Here is a link to an interesting tool that can help you understand the overall changes with different tire sizes.

http://www.rimsntires.com/specspro.jsp

I think that you want a wheel and tire package that gives you the desired overall tire diameter for apperance and speedo calbration and/or effective gear ratio. I'd say you do not want to lower the vehicle by using low profile tires because there is not much room for that in tire sidewall. If you want to lower the car do that with suspension set up, select the tire based on what looks good/right in the wheel well. I can't recommend a width and offset to avoid rubbing on steering and suspension components, but I'm sure I've read threads on FMF that may apply to the 61.

FMF is Ford Muscle Forum... in other words the competition  I have 17" rims on the rear of my 62 and 16" rims on the front,  I took some measurements but they will not be exact as rims are officially measure inside to inside and I measured outside to outside as best as I could.   I too have disc brakes all the way around.   The back tires are quite wide, the rubber is about 10" and the rims are about 9" outside to outside so I am thinking that are 8" X 17".  Looks like the backspacing on my rims is 4".  The fronts are looking like 7" X 16"  also 4" back spacing.  I could have gone wider in the front with out issue, but the back ones are so wide that they are a little difficult to get on and off the car.  There is about 1" of space between the walls of the tires and the springs and about 1" between the walls of the tires and the fender lips.  Seams like a lot but but the fender hangs down so low on our cars that the tires are difficult to get on and off.  Hope that helps.

Sounds like our cars are very similar, R&P, 4-wheel discs, hydro boost, and so on  

dan_6776 wrote:

Thanks for the link...I checked it out. I switched to disc brakes all around, would that make any difference? I am guessing that the information provided on that site is based on OEM(drum brakes)?
Currently I have 225 60 R 16 tires on factory aluminum Mercury Grand Marquis rims and in order to get those on the rear, I have to let the axle hang down as far as it will go and jack up the body of the car. I am really concerned about how much larger I can go?
Not familiar with FMF...please explain.
Thanks

That tool on the rimsandtires site does not consider the brakes, or anything eles, as far as I can see. I just see it as a tool to show you what the tire sizes measure to and it may help you slect rims when you know how much back space and offset you need. I primarily use it to find alternate tire sizes that are similar in overall diameter to the OE size, working backwards from that point.

It sounds like your vintage of car has a low rear wheel arch. If you want to go real wide on the rear it would take narrowing the rear axle to get the wheels in there. It would seem that getting the rear wheels on will be the determining factor on your width selection, not steering component and suspension interference issues. I've seen a fancy adjustable aluminum jig on TV in the past that lets you test wheel diameter, offset, backspace and tire size on you car. I'm sure they are expensive pieces, but I also seem to recall seeing one home made out of plywood as a DIY option.

Check this. http://www.thewheelfit.com/home.html

Last edited by 70XL (May 1, 2015 2:39 pm)