225/45/18 vs. 245/40/18
 

I used a tire calculator and it doesn't seem to be to big a difference on the actual speed, etc. How does this effect overall drive ratio? Acceleration? I am interested in going with the Pirelli PZero Nero MS and returning my BF KDW's. The tread wear rating is even higher on the Pirellis but I don't want to slow the car down, mess with mpg, etc. What about 235/40/18's. Can I get some feed back. How much wider are they? More hydroplaning? It rains a bit in Houston. I was using this calculator: http://www.miata.net/garage/tirecalc.html

One more question if I may. I know these are wider, but how do they look? I have seen some cars where someone has gone wider on the stock wheels and it just didn't look right.

A bunch of us have the Pirelli PZero Neros in 245/40/18. I think the extra width looks great. Someone did speedo checks with GPs and verified that that readings were not affected. I changed tires right after I bought my 8, but some that have driven appreciably on both the stock Potenzas and the Pirellis found no difference in mileage. Wider tires are known to have problems with skidding in snow and hydroplaning than narrower ones, but with the change you are considering, the width only increases by 9%. I'm not a physics expert, but unless potential for slippage is inversely related to the square of the surface area, this seems like s small change.

I suspect the 235/40s will make your speedo reading higher than actual by about 2%, I would guess.

I'm curious I had a hard time telling from the photos on the Pirelli M+S post but do the tires come out of the wheel well at all? If they stick out a bit they might start throwing rocks and such and mar the paint.

Hadn't noticed. I think a few have posted pics of them on the car from the rear. Check out that dedicated thread, which is now five pages.

I just went to 225/40/18's and I love the look. The tires look much more aggressive, and there is no appreciable overhang over the wheel. If anything, you may get a little protection with the overhang to protect your wheel.

This is a good tool for checking tire size change:

http://autocrossing.com/cgi-bin/tiresize.cgi

btw, You lose about 1% going to the 245-40-18.

My dealer is refusing to put the Pirelli pZero Nero M+S 245/40/18s on my car (stock wheels). They are telling me that the extra width makes the tire a risk for a blowout. Just spoke with Connor at Tire Rack who seems as mystified as we are. I am so pissed off about this I can't even tell you. Argh.

I can't be the only person running into this problem, can I?

i just ordered myself some goodyear eagle f1 gsd3s in 235 40 18. I talked with both my tire guy and connor. My tire guy said that the ride will get marrginally rougher but grip will be increased immensely, he said that the difference in sidewall size was .3 inches and not too much. The extra width isnt enough to rub on both sides. Connor said that as long as i keep the psi above 36 i should be fine with the tires.

so im feeling good about the purchase and cant wait for the performance.

elara id just bring the tires to an aftermarket tire shop and have them install them, like a tires plus or a goodyear should be fine.

I've been riding on the 245/40X18 PZero M&S for 4 months now. There have been no problems. The tires stay in the fender well with the stock wheels. I also have been to 100+ with no handling issues. Dispite what you may have earlier read, the squishy side wall feel dosn't go away. I bought the Pirelli's to get me through the winter. The Bridgestones only have 8k miles on them and will soon be going back on the car.

Had a friend's shop do it and all is well. I've learned my lesson- never go to Mazda for anything except free maintainence and warranty work.

Have any of you guys autocrossed with the 245/40-18s on the stock rims? I was checking into the Azenis RT-615 in both the 245 and 235 widths and a couple of guys at the track mentioned if you get too wide the handling can get squishy and unresponsive..

I just ordered 245/40/18's (originally had 225/45/18's) for my 8 and the salesman did a calculation on some website and said it would increase efficiency by 1%.

I know he might be lying, but I had made my mind up and I wasnt going to change it.

Frankly 20mm wider and 3mm shorter shouldnt make much difference.

ok, we are discussing 225/45-r18 vs 245/40-r18
first off is total tire height.

w=width
s=side wall ratio (the /45 or /40 or /whatever your tire is as a decimal or percentage, ie /45 =.45 on the calculator)
d=wheel size

(w*s)*2
---------- + d
25.4

this may be confusing, but it is

225 *.45 = 101.25 (this number is the height of 1 sidewall in millimeters)

101.25 * 2 = 202.5 ( this number is the total height of both sidewalls in millimeters)

202.5/25.4 = 7.972 (this number is the height of the sidewall in inches by dividing height of the sidewall in millimeters divided by 25.4 which is the number of millimeters in an inch)

7.972 + 18 = 25.972 overall diamter for stock tires.

using the same formula for the 245's we come up with 25.716 inches in diameter.

going with the 245s, though a wider tire, will be just a little bit shorter, meaning that your speedometer will read just slightly more than the actual speed you are traveling.

the difference in size is something less than 1% in total diameter.

what this means in real world terms is, slightly better acceleration, but a slightly lower actual top speed (not referenced from your spedo, but from a radar gun) as you car will still read x mph for y rotations of tires.

I tried to get to my user page, but the site appears to be bogging down. :mad: Tomorrow I will send a link with pictures when I also asked how the 245/40/18 tires looked insalled.

So you are saying that my speedometer will read 125mph when I might actually be doing 124mph?

If so this means my car will shut off at 124mph instead of 125mph. lol

stupid governor *sigh*

basically yes, the spedometer reading is taken from the transmission, usually from the output shaft on the transmission, and what the speedo does is compute x number of revolutions on the output shaft equals y speed.

because the tires are "slightly" shorter, that means that during the course of one revolution of the tire, the car wont have traveled as far. so the car thinks it going the same speed based on the revolutions of the driveshaft, not the actual distance traveled as would be referenced by an outside measurement (ie radar gun)

the difference in the 2 tire sizes is really negligible, so nothing to be really worried about. only catch is that going with the wider tires (245 vs 225) the tire is almost 1 inch wider (actually about 4/5ths of an inch) so putting them on the stock rim may cause problems as i believe it is only 8 inches (i could be wrong) a 225 tire is approx 8.85 inches wide, whereas a 245 tire is 9.64.

the wider tire may not seat correctly on the wheel and does present the possibility of "blowing out" because it doesnt fit optimally, especially considering that its a low profile tire.

the shorter the profile in relation to the width, the closer the width of the tire should match the width of the wheel. on larger tires this is not so much of an issue (larger profile) as there is more flexibility in the tire to absorb any impacts.

The sugested wheel width for the 245/40WR18 Pirelli in question is 8-9.5". Tread width is 8.5 and section width is 9.6 when mounted on a 8.5 inch wide wheel. They will fit fine.

Here is the link with pictures of the 245s

https://www.rx8club.com/showthread.p...0&page=3&pp=15

just as further food for thought, here is the number of rotations of your tire you need to travel one mile. keep in mind that these are approximate numbers due to differences between tires, not only from different manufacturers but even from same manufacturer due to slight differences in producing the tires as well as my calculator would only go to 28 decimal places................ :) :) :)

225/45-18 is approximately 776.5426922384597863526148104448 revolutions per mile

245/40-18 is approximately 784.27007665295762562281076309012 revolutions per mile

this results in a net difference in 7.7273844144978392701959526453 revolutions per mile.

edit: i made a mistake with my math earlier. hehe