Mismatched tire widths for gravel

I’m revitalizing my old cyclocross bike for gravel. The largest I can fit in the rear is 35mm (measured), but I can fit a 45mm in front. Is there any good reason not to run a larger front tire?

The cons I see are:
-I can’t swap/rotate tires
-It may adversely affect handling by raising the front end

Pros:
-additional corning traction
-additional suspension of larger front.

For reference, I plan to use this riding from home on pavement to local gravel as well as XC trails, in the PNW.

I’ll take any tire recommendations while you’re at it!

Not really. I understand it is very common to do so in the MTB world. I do this on my gravel bike with a 32mm rear and 35mm front and can’t notice any difference to the 40/40 I ran before.

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that kind of front size bias is a hell of a lot more appropriate for MTB where your doing a lot of more technical riding with significant braking. imo, the big tire belongs on the rear for gravel, and especially road bikes.
I realize the following example is only a sample of one and on a mild race course (but for many good reasons); Vermeersch who won elite men ‘Gravel WC’ was running 35R/33F, and not by accident.
and fwiw, I’m a believer in running a 2-3mm fatter rear tire on road bikes 'cause it makes all kinds of sense, but more importantly it feels good.

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I can only recommend it. You’ll most probably like it, especially in the PNW where you would often like to have more traction in turns that what is available.
It’s a legitimate thing to do when the weight distribution on your bike is less than ideally balanced, and you want to make sure that the rear wheel is the one which is losing traction first in turns.
45/35 might be a little too much of a good thing though. I’d go with 40/35 mm.

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I’ve done it for a while, for similar clearance issues as you. Quite liked the extra comfort and traction. I’ve got more clearance on my current bike so I’m on ‘evens’ now.

Potential negatives might be a change in handling as the larger tyre will increase the trail number (this might not be a bad thing on XC trails). It will also change your saddle angle as it raises up the front end. 35-45 is a pretty big jump, maybe try 35-40 as the effects won’t be quite as pronounced.

I’ve ridden 45F/40R* since I got my gravlocross bike years ago & nothing’s caught on fire & my portfolio is in decent shape, market factors not-withstanding. Give it a shot. It’s fine.

*I like to hit singletrack on occasion & appreciate the wider footprint up front.

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Meh…you’ll be fine and the extra traction upfront will be helpful on both gravel and trails. It may slightly affect handling, but chances are it will be pretty negligible and easily adapted to.

I guess the bigger question is do youhave to run a 45? A 40 will get you most of the same benefits, but less handling impact.

No I don’t have to run a 45, I only meant that I can fit up to a 45, mostly to explain my options and garner responses. That said, I have a 40/35 combo on the way.

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I’ve done this on the road bike many times to zero ill effect and I trail brake corners in the wet all the time on that set up (I spend a lot of time riding motorcycles on the track and twisties). Only caveat is that when you brake into a corner the grip on your front tire increases with the load but the grip on the rear decreases obviously with less contact patch in the back that’ll be exacerbated but if you’re a competent bike handler you’ll probably figure out pretty quickly how much you can push it probably would keep it to 5mm difference unless you’ve got a 45 just laying around you want to use.