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speedometer calibration
Posted by: DOUG WARD ()
Date: July 09, 2021 09:28PM

Since I have my instrument panel taken all apart, It is a good time to correct an incorrect speedometer reading.
When I was restoring my 65 Corsa, One of the things I " farmed out " was a speedometer rebuild & calibration. At that time, the only tires I had were Michelin P 175 /70/ R 13. They are 22" in diameter. The speedometer was set to 860 revs / mile. With that setting, my road speed was spot on, according to GPS.
I now am running front tire size 185 / 65 / R 14, & they are 23 1/2" diameter. My speedometer now reads about 55 MPH at actual 65 MPH, since the tires are turning slower than if they were only 22" dia..
I don't know how to calculate to a new " revs per mile ". The speedo shop will probably know how to adjust the instrument but in case He doesn't, can anyone help me out here ?

Doug Ward
Orange, Calif.
65 Corsa 180, 64 Monza convertible, 61 Lakewood, 61 Monza


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Re: speedometer calibration
Posted by: RobertC ()
Date: July 09, 2021 10:02PM

Revs per mile is in the specs section of a tire.

The web site of your tire manufacturer will have the info.

Possibly you can find the info on a site for Tire Rack?

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Re: speedometer calibration
Posted by: American Mel ()
Date: July 09, 2021 10:14PM

Most "Tire Size Calculators" will give you that info.
Try this one.

> > > Tire Size Calculator < < <

I know that different tire makers end up with different actual sizes for the same tire number, but are you sure about what you have stated?
Using that calculator, it says that

175/70/R13=22.6" and 891 revs/mile
and
185/65/R14=23.5" and 860 revs/mile

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
WA. state, 1 mile south of the Canadian border, I am not at the end of the world, but you can see it from here.

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Re: speedometer calibration
Posted by: jjohnsonjo ()
Date: July 10, 2021 05:16AM

The revs per mile charts are fairly close, but if I were going to pay a speedo shop, I would paint some marks on the tires and roll them a few turns, then do you own math.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: speedometer calibration
Posted by: Caraholic4life ()
Date: July 10, 2021 05:52AM

Many years ago I worked in a business that also had a guy that rebuilt and calibrated speedometers but even back then there was not much demand for the service.

I seem to recall having the understanding that the calibration was done using some sort of magnetic device which might be difficult to recreate without having the experience and or equipment available. I could be completely mis-remembering this so do not take what I have said as an absolute.

1962 95 F.C. Van
1964 Greenbrier Deluxe
1965 Monza Coupe
1965 Monza Convertible
Westminster, Maryland

MID ENG enthusiast &
prior Kelmark owner.

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Re: speedometer calibration
Posted by: 1STLM ()
Date: July 10, 2021 06:04AM

After having my speedometer rebuilt it was inaccurate also. The rebuild shop told me to document the speedometer reading and a GPS reading about every 20 MPH. I sent the speedometer back to them, along with the comparisons, and they readjusted it. It is now spot on.

Brice

66 Corsa Turbo Convertible

Bozeman, Montana

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Re: speedometer calibration
Posted by: 66vairman ()
Date: July 10, 2021 09:13AM

jjohnsonjo Wrote:
-------------------------------------------------------
> The revs per mile charts are fairly close, but if I were going to pay a speedo shop, I would paint some marks on the tires and roll them a few turns, then do you own math.

Best advise so far!

The Corvair LM speedometer was factory calibrated to 825 revolutions per mile which meant the speedometer typically indicated 60 MPH at about 57 MPH - typical back then. When tire size was increased to 7.00 X 13 the indicated reading was about 58-59 MPH (you can look at the old magazine road tests to verify).

I rebuiltmy Corsa speedometers and took them to an old time speedometer shop for calibration. There are TWO adjustments. First the magnet is "reset" for a stronger field then it is "de-gaused" with minor bursts of EMF AC until the needle indicates 60 MPH at 825 revolutions. Then the speed is checked at other indications (usually 30 - 90 MPH) and if it is NOT LINEAR then the needle spring has to be reset (BTW - NEVER ADJUST THE FACTORY SET NEEDLE SPRING as it increased the time and COST to get it reset properly - BTDT) and the magnetic calibration repeated. This can take numerous tries and I got a lecture from the speedomter technician to "NOT TOUCH THE NEEDLE RETURN SPRING".

Finally -- there is only so much leeway in tire size before the speedometer can't be calibrated to be linear (proper speed indication at different speeds). If the tire size change is great enough the speedometer shop can build a mini-gear box to attach between the cable and speedometer. That said it's getting very difficult to find someone with the parts to do that now.

