I have an ‘08 Sienna that had started slipping in wet pavement when I was starting on a hill. My tires were pretty worn so I replaced them about 6-9 months ago. Recently it has begun losing traction when I start on a hill but on dry pavement. Tire tread is good as new so I can’t figure out what would cause this. The tires slip and the traction control comes on. Any ideas? I have Cooper Tires, nothing fancy because my Sienna eats them up whether they are top of the line or bottom.
Brake caliper/shoes hanging up?
Parking brake not fully released?
CSA
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Have you checked on a site like Tirerack.com, to see how those tires are rated for traction?
Have you checked the date codes on the tire sidewalls, to see how old they are?
(Hint: Some unscrupulous tire shops will unload their old stock on unsuspecting customers.
The rubber compound of tires hardens as they age, and as a result, old tires can have much less traction than new ones.)
This might help you:
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=11
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Tires sold in hot climates are harder than tires sold in cold climate areas. You may need to make sure you have the right application of tires.
On dry pavement, tread makes very little difference in traction. Tread is to channel water away, not grip the road. The rubber itself is for traction, wet or dry. Notice racing tires have no tread unless they are designed for rain.
Take your fingernail and try and make an indent into tread rubber. If it is hard to make any mark at all, then there is your problem. This is an over-simplification, but cheap tires advertised as “high mileage” are made with harder rubber that gives poor dry traction. Old stocks of brand-new tires will have rubber that has hardened, so traction will also be poor. Cheap, old tires are a double whammy, double hard and super-poor dry traction. I’d also bet these tires are horrible (unsafe) in the rain.
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Or they’re mid-2000’s Formula 1 tires because F1 likes to make weird, arbitrary rules. 
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I’d check how old the tires are, and that they are inflated to the right pressure (not the pressure shown on the tire sidewall).
+1… THAT, my friend, is the understatement of the day… and maybe the week!
Rivals the rules that LeMans race cars be required to carry a spare tire and have a luggage compartment for an FIA “spec” suitcase!
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I recently put air in the tires because my pressure light came on. I used an automatic air pump (where you set the pressure and it fills it until it reaches that pressure) and filled to the pressure on the door sticker. How do I tell the age of the tires themselves?
There will be a 4 digit number on the sidewall . Such as 0417 - the first 2 numbers indicate the week of the year fourth week (04 ) the second the year made ( 17 ) 2017.
Have you talked to the place you bought tires from ? Maybe they can sell you a different type for a decent price adjustment. You can also go to the Cooper web site and read the specs on the tires you have.
Here’s how to tell tire age:
https://www.tirebuyer.com/education/how-to-determine-the-age-of-your-tires
Also, buy a decent tire gauge and check them in the morning, before you drive, to make sure the pressures are correct.
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By following the link that I provided in the second response to your original post!
+1
If these tires have very poor traction on a dry surface, they must be absolutely hazardous on a wet surface.
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Yes, well… we don’t talk about that anymore! 
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Isn’t the Sienna AWD? Is there any possibility that system is having some issues versus a tire problem?
No mine is FwD. what could it be if not a tire issue?
I haven’t had a problem on wet pavement since I changed the tires 6 mo ago which is why it’s weird. Only just started happening on dry pavement and only when starting up on a hill.
I would suggest looking into CSA’s suggestions above; sticking brake caliper or parking brake partially engaged. Ruling those out first would be high on my list…
Most minivans have this issue, the combination of a relatively powerful engine, front wheel drive, and an overall heavy vehicle all make for traction issues at takeoff. Tires with higher mileage ratings are generally more prone to this issue, as the rubber is generally harder to reduce tread wear.
I would have to disagree on that. I’ve never had a traction problem with my FWD Dodge Caravan, dry roads or wet roads, hot roads or cold roads.
CSA
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