RPMs Tire Size conundrum

[Chiclassics]

Autobots, I got a tricky situation I cant figure out, need some expertise

3.5 gears in rear
C4 automatic
33inch tires

I am getting RPMs 1440 - 26mph, 2200 - 50mph, 2800-3000 - 65-70mph.

Based on calcs for 33 and 3.5 I think at 70mph i should not exceed 2500.

So is my tach wrong? What else is off that is causing above. In idle my tach reads 1000 and when in I put in drive 800 at idle.

 

[Broncobowsher]

Is the speedometer correct? Unless it was changed from stock, it is wrong. And I don't think there is even a gear that will work with that tall of a tire and axle ratio.
Is the gearing confirmed to be 3.50, or is that just what the glove box tag says it was built with?
Tachometer error is possible as well.

 

[AZ73]

I'm not going to post all the math here, but you're not doing 70, you're going roughly 85 mph when your speedometer says 70.

Your speedometer is set for 195/ R15 tires, which are about 26.5". Now you're running 33 inch tires. You only need your 2500 RPM based on the math to reach 70 MPH. Because your 33" tire has LESS rotations per mile, your speedometer should read 55-56 when you're actually going 70. And it's reading 70 when you're going 85.

If you need the actual math, ask, but it seems like you already know how to figure it out.

And pretty impressed you can get your brick up to 85!

 

[Chiclassics]

Gears are 3.5 i did the standard jack up rear and turn the tire thing. Its posi so one full tire turn resulted in 3.5 turns of rear drive shaft.

My speedometer is not yet connected so I used a Phone App...i know its pretty accurate because i was on fwy on a good moving traffic day and i was on second lane from right doing 50 then up to 65-70. Local drive when I hit 40 close to road traffic on a Sunday i see my RPM is closing on 2000 already. So im pretty sure speed is correct.

Can tachometers be off by 300-500rpm?

Overall drivability is great though no issues

I used this Engine RPM = (MPH × Rear Gear Ratio × 336) / Tire Diameter

 

[Master Chief]

What is your wheel width? 8” or 10”? My 33” Goodyears are 31” tall on a 10” wide wheel.

 

[AZ73]

WAY too many variables.

Your tach could be off, your app could be off, your C4 could be slipping (although it should lock at highway speeds), your tires aren't really 33" in real life situation (they don't touch the road at a point, they're flat at contact) so, if you have the correct air pressure in them, you should use 32". The tires themselves that say 33" are usually 32.5". Take off the 0.5" for the reduction of diameter at road contact and you're down to 32". Standard Bronco differential ratio is 3.54. This all adds up.

 

[Chiclassics]

I see ill measure tomorrow whats my effective tire height.

@Master Chief my wheels are 10inch I believe as my tires are 12.5 width and i see 1-1.5 bulge from the wheel edges so guessing.

 

[El Kabong]

your tires aren't really 33" in real life situation (they don't touch the road at a point, they're flat at contact) so, if you have the correct air pressure in them, you should use 32". The tires themselves that say 33" are usually 32.5". Take off the 0.5" for the reduction of diameter at road contact and you're down to 32".

I've heard that. But I've also heard that it's the circumference of the tire that matters. So even though the tire is flat at the bottom, the full circumference based on a round tire travels it's full distance per every revolution of the tire.

Which is correct? Does the tread of the contact patch get shorter when it's on the ground? I don't know.


I do like the grimmjeeper calculator. Enter your info & see the results. And it has 2 tables side by side to compare variables.
https://www.grimmjeeper.com/gears.html

 

[DirtDonk]

Your tach could be off

Very easily!

your app could be off

Is this possible? I would think the GPS apps would be extremely accurate.

your C4 could be slipping

Almost guaranteed to be 200 to 400 RPM higher than a one-to-one manual transmission.

although it should lock at highway speeds

Don’t think so. To my knowledge old torque converters might get close to zero slip, but still slip.
An actual “lockup” converter would’ve come much later. Did they even have lockup converters that are not computer controlled?
I don’t know so I suppose it’s possible.

 

[Madgyver]

You should be 4:11 with 33"s and C4. stock 302? it's not in the right powerband. Many get away with it because upsizing tire size is easy, replacing gear ratios is another story.
With a torguey 351W, 3.50 gears and 33"s works but going to 4.11 and you will notice a big difference.
My opinion and experience.

 

[Broncobowsher]

Tachometer error, very possible. Especially with a basic analog meter.
Error on tire size, there will be some. The nerdy people will actually go with the revolutions per mile spec for the tire. If you look up the tire specs for your exact tire, it should be published. Even the same size and the same brand, a different tread pattern will be a different revolutions per mile. I recall it is even different even in the same tread pattern and size just by changing load ratings.

Now for convertor slip. That will be a lot of it. That tall of a tire, that tall of a gear ratio. You are working the convertor hard, it will be slipping a lot, and as it slips it makes heat. The gear ratio through the transmission is 1:1. but with that load the convertor isn't 1:1. Big tires and tall gears are a good way to cook a C4.

As Mad stated, 4.11s are a great choice for 33s and a non-overdrive transmission. Just driving it around you will think you just bolted a supercharger on the engine. The transmission will get nice and happy as it isn't stressed. The engine will turn a higher RPM, but also a higher vacuum. Less stressed.

 

[SLOWnEASY]

If I remember correctly the C5 is just a C4 with a lockup converter. So with a C4 you definitely have some slip at highway speeds.

 

[AZ73]

I've heard that. But I've also heard that it's the circumference of the tire that matters. So even though the tire is flat at the bottom, the full circumference based on a round tire travels it's full distance per every revolution of the tire.

Which is correct? Does the tread of the contact patch get shorter when it's on the ground? I don't know.


