MTFCA Forum

Has anyone run Firestone 5.25 X 21" tires on Model T wire wheels? (OD 31.5", tread 4.25")

Pat. Years ago a friend did have those on his 27 touring car, tires did look oversized road just as well as mine with regular size tires. I'm sure if the car was overloaded the tires would have rubbed on the inside of the fenders but I have no proof of that.

I have been running 525/21's for years on wood and wire wheels and I love them, the Firestones are a little taller than some of the others and they are 6 ply rated, I also like that there is more tire on the ground for braking. It takes a little bit to get used to the "Hobo overdrive" and if you don't have a Ruckstell you will be in low more, I run 36psi on my roadster (it weights about 2600 lbs when loaded) and 20psi on the Funster.

If the tires are oversize, you will go a bit faster on level ground at the same engine speed than you would with the standard size, however, if you have hills in your area, you might need to shift down to a lower gear to keep from lugging the engine. So one of the important things to consider is how the car will be used.

My present tires are Lucas 4.40/4.50 21. They measure very close to 30.50 overall using a large square across the tread. Pavement to center of hubcap is about 14.75 with 28 psi. Tread is about 60 %. Firestone shows 31.50 OD, which I expect would give an effective diameter of around 31". That's about .033 OD, which would not affect the final drive much. If I'm doing the math right, it would change the effective final drive from 3.64 to 3.52. That would help in low, and be a benefit (most of the time) in high. I think the wider tires would handle the non-paved roads around here better than the narrower tires, especially if I could get by with lower pressure, like maybe 25 PSI. My car weighs around 1900 lbs with me in it. I don't know if there is any set standard for measuring tire OD. My tires showed about 14.25 from pavement to the center of the hubcaps after sitting about one hour. I pushed the car off the flat spots and the same measurement went up to 14.75+. That's probably a little less than the running diameter at road speed. I'm tempted to try the Firestone USA made 5.25s. Lucas offered a tire in that size at a better price, but I cannot find that tire listed on a Lucas website.

A friend went with the Lucus 525/21 tires and they are about 1/2 in shorter than the Firestones and he didn't get very good wear out of them. I average about 35,000 to 40,000 miles out of a set of Firestones and there is still some tread left and I tend to torture my tires more than most.

Do you run 20 PSI on the road with the lighter car?

Pat, I do run 20 psi sometimes if I know that it will be rough like when I take it to the Bonneville Salt Flats for Speed Week however on the pavement I run up to 30psi, I get less side wall roll and the rolling resistance is much less, seems to drive better with a little more air.