I shoulda listened to Gabe.

Pentalab

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Recently I removed my front 275-40-18 NITTO NT-555's (on 18 x 10 rims), and temp stored em away.
The rears were 285-40-18's (also on 18 x 10 rims). I moved the rears to the front. Front is good.

Then bought new 305-35-19 MPSS's...and mounted em to new 19 x 10 rims..... and installed em on the rears.

The section width of the MPSS 305-35-19 ( when used on a wider 11" rim) is listed as 12.31".
Section width is actual side wall to side wall dimension. 12.31" x 25.4mm = 312.75mm... IF used on a 11" wide rim. On a smaller 10" wide rim, the section width drops to aprx 302.75mm.

Actual tread width for the MPSS 305-35-19 is listed as 10.7". Well, on a smaller 10" wide rim, it's no where near 10.7". It's barely 9" !! And that's with aprx 900 lbs on each rear..and also with just 30 psi. (this is the actual contact patch width on the ground).

(a 275-40-18 on a 18 x10 rim is 9.75". And a 285-40-18, also on a 18 x 10 rim... is almost 10"..that's contact patch on the ground.

I shoulda listened to Gabe. Rears stick..."ok", being they are brand new MPSS, and just 30 psi.

Moral of this failed experiment is to use at least the...'measuring width' the tire maker lists..and preferably a 1/2" wider than that.

You woulda thought I had learned from the last fiasco... using a 275-40-18 on a 9" rim...(these were on the fronts, 2 summer's ago). Sluggish, and un-responsive, underwhelmed to say the least. The 275-40-18's were swapped onto wider 10" wide rims..then all was well, worked superb on the front, being stretched 1/2". (I had gone from 1/2" under... 'measuring width'.. to 1/2" over measuring width.) I should listened to Norm on that one.

Bare min for a 305-35-19, IMO, is 11". Even a 10.5" wide rim is a wasted effort.
On a 10" wide rim, they are a full 1" narrower than measuring width. The MPSS rolls off on each side, at an angle. This gets accentuated even more when using a narrower rim. I have zero sidewall bulge, tires look good, rims look good, fits good..with it's 48mm offset, but just not enough actual rubber on the road. Another high level experiment gone amok....more Fubar on my part.
 

Gabe

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Lol ... I can only advise, can't MAKE people do the "right thing".

I'm still running "wrong-fitment" wheels/tires myself (at least my current AMR's are 10" wide, not 9.5 like the GT500 wheels).
A lot of my advise is from failed experiments of my own.
Like the 305/35/20 tires I ran on 20x10 rear wheels with that lovely 40mm offset that so many manufacturers are proud of.
Tires bulging and the offset being a few mm too aggressive: 2 things that came together and resulted in shitty tire rub.

What's even worse is seeing all the people with '15+ cars running 305/35/19's on their stock 19x9.5 rear wheels .... or Shelby guys running 305/35/20 on the stock 20x9.5 rears .... ugh.


So what's your plan?
 

Pentalab

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Plan C is....I'm gonna reduce the rear tire pressure a bit. It was originally 33 psi. Then I reduced it to 30 psi (which is when I took the measurements). Still zero sidewall bulge. I will try dropping it to 29 psi... then maybe 28 psi..then maybe 27 psi.

I have way too much $$ tied up in this fiasco. I already have the 4 x oem rims + 4 x new nitto Motivos' I use in fall / winter for rain use. Then 8 x chrome bullitt rims. Then pairs of 305/285/275 rubber.

IF I was doing this from scratch, the optimum setup would be 4 x 285-40-18's..... on 18 x 10 rims. Then I could rotate front to back. IF MPSS rubber was used, then you can also rotate side to side.

And, no, I'm not about to replace the 305-35-19's with 285-35-19's either. (then u end up with a 26.85" tall rear..and 99.75 mm sidewalls.

This latest gong show started with me almost putting it into the center concrete barrier on the local hwy..twice within 6 secs... last summer.
That was the result of me installing the 285-40-18 NITTO NT-555's reversed on the rear ! Those are directional tires. 2 x issues with reversed on the back. Water converges..instead of diverging... hydroplane city in the wet..dangerous as hell. Car pulls violently to the left when approaching the top end of 2nd gear with the blower on...also dangerous..and is more than an eye opener. Swapped the rears back to normal..and all was well again.

Then I get this idea in my head about 305's on the rear..for street use. I did not want drag radials..and that's all you can get in the typ 305-40-18. So plan B was either MPSS / NT-555-G2 in a 305-35-19...which requires a pair of new 19" rims. IF I coulda got new chrome bullitt rims in 19 x 10.5", I coulda lived with that.

