The original 14" rims now have snows on them. I need new summer tires to put on the 15" wheels (from 740). Old tires not worn out, but rubber is cracked and one tread starting to come apart (I said old!).

I want large diameter, but not so much the width (daily hauler, not sporty).

When I put them side by side, the new last year 14" snows(185/75) look like 1" bigger diameter than old 15" (185/65). Is this just wear or real difference?

What do you recommend?

I'm running 195R65x15 on both my 86 245 and my 93 940. The speedos are accurate. I run 195R60x15s on my 88 760 and 205R60x15s on my project 86 244 with no rubbing.

Regards,
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Will Dallas, www.willdallas.us, www.willdallas.org, 86 245 DL 205K miles, 93 940 250K miles, 88 765 GLE 149K miles, 87 745 , 86 244 DL 200K Miles

Tire calculations notwithstanding, when the 240s were fitted with 15 inch wheels, which is just on the 80 242GT and the 81-85 GLT/Turbos, they all received 195/60-15s.

This is the same size used on the 740GL, 740T, and 900 series when fitted with 15 inch wheels. 185/65-15 is usually used just for snow tire fitment.

I've always used this size, and have them on my 82 245T, and the summer wheels for my regular 240.

For a normal 245, that came with 185R14 or similar standard tires, a 195/65 or 205/60 is probably closer, and there should be no clearance problems with that. Wider tires may rub inside the rear fenders, hitting the inside of the tire on the cargo floor.
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Rob Bareiss, New London CT ::: Roterande Fläkt Och Drivremmar!

First, re: "...I need new summer tires to put on the 15" wheels (from 740).... I want large diameter, but not so much the width (daily hauler, not sporty)...."

The larger the diameter, the more your speedometer will under-report your speed, and your odometer (similarly) your elapsed mileage.
Also, the section width (185, 195, 205, etc.) of your tire is limited by the wheels' width -- you didn't mention what 15" 740 wheel you have, the standard wheel or one of the wider alloys?

re: "...When I put them side by side, the new last year 14" snows(185/75) look like 1" bigger diameter than old 15" (185/65). Is this just wear or real difference?..."

Not surprising.
Your 185/75-14 has a nominal overall diameter of:
(185 * 0.75 * 2) + (14 * 25.4) = 633.1 mm or 24.93 in.

The (185 * .75) is the section height of each sidewall based on its section width, 185 mm, and its aspect ratio, 75.
That time 2 is because the diameter spans two sidewalls.
The (14 * 25.4) is the conversion of the 14" wheel to equivalent mm.

In contrast, your 185/65-15 tire is a "low aspect ratio" tire, meaning it has shorter sidewalls:
(185 * 0.65 * 2) + (15 * 25.4) = 621.5 mm or 24.4 in.

Thus, the different sizes account for about half and inch, and the other half inch that you observed may come from the deeper lugs of the snow tires' tread.

I recommend you spend some time playing with the Tire Size Calculator

There are about 6 sizes that will work on your car. You need to think about the tradeoffs.

If it was me, for my driving style and local conditions, I'd do 195/65R15.
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'73 142, '75 242, '75 245, '80 245, '86 244, '87 745T