Possible to swap big car speedo head parts for improved accuracy with 15" wheels?

cfair · July 15th, 2009, 07:15 AM

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 miles per hour 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.

72 cutlass455 · July 21st, 2009, 10:08 AM

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

Redog · July 21st, 2009, 10:36 AM

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

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

72 cutlass455 · July 21st, 2009, 11:09 AM

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

July 15th, 2009, 07:15 AM	#1 (permalink)
cfair Registered User Join Date: May 2009 Posts: 44	Possible to swap big car speedo head parts for improved accuracy with 15" wheels? 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 miles per hour 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.

July 21st, 2009, 10:36 AM	#3 (permalink)
Redog Alero Chapter Secretary Join Date: Aug 2006 Location: Newtown, PA Posts: 1,145	http://1010tires.com/TireSizeCalculator.asp and speedos are NEVER off by a mile per hour, they are always off by a percentage __________________ Me 2000 Alero running 14's 2005 Nissan Altima (her's) 1967 Olds 330 motor sitting on a stand 1985 Delta 88 w/1968 350 rocket ( MURDERED )

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July 21st, 2009, 10:08 AM	#2 (permalink)
72 cutlass455 Registered User Join Date: May 2009 Location: Moreno Vally CA Posts: 142	2 to 4 I could live with my 72 cutlass has 205 70r14 on it and I am off by over 15 miles per hour at 55 miles per hour was thinking putting taller tires on would bring it up a bit. guess i need a new speedo on the old yeller beast.

July 21st, 2009, 11:09 AM	#4 (permalink)
72 cutlass455 Registered User Join Date: May 2009 Location: Moreno Vally CA Posts: 142	Wow i would have to put on 50s to get close so I guess need calibration.