Speedometer Swap

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Speedometer Swap

Post by Mcarlo77 on Fri Feb 12, 2016 7:36 pm

Can anyone confirm whether I would be sacrificing accuracy if I were to swap from a 100 mph speedo to a 120 mph when I get around to installing the tach cluster? This would be for the '75 Laguna and would, of course, be the round gauge.
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Re: Speedometer Swap

Post by Roadcaptain S3 on Sat Feb 13, 2016 7:36 am

Probably.
How accurate is it now? I haven't had an old car that wasn't off by at least five miles an hour.Smile

I would guess that it would still be close since the speedo gear in the transmission won't change.

Bowtie overdrives used to have some formulas on their site to help with speedo gearing.
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Re: Speedometer Swap

Post by Bruisr on Sat Feb 13, 2016 9:23 am

All things being equal the 120 speedo will be correct because the gearing inside the speedo gauge will setup up for the appropriate markings on the 120 speedo face. If your 100 speedo is not accurate the 120 will not be accurate either. As long as you change the entire speedo assembly it will work, just giving you more mph before "burying the needle".
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Re: Speedometer Swap

Post by Mcarlo77 on Sat Feb 13, 2016 11:20 am

Thanks, gentlemen...pretty much what I was thinking, too.

I haven't really had a chance to check the accuracy of my existing speedo. Only hint that it was off was a time when I was being followed by my daughter in her car and, when I asked how fast we had been going on the highway, she indicated about 5 mph slower than my speedo was indicating.

I remember back when the 55 mph law was passed, there was a large speed sign mounted on an Interstate overpass outside the college town where I was attending that would give you your actual speed as a reminder to obey the new law. We'd go out and try to ring up the highest score...when it was safe to do so, of course! It didn't stay up there very long...
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Re: Speedometer Swap

Post by Roadcaptain S3 on Sat Feb 13, 2016 2:56 pm

I THINK different years used different gearing as well. So if you are using round 73 gauges in place of 75 sweep gauges...

I normally will pace my wife en route to a car show or something and call her phone. She'll tell me how fast were going and I don't think I've had a car yet that wasn't off quite a bit.
I've also used the GPS on my phone when I was by myself. i know a couple of guys that just set their phone on the console and don't ven check the cars speedometer.

Every old car I've had either had transmission swap, bigger or smaller wheels, rear gear changes, different gauges et al. I figure if I'm within 10 mph, I'm doing good.
I put the 700r4 in my Laguna and spent all kinds of time swapping tranny gears and working out wheel dimensions and still couldn't get better than within 5 mph.

I've heard you can upgrade to digital guts in old speedos somehow and they're dead on accurate without taking a class on physics to get them to work properly.
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Re: Speedometer Swap

Post by 1973montec on Sat Feb 13, 2016 3:37 pm

theres a speedo ap for smart phones
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Re: Speedometer Swap

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