I was shopping around for a tire air inflator for my car, and noticed that for a Viair 84/85P Model, it can pump 31" inch tires but has a work pressure of 60 PSI, compared to the 70/74P Model, which can pump 80 PSI, but are only rated for 225/60R18 sized tires. It seems as though with this product modeling, it's inferring that size affects the working PSI of the device. This is just my guess though.

I figured that it doesn't matter what the tire size is, because the size wouldn't affect the PSI. It will still take the same amount of effort to push 35 PSI in bike tire as it would a car tire, right? Not necessarily the same amount of energy, but in terms of effort, shouldn't it be the same? The only thing I can think of, where the tire size affects the air compressors operation, is that the device would have to sustain pushing 35 PSI for longer periods, because the size is larger, but why would that make any difference? Shouldn't an air compressor be able to sustain pushing 35 PSI for a prolonged period of time?

I must be missing something.

1 Answer 1


Checking the spec sheet on line it appears the duty cycle of the Viair 84/85P Model is only 20 minutes. This is the reason for the tire size limitations. Basically they are trading off a low performance long running duty cycle for short term performance. You have to ask if you want a pump that will take 60 minutes to fill a tire, but could do it all day or a pump that can fill up a tire in 20 minutes but then has to cool down before the next 20 minute cycle.

  • How is it higher performance if the working PSI is only 60 compared to the 70/74P? I understand the trade offs in time, but I don't understand why the rated PSI is lower if its supposed to be the better model.
    – Narcotixs
    Dec 31, 2017 at 1:18
  • The volume of air is different in different tyres - which is why Mikes pointed out duty cycle. You have obviously pumped up a bicycle tyre with a hand pump, so now try and pump up a car tyre (15”) with that same pump - that will give you an idea of duty cycle.
    – Solar Mike
    Dec 31, 2017 at 8:30
  • note also on the spec sheet viaircorp.com/portables/84p the fill time for a 31 tire for the 84 is 4 minutes. You can extrapolate the data for the 70 and estimate 5-6 minutes fill time, well within the duty cycle. (But not if you need to air up all four on your off road vehicle)
    – agentp
    Dec 31, 2017 at 16:06
  • Again, I understand the difference in time and speed, but what about the tire size? Why is there a max tire size for these models? Why does it matter?
    – Narcotixs
    Jan 1, 2018 at 4:37
  • Tire size affects the volume of air required to fill. Note a 31 inch tire is nearly twice as big (volume-wise) as the 60 series/18. Volume means time and in turn running time determines whether or not the little pump overheats before the job is done.
    – agentp
    Jan 4, 2018 at 20:09

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