Dave---Congrats on getting your '69 up from its long sleep! We will all
try to help you keep it awake, should the need arise..
On to the diferential ratio...The stock diff on most US TR's have the
3.70:1 as you probably already know. When the tach and speedo are
properly calibrated, as when the car was built, the tach should read
1,000 rpm for every 20.7 mph in fourth gear. This should mean that when
you are going 65 mph, the tach should be reading 3150 or there about.
You didn't say just what you saw, only that the engine speed seemed
excessive at that road speed. If your speedo and tach shows this
relationship, all is well. If you were to install one of the
hard-to-get 3.45:1 ratios, the difference would be about a 7% reduction
in true engine speeds.
While it is true that 'smaller rear tires" will cause the engine to rev
more to attain a given road speed, note that it makes no difference in
this tach-to-speedo relation by the size tires you have. This ratio is
determined before the drive shaft, diff gears or rear tires start to
turn.
Most of our TR6's will happily run all day at 3600 rpm (or approximately
75 mph). If you feel you have the wrong gears, or one of the gauges
might be wrong, it would be a good thing to compare your actual speed
with those roadside mileage checks.
Dick Taylor
|