Width or height of the tire that gives more grief?

[Kumba]

Looking at putting some new tires on my car and am wondering if it's the width or height of the tire the tends to be more problematic.

The original 235/55r17's on my car had the following specs:
26.25" Tall
9.25" Wide

I'm looking to go to a 255/35r18's which would be the following specs:
25.03" Tall
10.04" Wide


So the tire would be wider by 3/4" or so, but around 1.25" shorter.

I am leaning towards the height being more critical then width but figured I would ask everyone who's put big rims on their car what they found.

 

[FDR]

I thought 17s had 50-series tires. However, 235*0.5*2+17 is 26.25, so I assume it was just a typo.

The closest tire size I found for the specs I remember for jag 18s (18x8 et42) is a 255/40/r18. That gives 0.85% difference, with the speedometer reading 60.51mph at an actual 60. Tire rack has 70 results for that size.

Not the most popular size, though, so I'd like to hear what others have to say since I should be getting a set of 18" Tritons some time this winter if the guy ever makes it down from his new residence...

 

[LS4me]

I thought 17s had 50-series tires. However, 235*0.5*2+17 is 26.25, so I assume it was just a typo.

The closest tire size I found for the specs I remember for jag 18s (18x8 et42) is a 255/40/r18. That gives 0.85% difference, with the speedometer reading 60.51mph at an actual 60. Tire rack has 70 results for that size.

Not the most popular size, though, so I'd like to hear what others have to say since I should be getting a set of 18" Tritons some time this winter if the guy ever makes it down from his new residence...

You are correct. Factory size is 235/50/17.

I'm on my second set of 255/40/18 on Jaguar Tucana wheels. No issues at all either at factory or Eibach height.

 

[Kumba]

I'm on my second set of 255/40/18 on Jaguar Tucana wheels. No issues at all either at factory or Eibach height.

Well then I should have no problems with a shorter sidewall then. Thanks for the input

 

[FDR]

So you're still going with 35s? Why?

 

[Telco]

Each has its own problems. Height messes with the computer. Width messes with traction. Thing on snow is, I don't know if a narrower tire is better since it concentrates the weight, or a wider tire since it has more tread to grab snow. I do know that too wide and the tire will balloon up in the middle, effectively changing the tire height and causing wear problems.

The proposed tire height will make your speedo read 5 percent faster than your actual speed, so take that into account on the road and when calculating mileage. Multiply your miles traveled by 0.95 to get the correct miles traveled before dividing by gallons used.

 

[02LincLS]

I'm not sure what issues the OP is referring to, but generally speaking, lowering sidewall height (aspect ratio) may allow more clearance for additional width. How much depends on wheel well dimensions.

Each has its own problems. Height messes with the computer. Width messes with traction. Thing on snow is, I don't know if a narrower tire is better since it concentrates the weight, or a wider tire since it has more tread to grab snow. I do know that too wide and the tire will balloon up in the middle, effectively changing the tire height and causing wear problems.

You want a narrower tire in snow, increasing pressure on the contact patch.
Changing sidewall height will not mess with the computer or advancetrac or traction control as long as all 4 wheels have the same OD or very close. It has to allow some percentage of difference in wheel speed since rear tires wear faster than fronts.
As long as your wheel width falls within a specific brand tire's recommended width (shown on tire rack), you should have no issues with funny wear. Inflation is a bigger factor.

stock is 235-50-17. My car is lowered about 1" (Eibachs)...
My summers are 245-40-18 (Conti Extreme contact DW) in front (8" wide wheels) and 285-35-18 in back (9.5" wide wheels). I wanted 255-35 in front, but OD is extremely closely matched to back when its 245 so no "computer" issues or silly looking un-level stance. Zero rubbing in fenders.
My winters are 225-55-17 (Conti Extreme winter contact). Under the rare occasion of full suspension travel, they barely rub the plastic splash guards/wheel well inserts with 3+ people in the car- and only in the rear. The only reason I went higher than stock on aspect ratio is that the tire size was MUCH cheaper. I suppose there is additional ground clearance which could be beneficial if the snow is high.

 

[Kumba]

So you're still going with 35s? Why?

A couple reasons:

1) I live in Florida, no snow just rain/water
2) No advancetrac, just traction control and ABS
3) Lowers the center of gravity of the car outside of the suspension
4) Gives you the same effect as going from a 3.58 to 3.73 gears
5) Shorter sidewall is usually stiffer with less roll-over
6) Stiffer sidewall means lower PSI and a bigger contact patch

TLDR: I'm doing this for handling and performance reasons. I understand that the ABS program will not be completely correct and that the speedo will be off.

