Question about Prius handling?

That is just how the GenII and to a lesser extent the GenIII are. Adding air pressure to the tires helps with more responsive steering and the car will feel tighter. The other option is to use a wider set of tires. Your mpg may drop a bit if you go wider but with a good set of LRR tires this shouldn't be too big of any issue. The car will handle better and be safer overall.

Options:

195/60/15 (a tiny bit smaller than OE tire)
..................
Ecopia EP422
AVID Ascend T or H
Goodyear Fuel Max
ProContact with EcoPlus

205/60/15 (a tiny bit larger than OE tire)
Goodyear Fuel Max
ProContact with EcoPlus

Or you can go with the GenIII OE 195/65/15 size which is larger but offers a ton of great tire selections! You're speedo will read accurate or maybe a tad bit slow. It currently reads about 2mph too fast.

 

As F8L said going wider will improve handling enormously. In the UK the gen2 Prius comes with 195/55/16 tires, I have gone to 205/55/16 and found this improvement, and with careful choice of tire type I have actually gained 2mpg over the OEM tire. Now running at 65mpg, (UK gallons) making 600mile plus tanks the norm.

Tire pressures 40/38psi.

 

Exactly what F8l said.

If you check the tire pressure it will be low. The steering on this car is so tight it takes constant correction to stay in the lane especially if your running the god awful worst tire ever made the Good Year Integrity that came stock with the car. Running low tire pressure on those exacerbates this dithering or constant attention you must give the steering. And Integrity's constantly lose tire pressure especially if there's a cold spell.

Unlike any other car where you can just relax with your hand lightly on the wheel and just roll.
Not this car. You have to pay attention and steer. You really notice it if you have another car and bounce between the 2. If I just drive my Prius straight for a week or so I forget about it and just deal with it. Its still pretty annoying.

Inflating the tires helps eliminate that dithering a lot. I run 44 front 42 rear. Your mileage will greatly improve and steering will be better but the car will be somewhat harsh over bumps and hit somewhat hard. Tradeoffs with our little car that's for sure.

Good Luck.

 

Absolutely agree with F8L and edthefox5 about OP comments. Keeping the air pressure at max 42 Front 40 Rear will help with the control of the Prius on the highway. If you drive just the Prius everyday you will get used to it, but if you go from one vehicle to another you will notice the "wander" of the car. I too believe it is due to the narrow tires on the vehicle. The stock Goodyear Integrity's do have a rather narrow footprint, and a slightly wider tire will help with the overall control. Just be conservative with your choice of sizes listed in the above posts, and go with a LRR tire. Ecopia 422 and Michelin Fuel Saver A/S are good alternatives.

Ron (dorunron)

 

You might want to have someone check your alignment and tires as this is not something that I have noticed in my 09 Prius, and prior to the Prius I had a VW Passat and my wife drove a BMW 528 - both were cars that handled very well especially on the highway.

 

Pressure only helps to a certain extent - there is definitely a sweet spot. At 38/36, I find that most Gen II and Gen III Prii drive noticeably better than the 35/33. However, once you go up to 42/40 or 44/42, the car becomes very twitchy and low-speed braking is noticeably inferior.

 

The Gen II is notoriously sensitive to wheel alignment and there have been many threads here on this subject.
If rear-wheel alignment is out of spec., inexpensive shims can correct. My '04 as-delivered was very scary at highway speeds in windy conditions. A front wheel alignment and an $8.00 shim at the right rear changed the driving experience from two hands required to two fingers. A $50 chassis brace helped a bit as well.

 

Glad that solved your problem, LadyJ. It was definitely the cheapest option. Juat remember, the Harmony tires are not the best for fuel economy despite being a good tire otherwise.

 

I never associated the two, but I also notice that the higher tire pressures do help on the highway. interesting, I wonder why that is. I run 42/40.

 

Higher pressure helps stiffen the tire sidewalls so they resist deflection. This makes the tire less mushy feeling and easier to control. The 185/65/15 tire on the GenII is a bit undersized for the weight and suspension dynamics of the GenII so that at 35/33psi the tire is just plain mushy!

In high speed applications is is recommend to raise tire pressure above placard levels for similar reasons. Tire Tech Information - Air Pressure/Load Adjustment for High Speed Driving

 

I first noticed this on our '04 the first time I rode in our then new Prius while a friend who was used to driving a 30 foot motor home was driving it. He was constantly turning the wheel like he had to in the motor home.He was constantly turning , correcting back and forth. His driving was very irritating. The Prius steering, on the '04 and the '06 we now have is very tight and no correcting is needed at all. If the driver is constantly correcting, the car will move around like something is wrong...it isn't. Just hold your hands still on the wheel you will notice that no correction is needed. If you must constantly correct like you are used to doing on other cars, then slide your hands on the wheel and let the car go straight like it was designed to do. It's not the car, it's you. Check it out.

 

The Gen 2 has a thin strap of metal bridging the 'hump' under the floor. Many have found that this does nothing in compression, it only works in tension.
Replacing it with an aluminum bar makes the chassis stiffer in compression. (side winds, truck aero-effects)

prius g7 | eBay

 

In most cases of high speed instability! I would suspect alignment! Toe and thrust angle! If toe different between left & right is off from each other causing one side to push left or right the higher the speed the more steering correction is needed! Since rear torsion bar are non adjustable this creates a issue! Example if your left rear toe is - 1 degress and left is + 28 degress within factory spec of 52 total but the rear is push to the left also consider the reading changes as the torsion bar is twisting going down the road. Getting the toe left & right reading to be the same is critical. Getting a well season experience aligment mechanic maybe able to resolve these issues. I don't recommend using those plastic shimming system! There no plastic parts on any suspension on any vehicles I ever seen! Custom making shims out of metal ( steel) not copper nor brass they are soft metal. .008 will change toe around 12- 18 degress. On the rear!