What is the correct air pressure for 21" tires? I've always heard it was 50 to 55psi, but I just read in a 2012 thread where Royce recommended tire pressures of:
60 psi for 30 x 3 tires and
65 psi for 30 x 3 1/2 tires
The is what Hap wrote at:
Some additional documentation:
In the Mar 1926 Service Bulletin Ford recommended different air pressure depending on the body styles:
All were 27 psi for the front but 27 in the rear for Roadster, Coupe, and Touring , and 30 in the rear for Sedan and Roadster Pickup.
But the Sep 1927 Ford Service Bulletin said to inflate all 4.40/21 balloon tires front and rear on all models to 32 psi.
And those same 21 inch tires on the 1928-29 Model A Ford were recommended to be inflated to 35 psi front and rear.
So you also have a range of 27 to 35 depending on which Ford source you want to use for the 21 inch balloon tire.
Hap l9l5 Model T Ford touring cut off and made into a pickup truck and l907 Model S Runabout. Sumter SC.
32 psi it is. Thank you, Jim. Jim Patrick
Isn't the mfg. recommended PSI printed on the sidewall of the tire? It is on mine...says 50 lbs. minimum
What size and brand tire are you talking about?
42 year old Universal 4.40 x 21 on split rims. Jim Patrick
The pressure a manufacturer puts on a tire is Maximum pressure for Maximum rated speed at Maximum load. Since you don't run at maximum speed rating or maximum, load, about 35 PSI tops is good for the 21 inch balloon tires. I don't know if T tires have speed ratings, but the lowest standard speed rating today is R, which is for 106 mph sustained speed. I'm not sure what the load rating of the tire is, but I'm sure it far exceeds the weigh of the car.
This is one of those areas where people fall victim to the All the Same syndrome, assuming that one thing or another is the same on all Model T's. New folks don't always realize the difference between clincher tires (60 psi) and balloon tires (32 psi).
I run 27 front 30 rear on my '26 touring, but if the car had shocks (like a model A) I would run 35 all the way around (like a model A}.
The tires tend to bounce if you put too much air in them when you have no shock abosrbers (dampers).
I can't imagine running these tires at 25 psi. My Firestone 21" are rated and were kept at 32 psi, but after sitting for a few weeks....the pressure would drop a bit.
If they were down that low (anywhere below 30 psi) I would have nothing but blowouts or sheared valve stems (rubber stems). I went through more tubes and tires in the first 3 years then on all my other cars combined.
When I got my new set of Firestone 21" for the latest rebuild, I ordered nickle threaded valve stem tubes and have been running them at 40 psi for years now......and have never had an issue. it also rides better.
I had an ol' timer tell me to "run at 45 or 50 psi and it's ride better" at a car show once....looks like he was right on.
Just my $.02
I run 35 pounds in both the '27 Tudor and '29 Model A.
They share the same size tires and ride very nicely.......