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

8,415

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

Question type: Maintenance & Repair

11 Answers

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

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

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?

245

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.

8,415

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!

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%

8,415

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.

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***

6,465

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.

2 of 2 people found this helpful.
Best Answer
8,415

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.

According to Edmunds, 9 /19hghway

Your Answer

Add photo

Related Questions

Grand Marquis

Looking for a Used Grand Marquis in your area?

CarGurus has 1,804 nationwide Grand Marquis listings starting at $2,250.

ZIP:

Search Mercury Grand Marquis Questions

Mercury Grand Marquis Experts

#1 Tracy Hooks
Tracy Hooks
Reputation 5,450
#2 kbro
kbro
Reputation 2,060
#3 Js08016
Js08016
Reputation 1,740
View All

Related Models For Sale

Used Ford Crown Victoria
12 Great Deals out of 1,069 listings starting at $2,995
Used Lincoln Town Car
32 Great Deals out of 1,820 listings starting at $1,750
Used Chevrolet Impala
521 Great Deals out of 31,725 listings starting at $1,450
Used Mercury Marauder
25 listings starting at $7,300
Used Ford Mustang
472 Great Deals out of 45,735 listings starting at $1,999
Used Dodge Charger
407 Great Deals out of 28,172 listings starting at $1,988
Used Ford Taurus
249 Great Deals out of 20,433 listings starting at $699
Used Ford F-150
2,194 Great Deals out of 206,316 listings starting at $1,499
Used Chevrolet Tahoe
550 Great Deals out of 29,235 listings starting at $1,699
Used Cadillac DTS
58 Great Deals out of 1,630 listings starting at $1,995
Used Cadillac DeVille
7 Great Deals out of 1,640 listings starting at $1,999
Used Chrysler 300
415 Great Deals out of 19,932 listings starting at $1,000
Used Chevrolet Silverado 1500
1,718 Great Deals out of 175,751 listings starting at $1,250
Used Chevrolet Camaro
225 Great Deals out of 30,943 listings starting at $1,217
Used Buick LaCrosse
226 Great Deals out of 21,522 listings starting at $1,100

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use.