I do like the grimmjeeper calculator. Enter your info & see the results. And it has 2 tables side by side to compare variables.
https://www.grimmjeeper.com/gears.html

Not true. It's known as the static load circumference (vs the catalog circumference). The outside circumference will be about 2% lower. Add in the fact that a 33" tire isn't really 33" to begin with and you can be up to 4-5% smaller on the outside circumference. Hard to draw here but the bottom of the tire doesn't travel in an arc where it touches the road. More of a straight line, which is shorter than an arc. Imagine a dot painted on the side of a tire as it's spinning. It would draw a circular pattern except at the bottom where it would draw a straight line. The shortest distance between 2 points is a straight line. not an arc. The additional "circumference" gets squeezed out the sides of the tire. That's why if you look at a tire where it touched the ground while sitting there, it's bulged out.

 

[Chiclassics]

OK. My tires measured 31. I think that gets me closer to 55mph 2100 RPM. My app measured 50, so im close. Thanks guys. I still think my tach might be off a bit..and well noted on the 4;11s just trying to avoid gear changes right now . Also I like 3.5s on highways, so might go towards tire direction if needed., i might downgrade to 32x10.5s , i have been monitoring my transmission its not overheating but i might just do the tires once i drive it around on flat surfaces for few months.

 

[toddz69]

V Did they even have lockup converters that are not computer controlled?
I don’t know so I suppose it’s possible.

The AOD and 700R4 both have lockup converters that are not computer-controlled.

Todd Z.

 

[DirtDonk]

Cool. Thanks for that Todd.
Can anything be retrofitted to a C4 to turn it into a lockup?

 

[Yeller]

Another loaded tire height vs revolutions per mile thread…… always amazed at the misinformation all over the internet, and the regurgitation of it. Guess it’s time to document a test since physics are difficult and magically become variables.

 

[DirtDonk]

Have been thinking about how to do that the best way.
Maybe a light duty 15 inch tire of maybe 35 inch height, would be a good candidate. Something that would squish down a lot when air down.
Run it at full pressure, and then run it almost flat. Maybe 10 pounds.
See if the speedometer changes.
Or is there a scientific, or electronic instrument related (there’s an app for you!) kind of thing these days?

I was always one of those that said to measure the rolling radius of the tire, but just made that up on my own because it made sense in my brain.
I think it was James Roney (unless it was you?) that wrote here about the revolutions per mile part being consistent, no matter what the tires physical compression was.

But don’t leave us hanging. What’s your take on it.
We’re not (well, most of us anyway) purposely trying to spread misinformation. But if that’s the way we learned it, or that’s the way it made sense to us in younger, pre-Internet days, then that’s the way we will keep spewing it out!
So clear it up, please. I know I would appreciate it.

But I’d still love to see that test!

 

[Broncobowsher]

Cool. Thanks for that Todd.
Can anything be retrofitted to a C4 to turn it into a lockup?

No, with a maybe...
The C5 had a lockup version, but it was a centrifugal lockup. Only existed for a very short time. did not go over well. You know, the mid 80's when they were testing ideas on the market.
The other maybe, but this one doesn't make sense. A LOT of very custom work. I know of a powerglide that was custom built to do a selectable lockup converter. It was for drag racing. Loose sloppy convertor to get it going, lock it up to send the HP to the wheels on the top end. Never actually saw the car run. It was crazy one off custom parts type of stuff. Just do a 4R70W and don't use overdrive would be a lot easier at a fraction of the cost.

As for the AOD, that is one of the more bizarre lockup solutions tested on the consumer in the 80s. There was the normal convertor input to the transmission. then there was a second input through the center into the transmission that bypassed the convertor. First couple gears were normal torque convertor. Overdrive 100% bypassed the fluid coupling and was purely mechanical link to the engine. You could not get any converter action in overdrive. 3rd gear is probably the most bizarre in it is a 1:1 ratio, but the gears in the transmission turn under load. The torque from the converter went into one set of gears and the torque from the bypass went into the other set of gears. Typically in 3rd gear you lock both sides of a planetary set together and it locks the gearset up solid. But this drives the two sets from seperate sources. the torque convertor is only passing 40% of the engine torque into the gears, so even under heavy load, it is only 40% of that heavy load. 60% goes into the gears from the direct drive. This gives a little bit of converter slip in 3rd, but not as much as 1st or 2nd. It really was a inventive way of not needing to add the converter clutch and circuit into the transmission. Face it, AOD came out in 1980. To even get the extra gear of an overdrive to function was amazing at that time. to add a lockup convertor that needs a control system to run it as well, that was beyond what the OEMs could do in mass production. The C5 centrifugal lockup didn't even exist at that time. Pretty sure that was the first time Ford had ever done any convertor lockup design of any kind at that time.

 

[Chiclassics]

@DirtDonk I didnt take into consideration of 200RPMs C4 adds due to non-locking aspect. I think my #s kind of make sense now. Awesome!

Being a math geek I can tell @AZ73 is right. A tire under load especially when not at full air pressure loses to hold the formula of 2*pi*r for circumference. Essentially its not a perfect circle any more. But a circle of radius measured from wheel center to ground. So effectively lets say my 33 tires results in a circumference of 103.62 (2*pi*16.5) inches ideally, under load lets say my overall tire height is 31- i wouldnt even say thats effective diameter because top part of wheel might be 16, bottom half can be 15(due to the squish), so effectively 30. So the effective circumference becomes 2*pi*15 = 94.2 that is 90% of original circumference. So in simple terms a 10% reduction in distance travelled hence the speed. Meaning revolutions per mile will CHANGE when air is let out from tire effectively making it a smaller tire. I bet someone to test the theory though.