Significant Construction Difference between Radial Tires LR78-15 and 235X75X15?

[EAM 17806]

What are the significant differences, if any, between factory installed radial tires LR78-15 and their replacement radial tire 235X75X15 (1975-76 Deviles, etc.).  If there isn't any then the factory recommend tire air pressure recorded in those car manuals and on the glove box lid front and back reads 23psi on all fours and up to vehicle capacity 24 front and 28 in the rear tires.  Why would the factory engineers be so wrong based on most owners actual air pressure being used are 30 psi and over?  Comments and thoughts requested.    EAM

[suchan]

I recall reading somewhere that max pressure on the "letter" series radials was 32 psi. On the numeric series, you're just getting started at 32 psi; 35 psi is more of a happy place. Clearly, there are construction differences in the two series. The 235X's I had on my '71 Eldo were good to 50 psi. If you've got numeric series tires on your car, ignore the glovebox recommendations.

[35-709]

Right.  Basic radial tire construction may be similar but construction techniques, materials, and knowledge about them has, as one might expect, changed in the last 45 years and more.

An interesting note from Wikipedia --- "The first Michelin X radial tire for cars was developed in 1946 by Michelin researcher Marius Mignol.  Michelin owned the leading automaker Citroën, so it was quickly able to introduce its new design, including on the new 1948 Citroën 2CV model.  In 1952, Michelin developed a radial truck tire."

[Clewisiii]

Tangential comment on this. It reminds me of the Ford Explorer recall with the Firestone tires.  Difference in air pressure between the tire manufacturer and Ford.

[Jon S]

Realize the H78 was a 78 profile tire whereas your new tires are 75 profile. That means your car is sitting a few inches lower. I attached a picture to illustrate the difference.

[Mike Josephic CLC #3877]

What are the significant differences, if any, between factory installed radial tires LR78-15 and their replacement radial tire 235X75X15 (1975-76 Deviles, etc.).  If there isn't any then the factory recommend tire air pressure recorded in those car manuals and on the glove box lid front and back reads 23psi on all fours and up to vehicle capacity 24 front and 28 in the rear tires.  Why would the factory engineers be so wrong based on most owners actual air pressure being used are 30 psi and over?  Comments and thoughts requested.    EAM

I speculate that part of the confusion can be traced to the
introduction of radials then later to the European metric sized
tires.

Radial tires were still an option in 1973.  I factory ordered
my '73 Eldorado with radials.  However, for whatever reason,
the sticker on the door / glovebox recommended the inflation
for bias ply tires that you correctly note as 23 front and rear.

I assumed that the door stickers did not account for the
optional radial tire pressure and always kept them at 30 psi.
I never had an issue doing this.  These were LR78-15 radials
the factory. That's prior to the introduction of the metric
sized 235-75R-15.  When I replaced my original tires some
years later wit the 235 types, I kept the inflation at 30 psi.

Mike

[64\/54Cadillacking]

The 235-75R are definitely smaller in diameter compared to an L-78 bias ply.

When I first got my 64, it had some old radial 235-75-15R WW’s tires on it. The car sat lower and looked more cool IMO, but it rode like crap.

I replaced them with  L-78 bias ply fat 3 inch WW’s and the car sits more leveled and looks proper now, plus when going over bumps on the road, I barely feel them. It’s like riding on cushy pillows.

[EAM 17806]

I speculate that part of the confusion can be traced to the
introduction of radials then later to the European metric sized
tires.

Radial tires were still an option in 1973.  I factory ordered
my '73 Eldorado with radials.  However, for whatever reason,
the sticker on the door / glovebox recommended the inflation
for bias ply tires that you correctly note as 23 front and rear.

I assumed that the door stickers did not account for the
optional radial tire pressure and always kept them at 30 psi.
I never had an issue doing this.  These were LR78-15 radials
the factory. That's prior to the introduction of the metric
sized 235-75R-15.  When I replaced my original tires some
years later wit the 235 types, I kept the inflation at 30 psi.

