MTFCA Forum

I’m ready to put new sneakers on my ‘26 split rims and am considering going with 5.25-21. My engine is strong and I do have a Rux in this pickup. I like the idea of the slightly larger diameter tires giving me a little lower rpm at the same cruise speed. What have others experienced? Also, what’s the current opinion of the various brands that are available and what about the radial tires? I probably put about 4,000 miles a year on the vehicle.

I would look to see if the larger tires are 6 ply. Model T 4.40/4.50 X 21 tires are usually 4 ply. A larger tire will be heavier, which will degrade the ride, handling, performance, and braking, especially if the larger tire is a 6 ply.
There is some variation in the outside diameter of tires with the same nominal size. For instance, 4.40/4.50 X 21 tires range from around 29" OD to about 31" OD. The Firestone 4.40/4.50 X 21 repro tires are close to 31" OD. I think 21" radials are made for Model As. I suppose they'd work on Ford T wire wheels with a drop center rim. (?)

They are 6 ply.

I have been running 525-21's on my roadster for at least 20 years and I love them. With the Firestones it gives you about 3% Hobo Overdrive, improves the ride (especially on wash board roads), with aux brakes it improves your braking with more tire on the road, they last considerably longer and with the 6 plies I have only had 1 flat in over 100,000 miles and that was the tubes fault. That being said, I do a lot of dirt, gravel or less and I does change the way it takes off from a stop and climb hills, if you have a Ruckstell you are in good shape, if not I would recommend going to 4 to 1 gears in the rear end. It will also change the look of your T.

Dennis, what is the OD of the tires you are using?

Hi Dave .... Dennis Prince knows what he's doing and other than being not the original equipment size, going to the little bit larger tire wouldn't hurt at all. I would stay with the black wall, even the white wall is available. It all depends on whatever vendor has in stock.

Pat, It has been a long time since I measures a new one but I think I remember between 32 1/2 and 33 inches.

That will give about 9 to 10% "overdrive"... similar to a 3.30 axle ratio vs stock. But the disadvantages of more weight and a stiffer carcass will come with it. Rotating mass works against acceleration, especially if it is located well away from the axis of rotation, which a tire is, and you have 4 tires.

The extra rotating mass may be a slight negative compared to standard size tire at the beginning starting from a stop, but.. with increased speed, don't you think that minimal extra mass would act more like a weighted governor/harmonic balancer to keep speed momentum going. ?¿?

I do run on loose sugar sand quite a lot here in the Pine Barrens and was liking the idea of a slightly wider tread. I have seen these 5.25-21 tires on a few cars on International tours and they definitely do change the look of a T, but I like it. Has anyone run the 4.50-21 radials?

See I told you it had been a long time, I looked up the diameter on the Coker web site and it says 31.5 in. Sorry for the bad info.

That's not much more diameter than the Firestone 4.40/4.50 X 21. Wider tires would likely be an advantage on sand, and if you off-road with a fair load of stuff aboard, the wider, 6 ply tire might be a good choice. As for wheel/tire weight and unsprung chassis weight, less is better. It would be ideal if the wheel/tire assemblies and axles weighed nothing.

If you want to add mass, add it to the body. If you don't mind slower acceleration, adding a hundred pounds or so to the trunk and 170 pounds to the passenger side floorboard of a late style roadster will give a marked improvement in ride quality, at some cost in acceleration. Top speed would remain about the same. Maximum braking might not be much affected, since you'd gain some rear wheel traction, but on average, the brake would have to work harder. A touring car would certainly ride better with a full load of passengers than with a driver alone. I would not want to add any unnecessary weight to any of the closed cars. Henry Ford was a practical minded man, and he insisted on minimizing weight. He probably assumed that his cars would carry several passengers more often than not.

Adding weight to the wheels and a wider footprint on the road will likely reveal any problems with the front axle assembly and the steering response, especially on loose surfaces. The wider the tyre the greater the steering effort required, even if you have the 5:1 gearing. Your choice.
You can reduce the revs by dropping the speed a little. Again, your choice.

Allan from down under.

I haven't measured them. but I have both Firestone 4.40-4.50x21 and Goodyear Diamond tread 4.50x21 tires and the Goodyears are definitely shorter in diameter.

Firestone 4.40/4.50 X 21: 4 ply, 18 lbs, 30.8" diameter.
Firestone 5.25 X 21....: 6 ply, 24 lbs, 31.5 diameter.

0.7" difference in diameter, or .35" difference in radius, somewhere around 1.4 % difference in diameter. (Insignificant)

In 2022, I replaced my 52 year old, 21” “Universal” tires, tubes and flaps (purchased from Mark Auto in 1970) on my 1926 coupe with (5) new 21” “Goodyear” tires, tubes and flaps and they are superb. I could not be happier with them. Good luck.

PS. Increasing the diameter of the tire will throw off your speedometer (if you have one). I have a period 1926 Stewart speedometer so I would not consider doing it.