[C2] Speedometer Question
Thread Starter
Safety Car
Joined: Sep 2011
Posts: 4,102
Likes: 1,221
From: Madeira Beach, FL
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Speedometer Question
Hello Forum,
I have a 65 convertible. The speedometer is off by 2-3 mph, it reads slower than the roadside electronic speed warning devices say.
I believe that is due to the P215-70-R15 radial tires.
When I accelerate the speedometer moves nice and smooth up until about 60 mph where the needle starts to bounce between 60-70mph until it settles down.
Any way to correct this. I'm not sure at 60 it is really 60 or greater as I have not had a way to check it. Above 70-80 cruising it remains steady just during acceleration it bounces.
I found a receipt in the paperwork from the last owner the head was removed and repaired by a speedo place in 1984 for a broken odometer.
Thank you in advance.
Marshal
I have a 65 convertible. The speedometer is off by 2-3 mph, it reads slower than the roadside electronic speed warning devices say.
I believe that is due to the P215-70-R15 radial tires.
When I accelerate the speedometer moves nice and smooth up until about 60 mph where the needle starts to bounce between 60-70mph until it settles down.
Any way to correct this. I'm not sure at 60 it is really 60 or greater as I have not had a way to check it. Above 70-80 cruising it remains steady just during acceleration it bounces.
I found a receipt in the paperwork from the last owner the head was removed and repaired by a speedo place in 1984 for a broken odometer.
Thank you in advance.
Marshal
Burning Brakes
Joined: Mar 2011
Posts: 789
Likes: 465
From: Council Bluffs Iowa
I can't help with the repair side of it, but just get a free speed GPS app on your cell phone to check your speed. I use one called Thunder in my Ram since it can't be calibrated with my huge tires. It seems to be very accurate to me.
Melting Slicks
Joined: Apr 2015
Posts: 3,069
Likes: 1,127
From: NEW YORK
As far as fixing the ops problem, the plastic / nylon gear at the transmission end of the speedo cable may need replacing.
Safety Car
Joined: Sep 2007
Posts: 4,454
Likes: 328
From: Indy Indiana
if your speedometer is too slow then you need to speed up the spinning speedometer cable cable by installing a one tooth less speedometer driven gear ( plastic) inside your trannys speedometer cable connection point.
or you can ultimately buy you a gear reduction box that they can build for you to the specs needed to speed up or slow down your spinning cable.
I have seen bouncing speedo's by having too many 90 degree angles in your speedo cable,
or it needs a tiny bit of oil to lube it up... just a little.
I have also seen a worn out plastic gear mounted on your tail shaft causing this. replace it with a steel gear ( requires tranny removal and rear housing removal.
finally speed up your speedometer cable by installing 1 size smaller tires.
i think 1000 rpm for your cable = 60 mph you can calculate from there, you want your cable spinning 1000 rpm at 60 mph. you adjust tires, rear end ratio, speedometer drive gear, and speedometer driven gear to get that.
or you can ultimately buy you a gear reduction box that they can build for you to the specs needed to speed up or slow down your spinning cable.
I have seen bouncing speedo's by having too many 90 degree angles in your speedo cable,
or it needs a tiny bit of oil to lube it up... just a little.
I have also seen a worn out plastic gear mounted on your tail shaft causing this. replace it with a steel gear ( requires tranny removal and rear housing removal.
finally speed up your speedometer cable by installing 1 size smaller tires.
i think 1000 rpm for your cable = 60 mph you can calculate from there, you want your cable spinning 1000 rpm at 60 mph. you adjust tires, rear end ratio, speedometer drive gear, and speedometer driven gear to get that.
Last edited by joewill; Feb 14, 2024 at 02:20 PM.
