Road bike gear ratio?

I'm building mine first "real" road bike and I currently got cranks with 52/42 chainrings and 12-25 rear cassette. I bought these used as a set and they come to me as is. Would that work fine or do I need to think again the rear cassette or chainrings?

depends on the strength in your legs and the terrain (gradients) you're planning to conquer.

if you're going to do many climbs in your area you maight want to consider a 39 smaller ring (if it fits, depends on the bcd of your crankset) and a 26 as largest cog

52/42 is very old school - nostalgic sigh

A straight through 13-18 block would complement it very well.

I'm on a 52/42 12/23 setup on my new (old) bike. The setup is fine for my daily commute as it's mostly flat, but I'm finding that hills over 5% are rather unpleasant to ride up. Not impossible, but not fun unless you're a bit of a masochist.
As a lot of the fun routes around Tokyo go up into the hills and mountains, I'm on the lookout for a 39t chainring to replace the 42t. 39/23 served me well for all but the most brutal of climbs. Might switch to a 25 once the mountains open up next year.

I'm really not looking for to change the cassette because it is brand new Record 10 and cassettes are expencive, so better option is to buy new chainrings if the current is not good. Would 52/39 to be good place to start messing around and trying how it feels?

As has been said, it depends on how fit you are, what the terrain is like and what sort of riding you are doing.

The reason for the switch from 52/42 to 53/39 is that modern derailleurs have a greater capacity than they used to; the greater the difference between the chainrings, the wider the range of gears: a lower low gear and a higher high gear.

I ride 53/39 with 13/23 which suits my 44 year old legs around a surprisingly lumpy Suffolk over distances up to 200km. Comparing that to your set-up, you'll see that my lowest gear is practically the same as yours, although I have lost an appreciable amount of the top.

Having said that, I only find a use for top gear when I'm throwing it down a long, steep hill.

The main difference, is that the gaps between the five smallest sprockets is much less than yours. As Sheldon explains, having closer ratios at this end of the cassette is of more use than at the other end: at higher speeds you need a lot more power to go a little faster, so small jumps between gears are needed.

When you're going uphill, your cadence will drop-off quickly and if the gap between gears is too small, you may well need to shift down two gears at a time.

All this theorising is all well and good, but the short answer to your question is that you won't know until you try it: either end of the range will cover almost all conditions, but you might find the jump between gears is something you want to improve on.

EDIT
I was typing the above when you replied and had assumed you had an 8 speed cassette: as you have a 10, then the jump between gears won't be an issue, but 53/39 is the norm from 9 speed onwards.

Just a thought: are your chainrings 10 speed?

Although you can (I think) buy 52/42 10 speed rings, it is an unusual set-up.

Personal chioce an all. But 42:25 seems a big, smallest gear. But if you're not doing long climbs then it'd be OK.

I tend to do the short sharp climbs on a normal ride 50:21or 23 (naughty cross chaining). Which basically means I dont use my small ring untill I hit the mountains proper. If I had your cranks I'd want a 12/27. On the plus side Your little ring is going to be more useful for day to day riding.

So you might as wheel see how you get on.

Going to convert 70's road bike to SS and wonder if 42/16 is a good ratio for 3-4 mile commutes on mostly (but not totally) flat area.
I'm 31 year old unfit person weighting about 230 lbs.

how about a test ride before conversion?

I use that ratio for training and I like it, 100rpm will get you about 33kph

You can check Sheldon Brown's gear ratio calculator to see if you need a bigger or smaller cog or chainring

how about a test ride before conversion?

No a bad tip. But it's getting cold where I am. About 0 ℃ atm :) Might try if it gets a bit warmer in weekend which is unlikely.

+1 for 42/16 - wasn't that hard to start with and now it's easy, 6 mile commute mostly flat with a couple of gradients

42/14

... but I was wondering, is there any difference if chain ring is bigger and driver cog is proportionally smaller? ie: 42/14 is a ration of 3, and so is 48/16. Is there any difference in terms of starting, hitting a cadence or spinning?

I'm trying to get my head around the physics. I guess you spin slower on a 48? Any ideas/experience/opinion?

... and what's the smallest driver cog you can buy?

So i'm going to bump this aged thread if I may, as I have a related question:

After decades of fixed I finally built up my dream road bike at the start of last year and have loved every second of it (still ride fixed too though xD) - although as a roadie n00b I installed a standard 53/39 chainset with 11-25 on the back.

It's useable... and has got me through my longest ride of London-Newhaven-London but I've had a fair few riders chuckle and recommended I go compact 50/34.

Now I see videos that 50/34 is dead, long live 52/36 and long cage derailleurs accommodating 11-32...

...Sheldon's calcs show me a ~1kph difference both cruising in a mid gear and in the lowest gear for the same rpm (standard vs compact) but I guess this isn't telling me about the change in effort involved?

Would going 50/34 mean also switching the cassette to 11-28 (max on my derailleur) to make it worth while?

Thinking I may just have to get the new chainset/cassette and bite the bullet, but anymore insight would be soo helpful!!!

(I would ask on roadie forums but I like this corner of teh internet)

I don't think it'll be an answer you'll hear a lot but I'd start with a 12-30 cassette (the rear dérailleur will be fine).

Are you needing lower gears? Are you finding yourself always changing gear to find a comfortable cadence? If the answer is no to both then why bother changing the gearing.
Or are you just looking to get whatever is in fashion at the moment?

I’m told I could be more efficient with saving energy on long rides with multiple climbs using the lower gearing, currently steep hills (ones with 10+ gradients in them) are tough but can be grinded through and eventually recovered from. If there’s is a better combo that would help dial down the effort on those so I can last longer, and cycle further - I’d be into that!

Interesting! Sacrifice some top end to allow the cassette to fit with the increased bottom end?

A cassette with a lower bottom gear as mentioned above is the simplest and cheapest first step. Depends on the capacity of your rear mech.