All
I've got a couple of 66 big cars which I've converted to 15" from 14" wheels and larger outside diameter tires.

Now my speedometers read a bit low -- maybe 2-4 mph under what they should be.

Have any of you tried the lo-buck route of swapping in later year speedometer parts in the earlier cases on the theory that in 1970 or 1971 the wheel and tire sizes went to 15" so the speedo heads would be calibrated differently from the factory?

I got a quote for like $300-400 to recalibrate. Frankly for that price I can live with a little inaccuracy. Also the guy wanted to add some kind of pretty obviously non-factory adaptor. If I could get boneyard parts and swap 'em in for like $20-50 bucks and a couple of hours of mechano fun, it might be worth it.

In 1965-66, you can disassemble the speedo heads pretty easily. They're just pressed together. What I don't know is if later years have more security to stop mileage readjusting or if the mechanicals changed a lot from 65 to 1970 or 1971 when they went to 15"'s.

In case it helps my speedos read "726 rev's per mile" or similar on the casing. Seems like a clue, but maybe not. I'm guessing the later year ones would be lower rev's per mile, but I don't know since I've never looked.

Thanks for any guidance you can offer. Reasonable vendor suggestions most welcome too. Sorry if this is the wrong part of the site to post this, I couldn't figure out quite where to put it.

 

2 to 4 I could live with my 72 cutlass has 205 70r14 on it and I am off by over 15 mph at 55 mph was thinking putting taller tires on would bring it up a bit. guess i need a new speedo on the old yeller beast.

 

http://1010tires.com/TireSizeCalculator.asp

and speedos are NEVER off by a mile per hour, they are always off by a percentage

 

Wow i would have to put on 50s to get close so I guess need calibration.