5.25-21 Tires

[Dave Young]

I’m going to be putting new tires on my RPU this winter. It’s got fresh power and a Rux. I’ve seen bigger rubber on some cars during the major tours and am considering going with 5.25-21. My goal is turning slightly lower rpm while moving along at 36 or so mph due to the greater diameter of the tire. Any of you guys have an opinion on this topic? Fresh engine, high comp head, 280 cam, standard 11/40 ring and pinion in the Ruckstell.

[TXGOAT2]

You should be OK, but performance will suffer a little due to longer gearing and higher rotating mass. Increased unsprung weight will tend to offset ride improvement on some surfaces, and if you go from 4 ply to 6 ply tires, the negative effects of increased weight will be more pronounced. Adding snubbers of some kind would offset any negative ride effects of the heavier and probably stiffer tires. My 26 roadster does best with 26 PSI cold in the 4.40/4.50 X 21 Lucas tires on Ford wire wheels. Check the actual OD of the tire you choose to gauge effect on RPM/mile. My car runs easy at speeds of 40-45 MPH on the Lucas 4.40X4.40 X 21 tires. Indicated RPM at 45 MPH is about 1750/1800. Axle is stock, no aux.

[Allan]

I would not be comfortable with tyres that wide considering the rudimentary nature of a T steering system., especially one that is not entirely up to spec.

Allan from down under.

[Moxie26]

Back in the day, going to a little bit larger diameter tire was a lot cheaper than rebuilding the differential with different gearing such as 3.25 or 3 to 1...... It was easier to stay in low gear for a little bit longer before direct drive, but slowing down to a stop resulted in a little bit more pedal pressure on the brake after retarding spark.

[Mark Gregush]

How much of a diameter increase is there between 4.40/4.50 and 5.25? The 5.25 is the width of the tire not the height and 5.25. The 4.40/4.50 would be in the ballpark of about 29/30" in diameter.
The other option would be to use 30X3-1/2, which you could still get in 1926. That could easily give you an extra 1+" in diameter but much less road contact.

[TXGOAT2]

Note that the 5.25 dimension refers to the maximum section width, and not the tread width. The actual tread contact with the road is probably closer to 4" wide and maybe less. A larger diameter tire will provide a slightly longer contact patch. Larger, heavier tires will diminish braking somewhat, due to flywheel effect and more distance between the axle center and the road surface. I suspect that an improved RPU with wire wheels would handle the larger tires OK, provided it was in good condition, but I haven't tried it. I'd want to use a 4 ply tire, if available, rather than a 6 ply, and avoid high inflation pressure unless you are carrying a significant load. Wheel/tire balance become more important with larger tires. It would be worthwhile to compare the actual weight of a set of balloon tire wooden wheels with demountable rims with a set of Ford wire wheels. The wire wheels and hubs might be significantly lighter than the wood ones, which would offset the disadvantages of a heavier tire. A heavy tire and rim is to be avoided, and the less unsprung weight, the better.

[Dave Young]

I hadn’t considered the unsprung weight topic. The tire is about an inch taller than a 4.50. This car does have disc brakes on it so braking should still be fine and 4 ply would be my choice if I go this route.

[Kevin Pharis]

My dad has 5.25-21’s on his RPU with 21” Buffalos. I’ll see about getting a pic…

[Norman Kling]

A couple things to consider. If you have a speedometer it will say your speed is different than with the correct size tires. If you plan to tour with the club, you might find that you will need to shift down more often unless you are the first in line, because the more standard T has more low speed torque but with the high compression engine and big tires you will have more high end speed. I have a standard engine in a car with 3-1 gearing and Ruckstell. I find that I have to shift down on hills where the standard geared cars can go along in high at about 25 mph I need to go about 35 to be able to keep it in high. If the area of your touring is mostly level, you will be fine but if hills, not so fine.
Norm

[TXGOAT2]

If you add an inch of diameter, that's about 3% of 30", if that, since the tire will "squat" somewhat when it is on the car. I don't think that will make much difference as far as effective gearing. It's not easy to get a very accurate figure on the true working diameter of a car tire, since so many things affect it, and nominal tire sizes and tire profiles seem to be pretty elastic, too. I put a new set of tires on a Packard, and they stand about 1" higher than the old one when mounted and inflated. According to my Garmin device, the car now runs about 1 MPH faster at a speedometer-indicated 50 MPH than it did before. In other words, the Garmin now shows 51 MPH rather than 50 MPH, whereas before the two devices were in perfect agreement at 50 MPH. That's about 2% difference, and it will be less as the new tires wear.

[Dennis Prince]

I have run 5.25 firestone 6 plies for about 20 years with no trouble at all, as a matter of fact I love them. They give you about20 % more tire on the ground for braking, about 3% Hobo Overdrive, unless you are carrying a heavy load they give you a better ride and they last a lot longer ( I get about 30,000 miles out of mine) and I run a lot heavier than most, they also give you a little more ground clearance. I have run them on wood wheels and wires without issue.

[Dave Young]

Dennis. Hobo overdrive! Love it. We run on sand roads in the Jersey Pine Barrens a lot and I want a little bit more float at low air pressure. Thanks for everybody’s input. Not sure which way I’ll go just yet but all the info is appreciated.

[TXGOAT2]

The larger tires would probably be a good choice for running in sand, small gravel, and loose soil. Loading the vehicle reduces the effects of un-sprung weight on ride quality. Tire and rim weight are always a factor when accelerating, but that should not be an issue in most off-pavement operation. Airing down the tires to about 22 PSI would probably give good results off-road, and airing back up to 26 to 28 PSI on pavement would probably be a good idea. I'd want to keep a matching spare aboard at 32 PSI. I'd want the 5:1 steering gears and a 17" inch steering wheel. I'd stay with a 4-ply tire if possible. The 21" radial tires made for Model As might be another choice to consider. You might be able to run them tubeless on a Ford wire wheel. If so, you could run ATV/dirtbike no-flat goo in them.

[John Codman]

It would seem to me that the disadvantages of the larger tire outweigh the advantages. I would definitely go with the four-plys and not the six-ply. There is no advantage to be gained with the six-ply. The advantage of the six-ply is load carrying capacity; if you are carrying enough "stuff" to need the six-ply, you are overloading the vehicle, which makes it unsafe whatever else you may do. You will also have to run the 5:25" tire lower in air pressure then the standard 4:40 - 4:50 tire or it will ride (I was going to say "like a truck") hard. It will also steer harder due to the greater tire contact with the road surface. Finally (no offense intended to anyone) in my opinion, they look lousy.

[TXGOAT2]

My 26 roadster with 4.40 /4.50 X 21 tires and standard gearing has no problem running 40 to 45 MPH on the open road. It will go faster, but I limit speed in the interest of durability.

[Dave Young]

I’m leaning toward the 4.50-21’s now.

[Dennis Prince]

I have to wonder how many theorist have ever run larger tires? They ride better, last longer and for the type of roads that I drive the 6 plies have never had a flat in over 100,000 miles. Just a personal choice and observation (not theory).

[TXGOAT2]

Physics is physics.

[Moxie26]

Dennis, surely makes sense. And besides, he shared his experience with the larger tires, and no complaints. Those tires get better resultsfor him on his car in the terrain that he travels.

[TXGOAT2]

"Those tires get better results for him on his car in the terrain that he travels."

I don't doubt it.