I would be more worried about the tire size if I had AdvanceTrac instead of just ABS.

 

[FDR]

If you have a chance of sinking through to the pavement, a narrower tire will give you a better chance of the treads cutting through to the underlying surface. If the snow is deep enough that you're definitely floating on it, width won't really make a difference. I'd think I'd also want fewer treads that dig deeper rather than more treads that don't dig is deep in the case of "definitely floating". My area rarely sees unsalted snow, so I'd lean towards thinner tires to cut the slush. The plan for me is to run down my 235 all seasons this winter, mount 255s to the 18s in spring, then mount 225 winters to the stock 17s by this time next year.

But that's just all my preference. I was just curious why Kumba still wants shorter sidewalls (assuming he meant shorter than 40s, as opposed to shorter than the stock 50s).

Anyway, here's some tire sizes within 2% of factory 235/50/r17. Negative percentages mean your speedometer will read low and your recorded mileage will be lower than your traveled mileage. Positive is opposite.

215/55/r17 (-0.22%)
225/50 OR 55/r17 (1.5 and -1.8%)
245/50/r17 (-1.5%)
255/45/r17 (0.8%)

215/50/r18 (-0.8%)
225/45/r18 (1.1%)
235/45/r18 (0.3%)
235/45/r18 (-1.61)
255/40/r18 (0.85%)

 

[Telco]

I'm not sure what issues the OP is referring to, but generally speaking, lowering sidewall height (aspect ratio) may allow more clearance for additional width. How much depends on wheel well dimensions.



You want a narrower tire in snow, increasing pressure on the contact patch.
Changing sidewall height will not mess with the computer or advancetrac or traction control as long as all 4 wheels have the same OD or very close. It has to allow some percentage of difference in wheel speed since rear tires wear faster than fronts.
As long as your wheel width falls within a specific brand tire's recommended width (shown on tire rack), you should have no issues with funny wear. Inflation is a bigger factor.

stock is 235-50-17. My car is lowered about 1" (Eibachs)...
My summers are 245-40-18 (Conti Extreme contact DW) in front (8" wide wheels) and 285-35-18 in back (9.5" wide wheels). I wanted 255-35 in front, but OD is extremely closely matched to back when its 245 so no "computer" issues or silly looking un-level stance. Zero rubbing in fenders.
My winters are 225-55-17 (Conti Extreme winter contact). Under the rare occasion of full suspension travel, they barely rub the plastic splash guards/wheel well inserts with 3+ people in the car- and only in the rear. The only reason I went higher than stock on aspect ratio is that the tire size was MUCH cheaper. I suppose there is additional ground clearance which could be beneficial if the snow is high.

Thanks on the snow aspect, but incorrect on the tire height. True, as long as they are all the same diameter the ABS and traction control will work correctly, because they don't know anything more than RPM per wheel and those systems operate based off wheel RPM.

The speedometer, on the other hand, does matter although the speedo doesn't care. The computer, or the speedometer on cars that don't use the computer to display the speed, is set to measure speed by driveshaft RPM. On a computer, it multiplies the RPM of the driveshaft by the number of inches it was told the tire is in diameter (which is tire height times 3.1416) and displays the speed. If the actual tire height is not the same that the computer thinks it is, then the computer does not know this and will continue to calculate the speed based off the number of inches it thinks the tire is traveling per revolution. If the tire is taller, then the speedo will display a lower than actual speed which means you are going faster than the speedo shows. If the tire is shorter, it will display a faster than actual speed which means you are going slower than the speedo shows. Not taking this into account also affects any MPG calculations you make as you will travel miles further or less. A 10 percent difference in height can mean a 30 mile or more difference in actual miles traveled.

If you tell the computer what the new inches per revolution traveled is, then you can correct the speedometer to read the actual speed with the different tires. This doesn't just affect the speedometer though, it will also affect shift points according to speed. I don't know if the LS computer shifts according to speed, but GM computers do. That handheld programmer you can get for the LS should allow you to correct this. It would be worth it as, say you're running 80 in a 75MPH zone on the interstate with 10 percent larger tires and pass a cop. He will clock you running about 88MPH in a 75. There's a couple hundred bucks in most jurisdictions.

It may also affect airbag operation, as in newer cars with the second generation airbag systems the system takes a lot more into account than just an impact sensor. I'm pretty sure (but not 100 percent) that vehicle speed is a factor since vehicle speed would help the computer determine how much impact there will be.