Bob:
Thanks for the help. I hooked a 1,200rpm drill up to the tach cable and
pulled the trigger, it read about 6,500rpm, definately way in to the red.
So I assume if you are correct with the 1:2 ratio that there is something
wrong.
I did not hook the drill up to the speedometer cable as I ran out of time.
I guess the next step is to find another tach to confirm there is a problem.
BTW: I am running 195 75 15 Bridgestone S408's.
If anyone has any other suggestions, please let me know.
Thanks again,
Bryan Lepley
Kalamazoo, MI
71 TR6
>From: "Robert M. Lang" <lang@isis.mit.edu>
>To: Bryan Lepley <blepley@msn.com>
>CC: 6pack@autox.team.net
>Subject: Re: Where is my SPEED?
>Date: Tue, 10 Sep 2002 14:21:10 -0400 (EDT)
>
>Hi,
>
>5900 RPM's in 4th gear with stock tire diameter (26.5 inches, more or
>less) and a stock TR6 3.70 differential should equate to over 110 MPH.
>
>100 MPH is something just over 5000 in 4th.
>
>If your tach registers 5900 RPMs and your speedo registers 69 MPH, either
>the tach is off or the speedo is off.
>
>THere are some tests you can do.
>
>1. as mentioned, compare your tach reading to a known good source. A
>dwell/tach is a good place to start, but that only works up to 1500 - 2000
>RPMs. After that, you need another form of tach. An "off the shelf" unit
>from Sun or AutoMeter for under $50 will fit the bill to get you a larger
>range of RPM readings.
>
>2. connect an electric drill to your tach drive cable. Spin the drill up
>to its highest RPM. The tach should move accordingly so 1500 drill RPMs
>should be 1500 RPMs on the tach. I'm pretty sure this is right, because
>the tach drive is geared 1:2 to give the proper cam/engine relationship.
>If this reads wrong, get it calibrated.
>
>3. do the same thing for your speedometer. 1500 drill RPMs should equate
>to about 30 MPH on the gauge. If not, then the "muppah" gauge is probably
>in need of calibration.
>
>Now, 5900 RPMs with a TR6 motor is a visceral experience. When you rev a
>TR6 motor that high, blelieve me - you'll know that you're turing some
>RPMs! I suspect that your tach is reading high and the basis of your
>incorrect reading. However, it might help us to help you if you told us
>what size tires you use and what your rear gearing is (if you know it).
>
>I suppose it is possible that you have some odd gears in the differential,
>the ratios that I have seen or used are 3.45 (PI spec cars and TVR
>2500's), 3.70 (stock Fed. Spec.) 4.10 (optional performance piece), 4.30
>(optional performance piece), 4.55 (ditto) and 4.88 (from some sort of
>milk truck :-)).
>
>I've worked out the numbers on all of these, and the 4.88 is about 30%
>more gear than the stock 3.70 thing. So with that setup, 5000 in 4th would
>be 100 MPH indicated but only about 70 MPH in real speed.
>
>So another possibility is that your diff. has been souped up with "gears".
>
>If the latter winds up being the case, contact me off list because I'm
>always looking for spare diff. gears for the race car!
>
>rml
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