I have a 78 Grand Marquis. What is the correct tires size for this tank? The door specifies an LR78/15

Michael Tutty

Asked by Michael Jan 27, 2013 at 01:53 AM about the Mercury Grand Marquis

Question type: Maintenance & Repair

11 Answers

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According to CrownVic,com "Serving the Panther Platform Enthusiast"... 215/70/15 sounds a bit narrow but that's what they say there

Not Active

However, that does not agree with the alpha numeric LR78/15, the conversion does not work from what's on you door Michael, the substitute for LR78 is 235 at the smallest width, I don't know if the dude on the site knows what he was talking about, I did think 215 sounded a bit narrow

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http://tireguides.com/TireTips/TireDocument/4 showed me that, and Michael, you know about these things, I can tell from your posts you are extremely well informed and intelligent .....whassup with this, a trick question?

Julian Vorgic

After looking this tire size on the chart, this is what you need P235/75Rl5, P245/70Rl5, P255/65Rl5, P265/60Rl5, P295/50Rl5. For a standard tire, go with the 235/75/15 but, 255/60/15 would look very cool on your car. With '78 being the last year for the ford/merc barge body, some people mix it up with the newly introduced panther platform. Panthers use a 215/70/15 tire as standard size.

Michael Tutty

I put the 78 on the road in a hurry. I was told when I bought the car for someone else, that the correct conversion was a 235/75R15. That is what it has on it now. They have the speedo out by about 20% which is how I know they are the incorrect size. I think it needs a 235/70R15 instead, and I will soon need to buy summer tires, so thought I would ask, in case anyone knows what is the correct size for the 78, which is the last of the very big Grand Marquis sedans. I know that the later ones used a 215/70R15, which is undersized. On them I always used a 225/70, and preferably H rated, which is the spec for the Police Package cars. This is the first of the very big, pre-panther, full sized Fords I've had. It will NOT be the last!

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The only thing is the difference between 70 and 75R is not 20%, it's millimeters so that would be, oh crap i can't convert it offhand but 5mm of 70 is sure not 20%

Michael Tutty

The difference between the 70 series and the 75 series is 5% of 235mms, which is 11.75 mm in the diameter of the tire. That makes a greater difference in the overall radius of the tire, which is what determines the reading of the speedometer. In terms of over all height, there is a difference of over an inch and that will affect the speedo for sure.

Not Active

Ok, got it. Call 11.75 1/2 inch...actually 1/2" is 12.7mm so double the radius, you have diameter, just a smidgen under an inch. Whew***


If you have a 1978 Grand Marquis, That is NOT the panther platform which came out in 1979. You have the older style big car (nice riding boat) and you should have 225/75R-15 as original equipment. tires on it. A 235 70R-15 would be the same as a 225/75R-15 . MrBlueOval.

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Michael Tutty

Thank you, MrBlueOval. I am more than familiar with the Panther cars. I can get Toyota Corolla gas mileage out of one. Bertha is my first giant, and I was obviously given bum advice at the tire shop when I got the snow tires. I had a feeling I needed a 225/75 or 235/70, but wanted information not guesses and feelings. I am wondering how far I can go with fuel economy out of my 5,000 pound behemoth.

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According to Edmunds, 9 /19hghway

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