Pls help me modify my tyre size

Although going to a larger size tyre will increase the ground clearance it will only increase it by half the size increase.
A 1 inch larger tyre will give you 1/2 inch extra ground clearance. This comes with other problems as the tyres may rub on the cars bodywork, and your speedo and oddo will under read.
Raising the suspension with spacers or fitting different springs would be a better solution to the problem. You do not state where you are located, country or area, so further help is difficult.

John (Britprius)

 

This is NOT a good idea.......for several reasons.
First tires are expensive.
Second, as mentioned, the bigger ones might rub and damage something.
Third, a different tire size might make the handling bad and the tires might wear out more quickly because of bad alignment.
And lastly, as was already mentioned, any tire size that will actually fit will NOT raise it enough to make any real difference.

You are new to cars but those people who are replying to your question here are NOT.
Ignoring their good advice is not a wise thing to do.

The easiest solution to your problem might be just to slow down.
Oh, and how important this problem is depends on what parts are hitting the pavement.
If it is only the rubber part under the front bumper you probably should not even worry about it.

 

As I said in my first post you could strike problems using tyre size to get ground clearance. I would think 205/70/15 is about as large as you could fit, but I cannot guarantee the tyre will clear the bodywork only trying them out will tell.
Going from the stock size 185/65/15 to 205/70/16 will increase the ground clearance by the figures below.

185/65/15 = diameter of 24.47 inches (621 mm)
205/70/15= diameter of 26.3 inches (668 mm)
this will raise the car .915 inches (23.5 mm)

John (Britprius)

 

I guess I shouldn't complain about DC potholes...

23mm of extra ground clearance ain't gonna help a prius much on unpaved roads. honestly, you need a small SUV if the roads are in terrible condition or are mostly unpaved. however, if you have a friend with a tire machine, you might test fit a used tire just to check clearances. also, rear shock replacement isn't too difficult, you might try rear coil overs or some type of aftermarket units which you might be able to get from japan. they have some cool stuff in japan for most cars.

 

I'm pretty sure the Prius uses struts, which are kind of similar to coil-over shocks. To add height, you'd want taller coils in the struts or taller strut assemblies. You have to be careful though. Without changing the geometry of the rest of the suspension, you can't change the strut ride-height very much without running into some problems. Reduced down-travel is one consequence. A suspension with insufficient down-travel is not a very good-performing system, especially on rough terrain. It also affects the wheel camber and tire contact patch negatively.

One of the most effective techniques of travel on unimproved surfaces is line-selection. It takes some practice, but you want to visually map the undercarriage of the vehicle, including the breakover (contours that approach the undercarriage between the front and rear wheels), approach (clearances and contours under the front overhang) and departure (under the rear overhang). As you drive, you want to visually map the contours of the road surface (low and high spots, ruts, camber (side-angle), syncline and anticline (valleys and hills) and how the vehicle will interact with them (points of suspension extension/compression, articulation, inertia, etc).

The main rules are to place the tires on the higher areas of the road surface, cross hills and gullies at an angle, not perpendicularly, and reduce speed on synclines that tend to compress the suspension as the vehicle traverses them. With practice, this line-selection gets to be automatic as you cover the terrain.

There are limits, of course. At a certain point, it makes sense to select a different vehicle, or do some extensive modification:

 

The coil is the spring itself. The strut is the assembly the spring is built into that also includes the integrated shock absorber.

Here's a strut assembly:

 

there are "coil over" kits available which allow you to change the springs to suit your needs and then adjust the ride height by turning the adjustment collar. justdidit has them on his car, and a few other folks have them as well. you can put on slightly taller tires which will correct the speedometer which reads high from the factory. using coil overs with slightly taller tires might get you 40mm of extra ground clearance; if that's not enough, you need to think about a different car.