Mike

MIKE:  The 1975-76 Deville factory tires were all radials and the air pressure recommended in the manual and glove box was as I stated above.  There wasn't any bias ply tires for these cars at that time put on at the factory so the pressure suggested was for the LR-58-15 radials.  I purchased my 76 deville  2/1/1977.  EAM

[fishnjim]

Door plate, just "recommended" tire pressure, most likely the minimum to keep the bead sealed.  Pressure will go up slightly as they warm during driving, do not want to exceed max.   Adjust for ride feel and tire wear and do not exceed the sidewall max pressure rating.   If they wear on the outside, it's too low, if wears in middle too high.   Tons of info on tires out there, just look.   I recommend to check/adjust pressure during the seasonal changes, warm to cold, cold to warm will vary a few psig.   Do a walk around at fueling or garaging for low tire(s).   Low is the worst condition for blow out.

[Eric DeVirgilis CLC# 8621]

EAM: I seem to recall tire pressures getting a significant bump on the door placard between 1978 and 1979 model year when they transitioned from the older "GR78R-15" size to the the new metric (in this case P215-75R-15 equivalent). This was due to the new "low rolling resistance" tires which seems to have coincided with the sizing change.

In this situation, I would go with the recommended pressure indicated on the tire rather than what was indicated on the door placard.

[EAM 17806]

EAM: I seem to recall tire pressures getting a significant bump on the door placard between 1978 and 1979 model year when they transitioned from the older "GR78R-15" size to the the new metric (in this case P215-75R-15 equivalent). This was due to the new "low rolling resistance" tires which seems to have coincided with the sizing change.

In this situation, I would go with the recommended pressure indicated on the tire rather than what was indicated on the door placard.

ERIC:  I use 29 psi on the front and 30 psi on the rear which is still somewhat stiff and comfortable; the pressure shown on the tires themselves is much too stiff for comfort and more of hard riding like truck tires and should never be used as psi recommendations.  EAM

[hornetball]

I would go with the recommended pressure indicated on the tire rather than what was indicated on the door placard.

There is no "recommended pressure" printed on the tire.  There is only a maximum pressure.  This is a dangerous misconception.  Furthermore, if you fill the tire to maximum pressure cold, you are likely to over-pressurize the tire as you drive.  Never fill your tire to this point.

It does make sense that recommended pressures went up about the same time that fuel efficiency and "handling" became selling points.

[Eric DeVirgilis CLC# 8621]

There is no "recommended pressure" printed on the tire.  There is only a maximum pressure.  This is a dangerous misconception.  Furthermore, if you fill the tire to maximum pressure cold, you are likely to over-pressurize the tire as you drive.  Never fill your tire to this point.

It does make sense that recommended pressures went up about the same time that fuel efficiency and "handling" became selling points.

I've never seen pressure between hot and cold vary by more than 5 PSI or so which isn't enough to cause a severe problem but I agree cold inflation to 5 PSI below the maximum is probably a good idea.

[MaR]

I just measured the OD of the LR78-15 spare that is in the trunk of my ‘74 Eldorado and it was 29.5”. The 235/75R15 tires on the other 4 wheels measure 28.5” on the OD.

[savemy67]

Hello Ev,

I concur with hornetball, that the pressure on the tire sidewall is the maximum inflation pressure, not the recommended inflation pressure.  From your most recent post, it looks like you have found the "sweet spot" for your car/tire combination.  If the tires wear well, you are probably spot-on.

Both Goodyear and Tire Rack suggest, that for a passenger car tire, a 10F change in temperature will result in about a one PSI change in tire pressure.  Keep this in mind when checking adjusting your tire pressure.

Respectfully submitted,

Christopher Winter

[TJ Hopland]

I just measured the OD of the LR78-15 spare that is in the trunk of my ‘74 Eldorado and it was 29.5”. The 235/75R15 tires on the other 4 wheels measure 28.5” on the OD.

Whats the difference in the 235's if you measure them horizontally vs vertically while loaded? 

[MaR]

Whats the difference in the 235's if you measure them horizontally vs vertically while loaded?

I probably would not get a good measurement as the spare is at least 40 years old and slightly dry rotted.

[Jon S]

Whats the difference in the 235's if you measure them horizontally vs vertically while loaded?

[MaR]

Well, I checked the loaded height of the basically new 235/75R15 vs the 40+ year old LR78-15 with almost new tread. Both tires are in the front drivers side of my '74 Eldorado and both tires are set at 30 PSI cold.

235/75R15: 27.25"
LR78-15: 27.75"

[TonyZappone #2624]

The Diamond Back radials on my '47 were wider than the stock tires.  I had to take each skirt and grind some stock from the skirt, no problem