Wider tyres on the rear

I had 28mm front and 32mm rear on my commuter with zero problems.

I run 25 front and 28 rear on my road bike for the same clearance reasons, since I swapped to that from 25/25 I have noticed it's definitely easier on my back on rougher roads and with the lower pressure and bigger contact patch there is a marked improvement in grip on slippery corners. I also feel, though I have yet to be convinced as to whether this is more psychological than physical, that I am getting a little more drive from the back wheel, i.e. I'm getting a little more power transferred to the road for the same wattage, but as I say I can't swear to that. Certainly hasn't detrimentally impacted comfort, handling or power transfer in any way, go for it!

I've been doing that for years. I've an old steel frame that will only take a 23 at the front and a 25 at the rear, no problems whatsoever. A more modern alloy frame takes 25 at the front and 28 rear with guards. All works perfectly well. If anything, it slightly sharpens up the steering by steepening the steerer, but it's pretty marginal. Go right ahead, you'll save frontal area and a bit of weight with a narrower tyre and the rear takes the majority of the weight anyway so a bigger air volume there makes sense.

Probably not relevant, but many MTBs have a wider &/or more knobbly tyre on the front, run at a distinctly lower pressure. The idea - & it definitely works in practice - is that the softer grippier tyre at the front provides extra control & stability over rocks or roots, while the narrower &/or less knobbly tyre at the rear digs into the ground more, so is less likely to slip when you're trying to put the power down.

I use a similar idea on my gravel bike, with 40mm front & rear (the widest the frame will allow) but luggy Conti TerraTrail on the front for improved grip & much smoother TerraSpeed on the back for faster rolling.

Basically, the moral is, if you've got your own ideas try them & see what works for you.

On the handling thing, I run 25F/28R clinchers on an old Lapierre in France.

Out of curiosity, I swapped the tyres over to 28F/25R, keeping pressures to about 65F/70R

In the twisties e.g. https://www.youtube.com/watch?v=3kogqzXUMPY&t=40s the bike is a little understeery with 28F/25R and a little oversteery (better) with 25F/28R. Your mileage may vary - have a play !

An old school BMX race technique is to have a 2.2 on the front and a 1.75 on the back. I run 1.75s front and back on my BMX 20 and 24 race bikes. It's easier using the same tyres front and back from a spares/replacement perspective.

I've been running exactly that setup for a few years now and think it is about the best combo for me.  No noticeable drop off in performance (or increase, I should add), but I think there is a slight increase in comfort and grip from using the 28mm at the back.  I also run relatively low pressures (70f and 80r) on clinchers but my weight of 65kg helps with that.  

The only problem with this is you have doubled your critical spares stocks.

As I recally my Aeroad was delivered with 23mm and 25mm tyres, now it has 25mm on both ends. I don't believe there are any issues with disimilar tyre widths. Just remember that required pressure is a function of system weight and tyre width and run the wider tyre at a lower pressure accordingly

I've ridden with a 30 front, 32 rear as I couldnt source a 30 rear one time I needed new tyres, it didnt cause any problems I recall, and arguably was a bit more comfortable. Theres a weight penalty if you are talking absolute performance impacts, and slight psi difference to be aware of, but it works, I'd be comfortable doing the same again for sure.

I wouldnt run it the other way around though ,wide front,skinny rear, as I think it would make the bike handling feel odd, and have a greater impact on absolute performance.

Shouldn't have any effect, certainly won't slow you down and the increased comfort may make you faster on longer rides due to less fatigue. You might also find it evens out the tyre wear front to rear. 

This did get me thinking of mullet MTBs running 29" up front and 27.5" rear.

Sounds like a good plan to me.
You have more weight on the back tyre so having a wider one there will be better.

I'm now using 25mm on my summer bike as it's the widest I can go - but when I buy a new bike it'll be sure to take much wider tyres.
My winter bike is on 32mms and the ride is so much comfier.

The only downside is you could have one tyre lose grip before the other if cornering very hard, but the difference will be marginal and only when riding on the limit. Other factors such as tyre pressure, tread condition and road debris will have far greater effect than 3mm in tyre width.

Come to think of it, most motorbikes have a rear tyre way bigger than the front, so it can't be much of an issue.

I'm glad someone asked (and answered) this as I've been wondering about the same thing...

Continental used to sell odd pairs like that - one 23 and one 25.

The theory is that the front has less weight and less work to do (as a guide, my front tyres last about twice the time of the rear), also that the narrower front tyre is more aero whereas the rear is protected by the frame anyway. From experience, there is a sweet spot, which may well vary by rider (weight of rider being one factor, weight of tyres being another) - I've got 32s on my winter bike - is what the bike came with and they are just a bit too heavy but I prefer riding them to the 25s on my other bike.

So as we are tending towards wider tyres (I've bought 28s for my next tyre change) if you can't fit 28s on the front, 28 rear 25 front is an entirely sensible thing to do.

I'm working towards 28s on my summer bike and 30s on my winter bike (gravel with mudguards) but I'm going to wear out what I have as I doubt the difference will be massive.

All tubeless.