Running tires with significantly different pressures?

I ordered a replacement for one of the old Schwalbe Marathon 700x28C tires on my ancient road bike, and it turns out that "new" Marathons of that size are meant to be inflated to 80-110psi despite that the older ones I have are rated to 80psi... so, now my front tire has 37.5% greater pressure than my rear tire. Principally, tires do not need to match, but will this significant difference in "hardness" affect the handling/fragility of my bike in any way?

• What pressure are you running in the rear tire? I would think that you'd want it near/above 80PSI anyway, when riding on reasonably smooth roads. Why not just inflate both to 80PSI? – Daniel R Hicks May 18 '16 at 20:46
• @DanielRHicks: I can't compare empirically as I've never had tires on this particular bike which could handle "high" pressures, but, IMHO, 80psi seems quite low for a road bike and, more importantly, after ~2 weeks they'll both be at around 70psi anyway --- which is "officially" outside the range of the newer tire (thus motivating me to post this question). – errantlinguist May 19 '16 at 7:33
• Then I don't understand your problem. If 80 is low for a road bike then you must have been running above that already, right? – Daniel R Hicks May 19 '16 at 11:21
• Note that a 28mm tire should be inflated at least once a week, probably every three days. And the pressure in the rear tire should be at least as high as the front, possibly higher. – Daniel R Hicks May 19 '16 at 11:23
• So what's wrong with just inflating both to 80? – Daniel R Hicks May 19 '16 at 12:03

1 Answer

The markings on the tires for the pressures can be essentially ignored. They're a combination of marketing and legal departments coming up with essentially arbitrary numbers.

Find a set of pressures that works for you so the tires are properly inflated -- it should prevent pinch flats, but keep rolling resistance low and absorb road hazards and irregularities to some extent. The tire will deflect a bit when properly inflated. There are charts which you can use as rough guidelines, but you'll need to play with it.

If the tire is over or under inflated, it can damage the wheel and/or compromise handling and/or lead to flats. Sheldon Brown has a good explanation.

Often, you want to run different tire pressures (for example, the rear tire has more load, so you may want to go 10% higher in pressure or whatever, even if the tires are of the same size). If you're running different sized tires (which is not uncommon, either for lower weight, better handling or comfort; your front and rear clearances for tires are often different), you'll usually be running different pressures.

Chances are the new tires you ordered are essentially the same as the old ones, so you should start with the old pressure combinations you were using, and make adjustments from there.