I'm gonna reduce the rear pressure.... in about 10 mins...and see what happens.

If shelby / 15+ folks want to use 305's on a puny 9.5" wide rim... they have their head up their ass.
 

Pentalab

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Plan C is....I'm gonna reduce the rear tire pressure a bit. It was originally 33 psi. Then I reduced it to 30 psi (which is when I took the measurements). Still zero sidewall bulge. I will try dropping it to 29 psi... then maybe 28 psi..then maybe 27 psi.

I have way too much $$ tied up in this fiasco. I already have the 4 x oem rims + 4 x new nitto Motivos' I use in fall / winter for rain use. Then 8 x chrome bullitt rims. Then pairs of 305/285/275 rubber.

IF I was doing this from scratch, the optimum setup would be 4 x 285-40-18's..... on 18 x 10 rims. Then I could rotate front to back. IF MPSS rubber was used, then you can also rotate side to side.

And, no, I'm not about to replace the 305-35-19's with 285-35-19's either. (then u end up with a 26.85" tall rear..and 99.75 mm sidewalls.

This latest gong show started with me almost putting it into the center concrete barrier on the local hwy..twice within 6 secs... last summer.
That was the result of me installing the 285-40-18 NITTO NT-555's reversed on the rear ! Those are directional tires. 2 x issues with reversed on the back. Water converges..instead of diverging... hydroplane city in the wet..dangerous as hell. Car pulls violently to the left when approaching the top end of 2nd gear with the blower on...also dangerous..and is more than an eye opener. Swapped the rears back to normal..and all was well again.

Then I get this idea in my head about 305's on the rear..for street use. I did not want drag radials..and that's all you can get in the typ 305-40-18. So plan B was either MPSS / NT-555-G2 in a 305-35-19...which requires a pair of new 19" rims. IF I coulda got new chrome bullitt rims in 19 x 10.5", I coulda lived with that.

I'm gonna reduce the rear pressure.... in about 10 mins...and see what happens. The only other possible..'fix' I can envision is to widen the rear wheels to 11".

If shelby / 15+ folks want to use 305's on a puny 9.5" wide rim... they have their head up their ass.
 

46addict

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At least you didn't install directional tires backwards this time. But yes I tend to listen to Norm and Racer47 when it comes to tire advice.
 

Pentalab

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At least you didn't install directional tires backwards this time. But yes I tend to listen to Norm and Racer47 when it comes to tire advice.

I reduced the rear tire pressure from 30 psi..down to 29 psi. This appears to be sorta optimum. I still don't see the contact patch getting much wider, nor do I see the contact patch getting any longer. Dunno how much lower I can go with rear tire pressure. I'm gonna leave it at 29 psi for now, call it a day... then test it for a couple of weeks....then re-evaluate. 29 is def better than 33 psi. Good enough for now. Never again.
 

eighty6gt

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Like I said splitting hairs with tires won't help much. Maybe just switch to drag radials, should be many that fit that rim size . 285's.
 

Pentalab

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Like I said splitting hairs with tires won't help much. Maybe just switch to drag radials, should be many that fit that rim size . 285's.

The min rim width for a 285-40-18 drag radial is 10"...same deal, should be wider. However a 285-45-18 wants to see a 10" wide rim. But then the sidewalls are massive, and overall tire diameter is too big for my liking. Ends up being 28.01". Then the effective gearing becomes a little too high for my 3.31 rear gear.

It's fine at it's current 29 psi, still no tire bulge. Another option is to have em widened to 11".... but that starts the beginning of an expensive gong show.
 

eighty6gt

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Change rear gears... My 285/40R18 nt555 g2's seem to fit my 18x10's perfectly.
 

Gabe

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Change rear gears... My 285/40R18 nt555 g2's seem to fit my 18x10's perfectly.

While Nittos always run a bit narrow, 285/40/18 seems to be a great size on 18x10 wheels.
It's the sizes I'm currently running on the front of my '13 and the tires are Goodyear Eagle F1's, which I find to run a bit wide, but they still fit great on these wheels.
They could still fit even better (more square) on a 10.5" or 11" wheel, but 11" I think would be stretching the tire a bit, and I personally wouldn't like that look.

