F100 tire size

[BigBlueIron]

While it is FE powered figured this was a better spot for this question.

What is the factory tire height for a 1974 F100. 390,C6 and I believe 3.73 gear. Just picked up a set of old turbine wheels 15x8.5 want to keep factory diameter for the speedometer. Cannot find a reliable source searching the webs, thought maybe someone here might know. 

[frnkeore]

I'm not sure that it will be any help but, I have 3.73 gears in my '72 PU. I have never ran factory size tires on any thing I have but, I ran 32" tires on that truck and my speedo was 10% off so. 32 divided by 1.1 is 29 and that's what I think the factory tires would measure.

I never changed my drive gear, as adding 10% to my speedo reading, was easy.

Edit:
I checked my MPH with a stop watch on mile markers, at least 50 times. Not much else to do on long trips.

[C8OZ]

Someone else will need to translate this into modern tire size language, but...

http://www.oldcarbrochures.com/static/NA/FMC%20Trucks-Vans/1974%20Trucks/1974%20Ford%20Pickups%20Brochure%20(Rev)/b_1974%20Ford%20Pickups%20(Rev)-16.jpg

[thatdarncat]

Yeah, so had the same thing to post. From the 1974 Ford Pickup brochure G78-15 would have been the standard tire, but there were optional tires available ( not specified ) as noted on the brochure. You’d probably need to find an old tire company catalog to see some actual spec sizes. Some things to consider beyond that though - the brand of tire you got on your new pickup usually could have been any of the major tire brands, unless the model specified something specific. For example a new Lincoln may have specified a Michelin tire. So even when new there could have been small variations in actual diameter, even if the size & type was the same. Also, it was pretty rare back then for the speedometer reading to be the actual speed anyway. Magazine road tests used to test the speedometer error and print the results, they were rarely close. Most vehicles of the last 20 years seem much more accurate, as long as the tire size hasn’t been changed. So even picking tires of the same exact size as new in ‘74 won’t guarantee you an accurate speedometer reading. My opinion is pick the tires you want and adjust the speedo driven gear as needed, they’re relatively easy to change and inexpensive. With the speedometer phone apps now it’s easy to get an accurate reading to compare to and adjust as needed.

[frnkeore]

There are two ways that you can check your speed accurately. As I edited above, with a stop watch and GPS, on your phone.

For the stop watch, if you know math, you just set your speedo at any MPH, hold it steady and time between mile markers. Most that I have used (West Coast) will have 5, in a row. 5 is more accurate but, just one will be close enough.

For convenience, we'll use 60 mph. Set your speedo on 60, hit the stop watch at the marker, hold it at 60 for all 5 miles. It takes 5 min, to go 5 miles. Covert your min and seconds, in to all seconds, divide that, into 300 (5 min = 300 sec) then multiply times 60 MPH and you have your exact speed. For one mile it will 60 sec, divided by measured time.

i.e. 4min 33 sec = 273 sec, 300/273 = 1.0989 X 60 mph = 65.93 mph. For 1 mile, 60 sec / 53 sec = 1.132. 1.132 x 60 mph = 67.92 mph

You can do it with feet per sec, too (lots of time to think when you on a lot trip).

Measure your time, in seconds, divide into 5280 ft (or multiples of that) that gives you the number of FPS, you where traveling. There is 1.46666 (1.47 if you like) FPS in one MPH.

i.e. 5280 / 53 = 99.623 fps, 99.623 / 1.46666 = 67.92 mph

[BigBlueIron]

Good reminder that those old speedometers aren't accurate at all speeds. I'll do a little driving with the current tire at 55 (speed limit here in IA) and see how close it is and adjust tire size to compensate. I'll use my phone for gps and grab my tom tom for comparison.