Old vs new tyres

[arcosine]

My worn out tires are not good in the rain, but they won the low speed coast down test. I guess that would apply to bald insight tires compared to new H rated tires also.

[litesong]

Originally Posted by redpoint5

On a test car fitted with 205/55R16 tires, a 100 mile test showed a 1.5% difference in odometer distance with worn tires when compared with new tires.

An old thread, but I had to affirm the truth of redpoint5's post. Often, my tires show an odometer error of plus 3%, which I use in my MPG calculations. As the tire wears, I have to reduce the error to 2% & sometimes to 1%.

[litesong]

Originally Posted by ecomodded

I remember seeing a chart of the stopping comparison many years ago and it stuck, I cannot find that particular article or chart but did come across the video companion to the link that " redpoint5" has posted, from the tire rack.

I am actually surprised that a worn tire performs better on dry roads. I always assumed the rubber compound in road tires would not grip as the race slicks do.

Here's ecomodded's post, well worth repeating about worn tire braking distances on wet roads.

[MeteorGray]

I changed out a set of worn Yokohama Avids for a new set of Bridgestone Ecopias a few months ago and was surprised that the new tires provided at least as good mileage as the worn set.

It was probably due to the Ecopias having a better rolling resistance factor, even though they had 10/32" treads to squirm around on compared with less than half that on the old Avids.

There was one other possible influencing factor: the old tires were 20560R16 and the new ones are 20565R16. So the new tires are several percentage points taller, which made it necessary to adjust my odometer readings because the odometer became measurably "off" by the added territory each tire revolution covered.

It is also possible the new taller size will yield a MPG advantage due to a lower engine RPM count per mile. I might be able to see this with time, although the difference may be too small to claim.

[litesong]

Originally Posted by MeteorGray

.... the old tires were 20560R16 and the new ones are 20565R16. So the new tires are several percentage points taller, which made it necessary to adjust my odometer readings....the new taller size will yield a MPG advantage due to a lower engine RPM count per mile.

Since I began driving with used taller & wider tires, I get better MPG, despite the tires having wider tread. It is wonderful that MPG goes up due to fewer rpms, despite heavier, wider tread. Of course, the wider tread & taller tires, now give my small Hyundai Accent, better cornering, a bigger feel on the road & makes it more highway viable. Some of my used tires are 8% to 9% taller & rpms have taken a big whack downward. Your tires appear to have 3% height difference. I might think you could even go with a taller tire yet, maybe another 4%.

One disadvantage of my tires, tho: My tires are too tall now, to give snow chains clearance, & keep them away from mudflaps & wheel wells. For regions needing winter snow chains, smaller tires need to be obtained.

One safety factor for driving used tires: I change my used tires with more tread than I changed my "new" tires, because used tires are 4 times(more?) cheaper than new tires.

[MeteorGray]

Yes, the taller tires do reduce engine RPM, which reduces internal friction by reducing how many times the revolving parts have to turn over for each mile. Up to a point, this reduced RPM factor will increase MPGs, especially on the highway.

However, a point of diminishing or reversed returns can be reached by causing the engine to labor too much if / when the tires (effectively, the gearing) become too tall to handle. Only by trial-and-error can one find that "cry uncle" point, as each car is operated in different environments and circumstances: weight, wind, hills, etc.

Another thing: the clearances for the taller tires have to be watched. It is possible that even though a car can benefit from a taller tire based on the factors above, it may not have enough clearance in the wheel housing to take advantage of a taller tire. This also takes trial-and-error to find out. There is a point that the ever-larger tires start hitting the fenders from suspension flex, and a point that the front wheels can't make sharp turns because the tires are hitting things there.

I'm glad I got the larger "65" aspect tires. They seem to be justifying the switch for me.

[Piotrsko]

It's really not all that hard to make a cardboard profile tire to check clearance. Get a rim that has proper offset, cut a chunk of cardboard that fits down in the rim 90 degrees to direstion of rotation. Where it hits on the body, take a measurement. That is the biggest tire you can fit. Make sure the axle is supported in the proper configuration. You should probably subtract a couple of inches for suspension travel, but I have seen people who constantly scuff their tires.

[litesong]

Originally Posted by litesong

Some of my used tires are 8% to 9% taller & rpms have taken a big whack downward.

Originally Posted by MeteorGray

.... a point of diminishing.....if... the tires.... become too tall......

Many older small cars have their engine rpms way too high, at highway speeds. Taller tires will never reach "dimiinishing" returns. On my motorcycle once, I changed the "sprocket/counter sprocket set" ratios by 15%. My MPG went up 10% into the 75+MPG region. Yet, I was able to accelerate past the designed "top speed" of the cycle, well into triple digits. I think the cycle would have continued to accelerate, till it destroyed itself...... & me, with it.

[litesong]

Originally Posted by litesong

.....I began driving with used taller & wider tires..... It is wonderful that MPG goes up due to fewer rpms.... a bigger feel on the road.....

AND the "bigger feel" really does work, which I mentioned somewhere else. With the Hyundai Accent, I traveled to eastern Washington from western Washington for a sweet 600 mile day trip. Normally I use the bigger Elantra, when driving longer faster distances. But the bigger tires was really nice, gobbling up 600 non-fatiguing miles. Just really sweet.