Are 255/40x18 tyres ok at rear + other mod info

[stevieturbo]

Mate has his new 04 Monaro 5.7, but the 235's on the std rims almost look too small.

Has anyone changed these for 255/40x18 ? Do they fit fine? any clearance issues ? It does look as if there is plenty of room.

Also looking a suspension kit soon. What are the options/reccomendations ?

Again same for braking. Fronts only. In part for braking performance, but also for cosmetic reasons.
No sliding caliper upgrades, must be 4 or 6 pots, big discs, and preferably something like AP.
A proper dust sealed road kit.

[P47ThBolt]

Don't know about tyres there have been some previous threads on this one recently where chaps are runnning 245's with no problems not sure about 255 though.

Brakes - Speak to Caspy as they are offering AP 6 Pot upgrades whole deal braided lines etc. as a distributor of Wortec's package which is the same as HSV AP systems in OZ. Fronts only are about £1600 all in.

Also Caspy has a new 6.0l with a fully adjustable suspension syustem that is amazing looks and performance again about £2500 all in. I believe thsi ia a tried and tested system for HSV's/Holdens from OZ as well.

Hope that helps. Its a start anyway

[bilton_d]

I guess if they were 235/40/18 he would be better going to 255/35/18 to keep the rolling radius similar.

[V8 OHV]

255/40 will raise the gearing, which is already too high on the 5.7, by 2.5%.

255/35 will drop the gearing 1.5%. A better bet but the ride will suffer slightly.

[booster]

Won't 255's be a bad fit (width-wise)? Not sure of the wheel width tho'.

[stevieturbo]

Not sure what width the wheels are. But the 235's do look too narrow.

I'll get him to speak to Paul about brakes and suspension. Need to have a word about exhausts etc too anyway..

A gearing increase would be more acceptable than a harsh ride due to very low profile tyres. Any gearing change is negligable IMO and wouldnt be a concern.

[caspy]

[quote]255/40 will raise the gearing, which is already too high on the 5.7, by 2.5%.

255/35 will drop the gearing 1.5%. A better bet but the ride will suffer slightly.[/quote]

Please explain. Too high in comparison to what ?

[booster]

I'm really quite interested in the technical details here as my rear tyres are due for renewal now. So please anyone with some knowledge here please let me know what the alterantives to a 235 tyre are that won't screw things up.

[booster]

[quote]
A gearing increase would be more acceptable than a harsh ride due to very low profile tyres. Any gearing change is negligable IMO and wouldnt be a concern.[/quote]

Yes, please explain!

[stevieturbo]

OK. Try the 2 different tyre sizes in this gear ratio calculator.

Its like a 3% increase, which has almost nothing and you would struggle to notice any difference.

[comm_SS_V8]

stevieturbo said:

Try the 2 different tyre sizes in this gear ratio calculator.

What's the URL mate?

On with the show....

Apologies if any of the following is teaching people to suck eggs, but I'm sure there's others out there like me who simply didn't know how complicated a set of tyres can be.

booster said:

Yes, please explain!

I was in the same boat mate. The comments made on this & the previous threads had me baffled. "Raising the gearing" or "dropping the gearing"?? What's that all about then?

As it's been an OK sort of night at work tonight I decided to research this highly interesting topic of discussion in order to answer some of these questions.

If you search for 'gear ratio calculator' in google you'll find a large result set. I had a play with a few of them and they are great at what they do, but as a rookie on this topic I needed more input. As an example, I wanted to try and understand the differences between putting 235/40/R18 Vs 245/35/R19 on a car (or similar changes).

If you remember back to your school days when ...

speed = distance/time

... you'll start to understand that in order to add larger wheels you need to keep the same circumference otherwise you'll be going faster than the speedo is telling you because you've just increased the distance travelled per revolution. To achieve this you need a lower profile tyre. This is where all the tyre width/ratio/diameter stuff comes in.

So why is it that if you add wider tyres and keep the same circumference it still alters the speed??? Well, it's all to do with the gear ratios of the gearbox & differential aparantly. The above equation get's slightly more complicated once we take these factors into account:

Speed = (Tyre Radius * Rotational Velovity)/(168 * Gear Ratio)

Oh! So now what? Well, now with something tangable to work with I went off to StarOffice armed with the data from Holden's website regarding the gear ratio's and came up with the following spread sheet calculator.

My Gear/Tyre/Speed Calculator and Comparator (.xls)

It's a 4 stage data entry spreadsheet:

Step #1: Enter the details of your current tyres; e.g. 235/40/R18
Step #2: Enter the gear ratio's of your gearbox & differential (Default values are for the CV8 Monaro & Commodore)
Step #3: Enter the details of your new tyres; e.g. 245/35/R19
Use this step if you only want to compare 2 tyre sizes, otherwise just enter data in to step #1
Step #4: This is a bit of fun. It calculates the theoretical road speeds based on the rpm when you change gears. It's not required for the main function but I'll be testing it out on the way home this morning to see if it's right. I added it for the benefit of those thinking of doing the differential gear changes as described by motomk.

Once you enter the details for 1,2 & 3 you'll notice the calculations tables change & with all the data complete the chart should display the changes. Try different tyres and you'll see exactly what changes it will make to the Road Speed Vs Engine RPM.

If anyone spots any mistakes, please let me know. I've compared the results with a couple of online calculators and I'm happy with the results.

Hope it's of use to somebody out there......

PS: Apples should be left at the front of the classroom

>> Edited by comm_SS_V8 on Wednesday 24th August 07:02

[caspy]

Fully understand the tyre size thing....just intrigues as to V8 OHV saying the gearing on the 5.7 is already too high!!!!

[booster]

Cheers Steve. That's one cool calculator - Now if only I could get 236 in 6th gear Great fun.

All in all this tyre size thing seems to be a real black art and no-one seems to have tried it on a Monaro so it's probably best to leave well alone and simply put the stock 235's back on? Ho hum.




>> Edited by booster on Wednesday 24th August 10:35

[stevieturbo]

oops

www.geocities.com/z_design_studio/transmission_z28_6.html

>> Edited by stevieturbo on Wednesday 24th August 17:06