Been offered a set of wheels with 17mm internal width- I see that things are slightly wider nowadays but would rather save money and get the bargain. Are 19/21mm wheels that much faster or better? This is for a race bike on the road. Any help much appreciated- if the difference is negligible I’ll go for it 
Sam
Do it.
I run 25mm all the time on this rim width it’s not a big deal. You’ll probably not notice any difference to a wider rim.
Assuming these are aero wheels you’ll want the tyre no wider that the external width so you might need to try a couple of different widths/tyres to get it right. GP5000 23s normally measure 25mm on 17mm internal width btw.
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That depends on a lot of things we don’t know! 
~17mm internal rim width could be a set of firecrest 404’s…so that would be a nice set of wheels to get at a bargain price, IMO.
17mm internal width vs 21mm internal width with a 25c tire is probably going to get you probably 1.5mm extra effective tire width (at pressure). So if that is important to you, there is that.
In terms of aerodynamics I’m not a big believer that wider wheels are faster but it is a popular notion these days. If you ride at low yaw angles you are better off with a narrower wheel IMO. The data from FLO wheels seems to indicate we spend the vast majority of our time riding at yaw angles less than 10% (heck, less than 5%).
Rule of 105 is real but, again, consider yaw angle…and also that internal rim width is not equal to external rim width. At mortal speeds and common yaw angles with a 25c tire you are probably looking at 10 to 20 grams of excess drag for violating the rule of 105. More yaw angle = more drag penalty.
I would probably spring for the bargain depending on the bargain and the wear & tear on the wheels.
Brennus brings up a critical point…what wheels are we talking about? That will help people give you a more informed opinion.
I would also agree that the 105 rule is not a rule that must be obeyed….yes, the data supports it, but it was also done some years ago and I don’t know that it still holds as wheels have gotten wider. I will disagree with him about wider wheels being faster…belief doesn’t factor in here and the data is overwhelming. Wider is faster (assuming you aren’t buying a crappy set of wheels, I suppose). How much faster is dependent on a lot of variables and whether that gain is worth it for you.
IMO, the biggest advantage of wider rims is the extra volume it provides, and to some degree a corresponding reduction in Crr. Again this is personal preference living in northern Chicago area with craps roads and frost heave across the pavement in Wisconsin.
All that said, I’m still on a wheel set with 17mm intervals, so……
I was running/racing on 28s with my Mavic MA40 back in 1990s. Pretty sure they were less than 15 internal. Not the most aero but definitely more comfortable than the 20s pumped up to… If it clears the brakes and frame, it all good.
Cheers for the advice- they were hand built unbranded wheels- put some extra money in and got myself some new Reynolds AR 58/62s