These are the Goodyears on the 18x10:

42897441211_6c272f4f8c_o.jpg


42849094492_06d6fe4a66_o.jpg



And a Nitto 555R 305/40/18 on the same wheel:

42849094182_641feaac6c_o.jpg



And showing that these two tires are almost identical width:

41997176445_26f71f46fc_o.jpg
 

Pentalab

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The problem with the MPSS in the 305-35-19 ..on a 19 x 10" wide rim is... it's only flat across the middle portion, then towards each outside edge, it drops off on a steep angular slope quick like. On a wider 19 x 11" rim, on the front, they would be good. That 10.7" tread width is sorta bs, the actual contact patch is way less. I can still see that effect when the MPSS rubber is off the rim.

I use Nitto NT-555 in a 285-40-18 on a 18x10 on the fronts..and they have loads of contact patch.

Nitto's work good, provided you don't install em reversed on the back..or front. Directional tires like the nitto, when installed swapped on the back don't work. It pulls violently to the left every time at the top end of 2nd gear.

Having said all that, I did a brief test with blower on in 2nd gear last night, and it stuck, throws you into the back seat, and stayed straight..... but I didn't have it wound out in 2nd gear. I found a place to test that effect...and will do a real test this week.

The nitto NT555- G2 is a good tire, and was my 1st choice, not the MPSS.

Gabe, on your very last pix, you can see the pronounced shoulder roll off on the nitto 305-40-18. My MPSS 305-35-19's have even more pronounced shoulder roll off. Bottom line is, they have LESS contact patch that the nitto 285-40-18's on the front..like quite a bit less.
And that's with 35 psi on the front..and only 29 psi on the rear.

I will try dropping rears down to 28 psi.

What these cars really need is a 10.5 to 11.5 on the rears...if using a 305 rear. A 285 on a 10.5 rim would be a winner. My quasi conclusion so far is... you want at least it's measuring width on the rear. Measuring width + 1/2" on the fronts works superb.

Rear gear is gonna stay put. It's a 3.31 But my 2nd and 3rd gear in the 5r55 auto tranny are 14/16.5% lower than 2nd /3rd gear in the manual. So my 2nd/3rd gear ends up the eq of a 3.73 /3.90 rear gear on a manual. It pulls like a mofo in 2-3-4.

IAT's and fuel temp was sky high last night. Way too hot out the last 2 days. This is the problem with running short range errands /coffee runs etc. Get back in..and it's all heat soaked pretty good. IAT was 140 F...and fuel temp was 132 F. Drops off slightly after a few blocks. Mash the gas and /or drop down some gears and IAT really drops a bunch. The air is going in so fast, it doesn't get as much of a chance to pick up heat. Get off the gas, and IAT shoots back up. Cops out in full force with the nice wx... which slows down my experiments.
 

RocketcarX

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Nitto tires run narrow for a given size, that's why they look so similar.
The 35 aspect ration makes the 305 effectively wider. The shorter the sidewall the wider the wheel needs to be for the same size tire that would normally "fit" in a taller aspect ratio.
 

Gabe

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The problem with the MPSS in the 305-35-19 ..on a 19 x 10" wide rim is... it's only flat across the middle portion, then towards each outside edge, it drops off on a steep angular slope quick like. On a wider 19 x 11" rim, on the front, they would be good. That 10.7" tread width is sorta bs, the actual contact patch is way less. I can still see that effect when the MPSS rubber is off the rim.

...

Gabe, on your very last pix, you can see the pronounced shoulder roll off on the nitto 305-40-18. My MPSS 305-35-19's have even more pronounced shoulder roll off.
...
.

The "shoulder roll off" is happening because the tire is mounted on a wheel too narrow for that size.
My 305/40/18's would prefer a 10.5-11" wide wheel.
Then the shoulder would be nice and square with no roll off.

Same with your 305/35/19 on the 10"-wide wheels ... You may not be noticing a bulge, but that roll off is basically the bulge
 

JJ427R

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The min rim width for a 285-40-18 drag radial is 10"...same deal, should be wider. However a 285-45-18 wants to see a 10" wide rim. But then the sidewalls are massive, and overall tire diameter is too big for my liking. Ends up being 28.01". Then the effective gearing becomes a little too high for my 3.31 rear gear.

It's fine at it's current 29 psi, still no tire bulge. Another option is to have em widened to 11".... but that starts the beginning of an expensive gong show.

I'm have a set of MT ET Streets 305/40/18 on 18x10 Roush wheels, love the stance and it fills the wheel well. I don't notice a big bulge, I run them at 30psi on the street 18 at the track.... 3.55 gear with automatic...
 
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ExSRT8Guy

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Jesus Christ. You are WAY overthinking this. "Bulge" doesn't mean shit. Go drive the thing and get the tires hot, pull over, and check temps across the width of the tread with an infrared thermometer.
 

RocketcarX

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I think a picture from the OP would clear up a of of the the "science" to this thead
 

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