Just me, but I prefer to find the correct tire size since tires that are too small tend to result in a lot of wasted engine RPM on the highway.

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Re: speedometer calibration
Posted by: DOUG WARD ()
Date: July 13, 2021 03:33PM

It's true that an undersized tire will cost more fuel & the engine wear to be accelerated, BUT, that is only if those undersized tires are in the rear. Smaller tires on the front of a late model will have no bearing on fuel, or engine wear. The speedometer is driven from the left front wheel.

Doug Ward
Orange, Calif.
65 Corsa 180, 64 Monza convertible, 61 Lakewood, 61 Monza


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Re: speedometer calibration
Posted by: jimc ()
Date: July 13, 2021 05:34PM

You can buy a speedo gear reduction box rather than having a speedo shop build one.

Ecklers, Ebay and Summit, among others, sell the units.

My speedo is off by 6-8 mph and linear, (shows slower than actual) and I am running 205/50-15 kuhmos. I use GPS on phone until I get a gear reduction box or change tires.

Jimc
1965 Corsa Coupe, two owner history


HACOA Member
CORSA 2017 Concours Senior Gold Award
AACA 2015 Award

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Re: speedometer calibration
Posted by: alan_smithee ()
Date: July 14, 2021 09:15AM

I'm of the just use an old cell phone with a GPS app to get your real speed and save the $$ on the speedometer. Unless of course you wanna go the route of one of those GPS speedometer adapter thingies that you hook up to your speedometer. I think they are what...like $300? Here is the thread here where it is discussed Speedhut GPS Device Thread



jimc Wrote:
-------------------------------------------------------
> You can buy a speedo gear reduction box rather than having a speedo shop build one.
>
> Ecklers, Ebay and Summit, among others, sell the units.
>
> My speedo is off by 6-8 mph and linear, (shows slower than actual) and I am running 205/50-15 kuhmos. I use GPS on phone until I get a gear reduction box or change tires.

--------------------------------
Was a 1962 Monza Convertible
Now a 1964 Spyder Clone/Tribute
Santa Cruz/Northern CA

--------------------------------




Edited 1 time(s). Last edit at 07/14/2021 09:19AM by alan_smithee.

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Re: speedometer calibration
Posted by: wittsend ()
Date: July 14, 2021 10:16AM

In Fred Phun's book "How to Make Your Car Handle" he states that tires sizes (meaning within a given size) can vary as much as 7%. The book was written the the 70's so that might be inaccurate today.

Hyper accuracy would factor the diameter of a tire being smaller at the contact patch (due to compression) than an 'off the car' diameter measurement. However, at speed centrifugal force likely alters the diameter too.

I'm not trying to split hairs on accuracy but rather asking there are so many factors (factory stated diameter vs actual diameter, compressed diameter and centrifugal diameter) that can there truly be accuracy???

Anyway, if I ever got a speeding ticket for being slightly over the limit that's my argument and I'm, sticking to it. smiling smiley

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Re: speedometer calibration
Posted by: joelsplace ()
Date: July 15, 2021 04:33AM

In my experience tires are very, very close to marked size in diameter but no where close in width. I've seen 225s vary 2" in tread width between a Goodyear and a China tire.

Joel
Northlake, TX
5 Ultravans, 138 Corvairs and counting...

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Re: speedometer calibration
Posted by: 66vairman ()
Date: July 15, 2021 09:39AM

wittsend Wrote:
-------------------------------------------------------
> In Fred Phun's book "How to Make Your Car Handle" he states that tires sizes (meaning within a given size) can vary as much as 7%. The book was written the the 70's so that might be inaccurate today.
>
> Hyper accuracy would factor the diameter of a tire being smaller at the contact patch (due to compression) than an 'off the car' diameter measurement. However, at speed centrifugal force likely alters the diameter too.
>
> I'm not trying to split hairs on accuracy but rather asking there are so many factors (factory stated diameter vs actual diameter, compressed diameter and centrifugal diameter) that can there truly be accuracy???
>
> Anyway, if I ever got a speeding ticket for being slightly over the limit that's my argument and I'm, sticking to it. smiling smiley

In the good old days ---- Car test drives in magazines almost always included a "indicated vs. real" speed number. Usually at 60MPH and almost always the actually speed when the speedometer indicated 60MPH was 56-59MPH. This was done deliberately by automakers to avoid liability for a vehicle going faster than the indicated speed on the speedometer.

Having said that ---- YES when radials first came out I know of folks that used the "Well officer these new radial tires must be making my speedometer read wrong". Sometimes that excuse even worked!!!!




Edited 1 time(s). Last edit at 07/15/2021 09:41AM by 66vairman.

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