but l do not wanna take the brakes off or go single speed.
but l do not wanna take the brakes off or go single speed. The bike is a Specialized P2 cro-mo

Lose weight. Just kidding.

Can you replace parts with lighter weight parts made of carbon or titanium?

The first obvious thing to do is take off the reflectors, & the pie plate if u have the tools. Then the first thing to look into upgrade r the wheels, it would make the biggest difference in acceleration, climbing, & the weight in total. How light depends on how big ur wallet is. Next up would be anything in the drive train, start with the cranks, derailleurs, chain, shifters. At this point anything other upgrades would be minuscule, again depending on how much money u want to plunk down to upgrade.

paint it white.

A lot of this depends on what bike u r lightening up. For some bikes it is more trouble that it is worth & cheaper & easier to sell it & buy a new lighter bike. Now aside from the simple items as reflectors & such the best way to make a bike lighter in weight & feel is in the wheels. Nothing will make a bike fell like it is 5lbs lighter than a good set of light weight rims & tires. If money is no object & u like ur current bike other items that can be replaced with lighter ones include handlebars, stems, seat post, seat, cranks, etc.

l would definitely second Ron S is suggestion about the wheels.
l have got a Spesh Allez triple, & after upgrading the stock Alex wheels to Shimano wh-r500, l was practically flying up the hills on my daily commute in hilly Bristol, England!!

For £99 (incl. both wheels) this has been well worth it!
You some more weight savings by upgrading the tyres as well, although u have to consider that puncture protection (spesh armadillo's, conti gator skins) come in heavier but u get less protection with lighter tyres(less kevlar on the tyre).

you can spend $500 making it 5 lbs lighter

how much faster will that be?

assume bike weighs 25, you 175 = 200 lbs

5/200= 2.5% faster
10 mph turns into a scorching 10.25
oh wait, that;s only uphill
downhill, you will actually be slower, on flats no difference

so yeah, what the other guy said
lose weight yourself, and get stronger

wle

first off what type of bike do u have dept store bike or bike shop bike. this is important because most dept store bikes r not compatible with most bike shop parts & if u have a dept store bike it is heavy & it will stay that way.not much u can do about it.
The bike store bike is a whole other story, u can change out several key components that will be much lighter but u can expect to spend upwards of $1000 or more depending on which group u buy. Alot of a bikes weight depends on the frame & what it is made from. If a magnet sticks to it it is likely chromolly (steel) or a mix of steel & lighter weight compounds & that is what u should check. The second heaviest part is the wheel set (again check for Magnetism)
You can go with better lighter Components (Shimano XTR for example) but remember to do this u have to change almost everything except the crank & cassette to do this as the gearing in the shifters & derailleurs need to be somewhat compatible with each other. the fork is the other key element the cheaper ones can weigh as much as 6-7 lbs or more & higher end forks can weigh as little as 3 so check all parts with a magnet & determine where it came from before u go spending ur $.

As has been covered before, it all depends upon what type of bike u have.

The cheepest place to start is on the tyres. losing 1lb of weight on a wheel will be equivilent to about 3lb on the frame. This has to do with the dynamics of rotation. So, start with thinner tyres. If possible, given ur rims, u might look at a tubeless conversion kit.

Next is to replace the rims with lighter jobbies. This is where it starts to get expensive. The reason race bikes r so expensive is because the parts have been machined as thin as possible, out of light, strong materials. As soon as u start doing this, funny things start happening to component prices.

However, another good way of losing rotational weight is to replace ur casset. Shimano XT is not bad. Light kit for mountain bikes. For road, go for Dura Ace.

Now it gets expensive. Replace the wheels. You want to look at about £500 for the set to get a viable pair. However, if u really want weight saving, DT Swiss. Look at spending about £1600 for a set of these.

If u have suspension, look at replacing it for an air system. Fox r my faves & u can get a reasonable fork with the Float, but again, this depends upon ur bike & what sort of riding u want to do.

Replace ur crank for hollowtec I l or similar. Failing that, ISIS style hollow crank.

Have l mentioned carbon fibre yet? Seat post & bars. You might do the stem at the same time. MBR just did a comparison & Easton Monkey lights still come out with good marks. £250 for the three.

Lighter saddle. Think a ruler, with a bit of covering & titanium rails.

l could go on, bearings, Titanium bits & bobs etc. Ultimately, u r looking at spending money on this. As soon as u start to make things noticably light, u r looking at spending a lot of money. The question is, is ur bike frame worth the expense? It might be cheaper for u to simply buy a lighter package.

Luck