Gear Box query

[HowardB]

My Tr7 is basically a standard 2l version, but there is one aspect that is most certainly non standard and I would appreciate any info on the subject.

The Speedo reads about 20% high - I have swapped speedos and the bevel drive gear with success - so clearly there is something amiss with the gearbox. Is there version of the Rover 5 speed gearbox that has different gearing? The only other notable aspect is that the clutch is heavier than on most 7's.

(I have currently put a fitted a small gear box in the speedo drive which brings my speedo back to the correct figures)

[Odd]

There's several different options to the speedo drive (and only one combination is <u>correct</u>
for any given rear axle ratio/tyre size). There are six driven gears, and two driving gears,
to choose from:
219001 ORANGE 20T
219002 GREEN 21T
219003 RED 22T
219004 BLACK 23T
219005 BLUE 24T
219006 WHITE 25T

TKC1273 WHITE 7T
TKC1274 BLACK 8T

To change between those last two you need to dismantle the rear of the gearbox.

[TR Tony]

Have you got standard 13" wheels? If different diameter wheels are fitted it will affect the speedo reading.

There were versions of the LT77 box with different internal ratios, as the box was used on many different BL vehicles. If you can get the gearbox serial number it may be possible to identify if it was originally on a different car.

Tony

<font size="1">1981 2L FHC Cavalry Blue
1980 3.5L V8 DHC Jaguar Regency Red - sadly sold!</font id="size1">

[HowardB]

Thanks Tony, will try & get the serial no when I am back home in a couple of weeks & post it then. Do you have any idea where I can find it?

A previous owner (about 15 years ago) tried to track the problem and eventually gave up. I think he discussed it in the TR Driver's club mag. FYI I am currently running with MGF wheels, but these are the same rolling dia as the standard wheels & give the same error reading.
I suspect that it is a gearbox from something other than a TR7 which is causing the problem & I do know that the gearbox has been replaced.

Odd thanks for your suggestion which is what I originally diagnosed. I fitted an orange gear (sourced from S+S as not easily available & now surplus to requirements) which made the situation much worse so refitted the original, which I think was black. I have not come across any references to different driving gears before but error ratio is 0.8414 with the black gear which is different from the driving gear ration(0.88). Robsport confirmed the rear axle is the usual ratio for a 2l manual 5 speed - sorry cannot recall the ratio.

[TR Tony]

Howard, from memory the serial number is stamped on the lower right hand side of the box.

The standard axle ratio for a 5 speed is 3.9 I believe, the 4 speed cars were 3.45 but it is a totally different diff anyway.

Tony

<font size="1">1981 2L FHC Cavalry Blue
1980 3.5L V8 DHC Jaguar Regency Red - sadly sold!</font id="size1">

[Underdog]

Late TR7 5spds also had 3.45, late 80s 81 I believe. I forget the com# break point. I have one in my 8 that came from a late 7.

72 MGB BRG
80 TR8 Persian Aqua
If women don't find you handsome, they should at least find you handy.

[Odd]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"> I suspect that it is a gearbox from something other than a TR7 which is causing the problem
& I do know that the gearbox has been replaced. <hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote"> What ratio(s) there are in the gearbox is not the least important. What <u>is</u> important is what driving gear is in there;
the white 7 tooth or the black 8 tooth. And that can easily be seen/determined when you have the driven gear
out of the gearbox, it's just a matter of peeping into the empty hole... (Use a mirror if needed!)

Next step would be to determine <u>exactly</u> what rear axle ratio there is in your axle, it could be any of the following:
2.84:1 , 3.08:1 , 3.45:1 or 3.90:1. My guess would be it's a 3.90:1...

Provided you have a 3.90:1 rear axle and original wheels (185/70*13 = 1800mm rolling circumference) the 'correct'
driven gears for the two different driving gears would be:
White 7 tooth should have a Red 22 tooth.
Black 8 tooth should have a White 25 tooth.
Then your fault would be within a few percent (-1.2 to -1.4 percent to be precise).

If you have any other combination the calculations will have to be altered accordingly...
/Odd

[FI Spyder]

Is not the MGF wheels 14"? Standard wheels are 13". I have seen a '79 build date with 3.45 diff whereas my July '80 build date FI has a 3.9 diff. Have heard the 3.45 was optional previous to being standard on FI late '80 and '81 cars. The 3.08 was the TR8 diff. Wasn't the 2.84 ratio from the Rover SD1? Lots of possibe combinations plus the speedo gear combos. I wish I had the FI 3.45 ratio as I want to grab another gear at highway speeds and have plenty of power in overdrive to go up hills.

PS you can tell the ratio by the stamp on bottom of diff. CM=3.9 for example.

TR7 Spider - 1978 Spifire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra

[Odd]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"> PS you can tell the ratio by the stamp on bottom of diff. CM=3.9 for example. <hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote"> You <u>USUALLY</u> can tell the ratio by the stamp on bottom of diff.
My 2.84 still say CM... <u>Never</u> trust a (D)PO!