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Mulling over whether to get some new gravel wheels in the sales.
I ride 700c wheels with 45mm tyres, but my current rims are only 19mm internal. The wheelset is about 1,630g and feels pretty fast.
Can get some wheels with a more fashionable 25mm internal width at a good price, but they weigh about 1,780g.
Will the extra width let me get more out of my tyres? Will I notice the extra 150g heft?
I ride about 80% off road BTW, and a fair bit of singletrack.
I'm in a similar spot, new bike came with solid Shimano RS171 at 2kg, 19mm tyre bed and heavy (745g each) oem tyres. Swapping to 450g tyres was definitely noticeable. If I switch to TPU tubes or tubeless I can still drop another 100g per end for very little money, so I'll do that during Black Friday deals. The thing that has stopped me buying Prime or Vel wheelsets in the current sales isn't the rim width but more the stupid noise the hubs make, I like silent hubs and clickity clack is in fashion.
It's the rim/tyre weight where it's noticeable. So take the wheel weights with a pinch of salt as a heavier hub won't really affect performance.
Just Riding Along do a Monitor 700c wheelset that's 25mm internal, 1600g & less than £400.
I've had them on my gravel bike for over 3000 miles, still straight as a die.
Just Riding Along
Excellent builders. Or they were 10+ years ago when I got some 29er wheels from them, they're still in regular use - tatty rims with frilly edges from impacts, but still impressively straight. Would shop there again.
It’s the rim/tyre weight where it’s noticeable. So take the wheel weights with a pinch of salt as a heavier hub won’t really affect performance.
My wheels are like that - a lighter rim on a basic Shimano hub, plus good tyre and tube = nippy enough.
I'm not sure of the weight & they are entry level wheels, but I got a pair of DT Swiss G1800 wheels for Wiggle for £170. They're 25mm rims.
Thanks for the replies. It's a factory wheelset (Prime Orro), so can't check rim weight alone.
I don't know enough about gravel to tell if 1,780g is considered heavy or medium weight?
And basically, is an extra 6mm internal per rim worth a notional extra 75g per wheel?
I'd say 1780 is on the heavy side.
Juts from looking around various sites, 1300 ish = racy, 14-1650ish = seems about the going weight 1700 & over = a touch heavy (might be worth it if they are robust).
I much prefer my wider rim wheels.
Those prime ones are lovely and a bargain at that price. 
1700-1800g is fairly standard, certainly not heavy. I don't think any of my road wheels are super light, but I've some factory Mavic Aksiums (on the CX bike), some Mavic Ksyriums on one road bike, a spare set of Fulcrum 7's and handbuilt Mavic CXP33 rims on old Dura Ace hubs. The DA hubs aren't that light, but are bomb proof (30 years on and been rebuilt once). I can't tell the difference really, only when a different tyre is fitted. The CX bike has some fairly light gravel tyres on
I'd look at tyre weight, and if you can get a rim weight too.
How about trying to hand build some - not difficult. I've done it a few times, usually for the commuter bikes as rim's wear out (rim brakes). Latest set were for my vintage Diamond Back MTB - 90's XT hubs in mint condition via ebay (easily serviced) and some fairly stock Ryde rims (easy to replace).
I wouldn’t trade weight for rim width unless you were on old school roadie rims, especially if it was costing a fair chunk of money.
I have the prime orro’s. They certainly don’t slow the bike down. Would recommend
I used hope fortus 30s for ages, worked fine on anything above 42mm
benefit of more worth is running at lower pressures, is that something want to do?
No
Yes
And basically, is an extra 6mm internal per rim worth a notional extra 75g per wheel?
Depends on tyre choice and actual rim spec.
If rim option A is 50-60% the width of your ideal tyres already, maybe not. If adding that 6mm with option B gets them to >50% <60% then yes.
Depends on tyre shape too but that 50-60% range is good for all the gravel, light off road and XC wheels I have.
In all honesty I'm not so sure the extra rim width is really worth it with ~40mm tyres but weight savings are a benefit.
My first thought on reading the OP was why not just throw some money at lighter tyres before changing the wheels?
You can easily lop a couple of hundred grams off the outer circumference of your wheels with a tyre change alone and then judge if saving weight is something to prioritise or not...