Thomas:
Sounds like you have a 4.11 differential. If the tach and speedo
are in synch, and registering the expected values, and you know that the
car is off by 20 MPH, then you definitely have some issues.
First off, your tires are causing roughly a 6% error. You need to
correctly size your tires, but this is only a part of the problem (sorry if
I am stating the obvious).
If you have a 4.11 rear end, then this will cause a 4.11/3.70 = 1.11
or an 11% error. The errors are compounded, so a 1.06 x 1.11 = ~1.18, or
an 18% error.
The other issue is that if your speedometer is bouncing around a
lot,
it is definitely time to get it overhauled and calibrated. I would do this
first, since it is easy. It might be that the remaining error can be
accounted for by simply calibrating the speedometer. A 5MPH error in
the speedo would give you back almost all of the problem.
1/1.18 x 60 - 5MPH = 46MPH. This is starting to get you pretty close to
your claimed 20MPH error at an indicated 60MPH.
One other item to check, and that is the speedometer drive gear.
If the DPO swapped rear end gears, who knows what else got swapped?
Just my random thoughts.... scrambled slightly by too much caffeine.
Cheers,
Vance
------------------------------
1974 Mimosa Yellow Triumph TR6
Cogito Ergo Zoom
(I think, therefore I go fast)
-----Original Message-----
From: Thomas Gauthier [mailto:td_gauthier@excite.com]
Sent: Monday, October 28, 2002 4:22 AM
To: 6pack@autox.team.net
Subject: speedometer error
List,
I am trying to figure out why the speedometer in my TR6 registers about 20
mph faster than actual in 4th gear. I have a 72 TR6 with no overdrive.
This is not a new problem, I have had it since I bought the car about 1.5
years ago.
The speedometer jumps around a bit, but in 4th gear at 3000 rpm it is pretty
stable, registering about 62 mph. The synchronization between the tach and
speedo are what you would expect based on TR6 gear ratios. I have verified
this for 4th gear only (I haven't checked the other gears).
The problem is that the actual speed (verified by driving alongside my
wife's car and by several passes by the local police speed sensing units -
you know the units that indicate your speed as you drive past) is about 20
mph less.
What is going on? The car came with 195 65 15 tires, but that shouldn't
make that much difference. Can I trust my tach? Do I have some kind of
crazy differential gear ratio?
Any help would be appreciated.
Thanks,
Tom
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