Strange such a difference.

Put 45 psi in them and see if the RPM drops at all. Maybe that 14" tire you have really sags a ton under load. Could be a comfort sidewall.

The thing that is hard to visualize is the tire tread actually lays down continuously like a tank track, and a soft tire does this more, but should lessen at speed as the tire grows.

Your numbers point more to a 3.36 gear, no? I've forgotten, was your Strat a 283/PG or 3 speed manual?

 

4SPEED427 wrote:

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Has anyone taking the time to measure the height of a tire mounted on a rim sitting on the shelf versus when it's actually mounted on the car with the weight of the car on it?

 

I'm playing around deciding what 15-in tire I want to put on my 66. It currently has 225 70 14 radials on it. I'm going to go to 15 inch so I can put on a taller tire. I'm looking at a 215 75 15.

 

The current 14" is listed on the websites as being 26.4" in tall. When I measure it on the shelf that is a very accurate measurement. However when I have it on the car with the weight on it that tire effectively becomes 23" in diameter. When I measure from the floor to the center it's 11.5", so a 23" diameter.  That's with 32 lb of air in the tire. That is a huge difference and I'm thinking now that my factory tach is more accurate than I thought. The chart said I should be turning 2346 RPM at 60 MPH with that tire and 3.07 gears. When I do the math  for what is actually the tire height (23'') when it's installed on the car with the weight on it then 2693 RPM is what it should be turning, pretty much exactly what the tach is showing. I'm amazed at the difference. It certainly has me rethinking what tire I will put on these 15-in wheels. 

 

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 It would still measure 13.2 inches from the center to the top of the tire plus the 11.5 so the diameter would still be 25.7 inches? The tire comparison chart I looked up indicates rpm calculated using circumference. Not sure if they allow for the distortion factor. Just my opinion but 215/75/15 have quite a skinny footprint for the big block power you have access to? Regardless your ride is worthy of some bigger meats!

I have almost every bias ply size mounted on 14 inch rims if you want to do some measurements. Most are on my cars for winter storage. You would be amazed how tall those old g78's and 6.50's 7.50's are even when sitting on a car.

LeeRoy wrote:

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 It would still measure 13.2 inches from the center to the top of the tire plus the 11.5 so the diameter would still be 25.7 inches? The tire comparison chart I looked up indicates rpm calculated using circumference. Not sure if they allow for the distortion factor. Just my opinion but 215/75/15 have quite a skinny footprint for the big block power you have access to? Regardless your ride is worthy of some bigger meats!

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 I know what you are saying but to me the centre to top measurement won't help me though, I want to get an RPM drop on the highway.

Just remember, the original tires on this car (if it were an original big block car) were pretty scrawny and that's the look I'm going for, to a point.

The 2 issues you are dealing with is looks- a bigger, wider tire fills the well better front and rear- and circumference- on the rears to change the rpm. I would suggest you ignore trying to compare an old bias to a new radial and trying to allow for that sag at the bottom and just decide which look and rpm you are shooting for? There are too many variables/guesses involved in using a measuring tape and trying to get a really precise accurate result. The only very accurate way is to install a bias tire on the back and a radial of the size you want and check the rpm. There are also small variations in same tire size in different brands? You may do all these calculations.

If you are running radials now, compare their advertised size to the advertised size of what you want to buy and from that, calculate your difference in rpm, imho that is as close as you will get on what your rpm will be.

Here's a low priced basic 235/70/15 blackwall touring that rates well. 27.9" tall. Significant enough of a tire that you'll no longer look like Magilla Gorilla on roller-skates.

https://www.tirerack.com/tires/tires.jsp?tireMake=Hankook&tireModel=Kinergy+ST&partnum=37TR5H735&fromSurvey=true

 

Would give you a calculated 2218 @ 60, certainly a reasonable 60 mph rpm.

https://purperformance.com/p-29669-rpm-calculator.html

 

 

I have 205 75 15 on my winter car. I can measure if you want. Also. I run 235 70 15 on the back of both my wagons.

Yup. I agree. Good size

4SPEED427 wrote:

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beaumontguru wrote:

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I have 205 75 15 on my winter car. I can measure if you want. Also. I run 235 70 15 on the back of both my wagons.

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 Thanks but I have pretty much made up my mind that the 225 75R 15 is the winner. I like the height of it, like the width of it too. It doesn't look too much wider than the original tire. I'm trying to steer clear of the "hot rod" look and keep the "Grandpa" look. I think it's the best compromise. 

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 I didnt know you were a grandpa? 427 and a 4spd, makes for one cool grandpa!

Nobody mentioned that going from 14" to 15" is going to throw out your speedo & odometer readings!!! you'll likely end up changing the pinion gear on cable end or if really out both gears [ 1 on output shaft] or use GPS for speedo??

I have a GPS and also a selection of speedometer driven gears. I suspect if I drop down to a driven gear with one less tooth it will be back to accurate again but I haven't actually done the math yet to see. And my speedo head is actually wonky anyway! The odometer is currently bang on but the speedo is weird. If I take what it shows and double it I'm almost exactly on for km/hr. There's something wrong in the head but I'm not willing to rip the dash out to repair it, I just live with it and drive by the tach and GPS.

LOL, you better believe there won't be whitewalls! If I can ever find a nice plug for the antenna hole in the fender the radio is coming out and that delete plate I got from you is going in. I want no options other than engine, trans, diff and chassis options. NO frills.