What size tires do you all run on your 1970s Delta 88s?

Glove box says H78 or HR78 I believe. The car has 225/75R15 on it right now, and the speedo is about 5mph under actual speed right now... I was thinking when I replace them I'd try 235/75R15 instead.

I can't see the tiny extra width being a problem?

Thanks.

 

The H78x15 series had a diameter of 28.4 inches. The 225/75/15 has a diameter of 28.3 inches, so that should not make any noticable difference to your speedo reading a'la factory, as long as your rear end has not changed. The 235/75/15 has a diameter of 28.9 inches, which would reduce your speedo reading by about 2% from factory at any driven speed.
Also, note that bias tires will deform at a different rate than radials as you increase speed, so the diameters above are approximate.

Check out this post:
Speedometer changes with rear end swap

it has stuff near the end about tires sizes.

 

225-75-15 should be correct.

 

I put 235/75x15's on my '73, because the reference I used put them a hair closer to the "correct" size than the 225's (I suspect the exact, practical, diameter is not a precise measurement and depends on load, air pressure, make and model of tire, etc.).

They look right, and my speedo is dead on.

235/70x15's are another possibility, with very minimal difference in diameter.

- Eric