometer readings and the ABS system are not impacted.

Which can vary by manufacturer's actual dimensions, as well as personal aesthetic preferences. For example, I like a wider tire on a narrower wheel to protect the lip of the wheel and have a "fuller" appearance. Others like narrower tires on wider wheels for the "dorifto" look.

It's a matter of personal preference, and since the width of the wheel does not vary, the width of wheel is used for clearance calculations. After all, it is wheel that will come into contact with suspension components, and not the tire. Fender lips and liners are another story, and that's why a 1/2" margin of clearance is advised beyond the edges of the wheel.

Not necessarily. As far as lock-to-lock clearance in the front, a wider tire will have potential clearance problems no matter what the profile is.

It assumes that at least 1/2" of clearance beyond the outside edge of the wheel's tire flange is preferable. It has nothing to do with the tire width, as explained above.

How would you compensate for variances between tire manufacturer's actual dimensions, let alone within their own lines? The spreadsheet is not meant to be definitive, it is meant to provide guidance.

 

This is not necessarily true in all cases. In my case, my rear tires were rubbing the shimmed RE trailing arms (10 and 7/8" wheels with 315 PS1s). I think people just need to use common sense and take the time to crunch some numbers before dumping money on rims and tires. These spread sheets are not precision tools, they are simply here to provide a good baseline based on setups that are known to work.

I also added the a 1/2 inch to the other sheet because every rim that I have measured in person is actually 1 inch wider than the advertised bead width. It is pretty much standard for the rim lip to extend about 1/2" beyond the inner edge which retains the bead on each side.

 

My calculations account for that.

 

Cool. Carry on then

 

do u have any more pics with the flares?

they look great,

I wana get 10.5, 18s with 15offset

I wonder if similar flares will cover them

 

I'm not sure what offsets I will need with my new bodywork. I'm thinking this:
Front:18x9.5 with 255/35 tires.
Rear:18x10/10.5 with 285-295/35 tires.
My arches are flared by:
Front: 2.5cm extra
Rear 2cm extra.

Any idea what kind of offsets would be right for this application?

Cheers.

 

+1

Those flares look nice! We need more pics.

 

Does any one know what car has the same bolt pattern as a 81 RX-7
Thanks

 

i suggest you post that question in another thread specific to that model Rx7. This thread was for the 93-95

 

Just used the Calculators and it seems like the wheels i want will fit.

18x9 + 34 235/40 Front
18x10 +42 265/35 Rear

That seem right on stock body mild drop?

 

Yes, but you might consider mounting some wider rubber up front.

 

There's these two setups that have 265/30-18 tires (look at the rear tire specs):Linky

 

I'm looking at 2 sets of wheels.

Option one is for track racing:

10X18 all around with offset 45: 255/35/18 tires on 4 corners


Option 2 (show wheels)

8.5X19 front offset 42

9.5X19 rear offset 43

tire size whatever is needed


What I find weird is that I see some people running 10X18's on 4 corners with offset 50 on all of the wheels (with coilovers) and the spread sheet tells me otherwise (possibly because of the coilovers)

Are the above offsets possible with stock FD body or lightly rolled fenders?

 

18x9.5
Bolt Pattern: 5x114.3
Offset: 38
Hub Bore: 73

255/40's

Would these fit?
- I have both front & rear fenders rolled flat, and the rears have a slight flare.

Thanks,
Geby

 

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