Old Tires, Higher RR?

[Bill in Houston]

I am currently driving on my 3.5 year old OEM tires with 40000 miles on them. They are not down to the wear bars, but are lower than I like to go. I have been putting off getting new tires because I am concerned that new tires will make my gas mileage drop due to the greater tread depth of new tires. BUT, I am wondering if it is possible/ probable/ likely that somehow the old tires have gotten stiffer as they have aged, maybe causing their RR to be higher than when they were new. Anyone think that it is possible/ probable/ likely?

[bbgobie]

Originally Posted by Bill in Houston

I am currently driving on my 3.5 year old OEM tires with 40000 miles on them. They are not down to the wear bars, but are lower than I like to go. I have been putting off getting new tires because I am concerned that new tires will make my gas mileage drop due to the greater tread depth of new tires. BUT, I am wondering if it is possible/ probable/ likely that somehow the old tires have gotten stiffer as they have aged, maybe causing their RR to be higher than when they were new. Anyone think that it is possible/ probable/ likely?

2 other possibilities to think about:
1. Your worn tires have lost rotational mass because you've worn them donw.
2. Its very possible new tires would be more efficient and lighter with regards to FE.

I would replace the tires if they need to be replaced. Unless they are say for example super sticky high performance tires, and you don't drive like that.

[Gary Palmer]

I don't think that you are going to see enough of a drop in mileage, by putting on new tires, to be worth fiddling with. If you'd feel more comfortable with new tires and greater tread depth, then I would suggest getting them. It's worth the peace of mind.

[rvanengen]

Takes a lot of MPG to pay the insurance deductible...and I am not sure that any amount of extra MPG is worth your life or health. Get new tires (sensible) before you hit a good puddle!

[zpiloto]

Originally Posted by Bill in Houston

I am currently driving on my 3.5 year old OEM tires with 40000 miles on them. They are not down to the wear bars, but are lower than I like to go. I have been putting off getting new tires because I am concerned that new tires will make my gas mileage drop due to the greater tread depth of new tires. BUT, I am wondering if it is possible/ probable/ likely that somehow the old tires have gotten stiffer as they have aged, maybe causing their RR to be higher than when they were new. Anyone think that it is possible/ probable/ likely?

Just get LRR tires. Lots of tread, high 51 psi max rating and the RR would have to be better then 4 year old technology even if they are wore out.

[Bill in Houston]

But Randall, I don't want to drag the Leftover Bananas down. :-)

Okay, you guys convinced me. I'll do it within the next few weeks... Thanks for helping me do the right thing. Good tires are the cheapest insurance you can buy.

The Long Trail TAs are supposed to be low RR according to Greenseal way back when. Maybe they will be a noticeable improvement... Hey, I can dream, can't I?

[thisisntjared]

i think as tires get older the contact patch shrinks. as that happens there is less friction.

[MetroMPG]

The Prius comments I've read suggest that people notice replacement LRR tires of the same brand/size etc. have more RR than the old tires. But they also say the tires break in, and get better within a few 1000 miles.

[slurp812]

I had some old really crappy worn out performance tires on my car when I first got it. They were shot, like crayons slippery as a banana. I got new all season fairly cheap tires to replace them, and my MPG dropped about 2 mpg or so. I didn't calculate as accurately as I do now, but it was noticeably less miles per tank. Of course mine was almost a worst case scenario. So YMMV!

[Bill in Houston]

Originally Posted by MetroMPG

But they also say the tires break in, and get better within a few 1000 miles.

Thanks for trying to cheer me up. :-)

[Hockey4mnhs]

yup i think im gonna invest in lrr tires soon

[thisisntjared]

Originally Posted by theclencher

Contact patch is largely a function of weight vs. tire air pressure, so, I doubt it.

and the elasticity of the treads has nothing to do with contact patch? no matter what compound, the lower the tread, the less elastic, right?

[Bill in Houston]

pretty sure clench is right. it would only change in extreme cases where the sidewall starts to support a lot of weight, usually possible only in a verrrrry low profile tire or a run-flat.

[caprice]

I replaced bald tires with a set of 2 year old hardened stock tires from a diesel pick up and I got worse braking traction. Also had other experiences with using older tires. I test drove a Mitsubishi eclipse with dry rotted back tires that made it want to slide out on turns. Usably the old tires had little visible cracks too.

[Bill in Houston]

Oh, yeah, the traction is DEFINITELY going away. Time for new shoes...

[brucepick]

One thing with new tires is the diameter is larger due to tread not worn away.
So actual distance covered per rotation is more - but your speedometer doesn't know this.

The difference vs. a half-worn tire isn't that much - but vs. a worn-away tire the difference is more. If you make speedometer stay at same speed you used to (with worn tires) the engine is actually working harder because it's driving the car faster. But the odometer shows slightly less miles.

If you stay off the gas a bit you likely will get better ACTUAL mpg but it won't show on the odometer. Like having a slightly taller final gear ratio.

[Bill in Houston]

Originally Posted by brucepick

Like having a slightly taller final gear ratio.

I can use all of that that I can get. :-)