Speedo Calibration

RJS · Postby **RJS** » 14 Feb 2008 20:08

I have a factory TR8. Previous owner put in a 3.90 diff for some reason (I assume the original went out). I changed it over to a 3.45 rear end (thanks for the rear end Peter N).

The speedo was off about 20 MPH at highway speeds when I had the 3.90. It is now off by 10 MPH at highway speeds.

Is #8 from the link below what I am looking for fix this? If yes (I suspect it is not), which one? Is it as simple as pulling the old one out and putting the new one in (I have changed the speedo cable in that area before)?
http://victoriabritish.com/icatalog/tr/0085.asp

Thanks for any ideas / suggestions,
Rob

jeffremj · Postby **jeffremj** » 14 Feb 2008 20:33

You have it in one. First check the 'colour' of the one in your gearbox. Then decide what percentage change you need. Each tooth is around a 5% change. You will probably need to increase the number by 2. The actual available range runs from 21 through 25/26 (?) teeth.

Workshop Help · Postby **Workshop Help** » 15 Feb 2008 00:09

Ahh, yes. First off, changing out the speedo gear is quite simple. After the cable and clamp with bolt are removed, using large pliers, grasp the end of the gear and wobble it out of the housing. A quantity of oil will also drool out.

The factory rear end ratio for the TR8 was 3.08/1. For that 3.08/1 ratio, the correct speedo drive gear is the red one with 22 teeth. However, if you are sure the current rear end ratio is 3.45/1, which would have come from a fuel injected 1981 TR7, the choice would be the green speedo gear with 21 teeth.

Mildred Hargis

RJS · Postby **RJS** » 15 Feb 2008 04:57

I will have to pull the gear out to find out how many teeth it has now, but my basic math skills tell me that the gear I need has to have more teeth.

Right now the speed reads 80 when I am going 70 (12.5% to fast). The more teeth on the gear, the slower RPMs it will turn. If the gear has 20 teeth now, each tooth is worth 5% (1/20=.05). If I add 2 teeth, that is adding 10% more teeth, so that should get me pretty close.

I am sure it had a 3.90 and that it now has a 3.45. Changing the rear end basically cut the speedo error in half. If I would have changed from the 3.90 to 3.08, then the change would have been more dramatic, and probably would have eliminated the speedo error all together.

Rob

jeffremj · Postby **jeffremj** » 15 Feb 2008 14:50

<blockquote id="quote">quote:<hr height="1" noshade id="quote">I will have to pull the gear out to find out how many teeth it has now, but my basic math skills tell me that the gear I need has to have more teeth.
<hr height="1" noshade id="quote"></blockquote id="quote">Indeed, if you have 22, you now need 24!

Workshop Help · Postby **Workshop Help** » 15 Feb 2008 15:17

Last night I was a little weary, so this morning let's discuss speedo gears and their relationship to rear end ratios. Over the years when changes are made to a car, which can include transmissions and differentials, often the alteration to the speedo gear is not made or is incorrectly made.

The TR7 with a four speed uses a 3.63/1 rear end ratio which gives right at 3275 RPM's at 60 mph in 4th gear direct drive. The factory installed speedo gear is black in color and has 23 teeth.

The early five speed TR7's thru VIN 202093 fuel injected and 202680 carburetted use the same speedo gear, (black 23 teeth), as the four speed cars. This resulted in a minor difference in readings of the speedometer, 60 mph reading while 57 mph actual, with the 3.90/1 ratio rear end.

With the change to the 3.45/1 rear end ratio, the green 21 teeth speedo gear was installed on the TR7's.

Please note this information comes from the Rimmer parts book, TRF, and Vict Brit catalogs. Vict Brit shows a blue 24 teeth speedo gear for 1980-1981. Well, if the 23 teeth black gear was a little off, the blue gear would go a ways to correcting that slight error. As for the 3.08/1 rear end ratio, the red 22 teeth speedo gear does seem to be going in the wrong direction.

Perhaps we should ask the other members of this community with genuine TR8's to advise their findings. What is the tach reading at 60 mph in forth gear for a factory TR8? What is the color of your speedo gear for the 3.08/1 ratio?

It would be of interest to know what the TR7 tach reading is at 60 mph in forth gear for the 3.90/1 ratios as well as the 3.45/1 ratios.

I'm now more confused than I was last night.

Mildred Hargis

Underdog · Postby **Underdog** » 15 Feb 2008 16:08

Mine did have the red gear with the 3.08. I swaped in a 3.45 and changed the gear. It's been a long time ago but I do believe it was green. Mines not quite ready for the road yet & it's snowing here so can't help with the RPMs.

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

jeffremj · Postby **jeffremj** » 15 Feb 2008 16:47

What has to be considered is the origin of the gearbox. A TR8 5 speed gearbox has a different INTERNAL gear to that of the TR7 5 speed:

TKC1273 (white with 7 teeth) stock in TR7 gearbox
TKC1274 (black with 8 teeth) stock in TR8 gearbox

I wrote this bit about 10 years ago on the WWW list - all info for the 5 speed box:

1) a standard TR8 gearbox driven gear has 22 teeth,
2) a TR8 gearbox driven gear with a 3.45 axle should have 24 teeth,
3) a TR8 gearbox driven gear with a 2.85 axle should have 21 teeth,
4) a TR8 gearbox driven gear with a 3.9 axle should have 26 teeth (have to use 25).

5) a TR7 gearbox driven gear with a 3.9 axle should have 23 teeth,
6) a TR7 gearbox driven gear with a 3.45 axle should have 21 teeth,
7) a TR7 gearbox driven gear with a 3.08 axle should have 19 teeth but the lowest value available is 20, so 5% under-reads, just like my TR7V8 :-)

a TR7 gearbox driven gear with a 3.9 axle could use 22 teeth (seems to be closer to reality in my TR716VSprint)

Item numbers:
219001 (orange) = 20 teeth - Rimmers price: Â£20
219002 (green) = 21 teeth - Rimmers price: Â£10
219003 (red) = 22 teeth - Rimmers price: Â£21
219004 (black) = 23 teeth - Rimmers price: Â£11
219005 (blue) = 24 teeth - Rimmers price: Â£7
219006 (white) = 25 teeth - Rimmers price: enquire

Note that some of these items may not be available.

Hope this makes things clearer.

Beans · Postby **Beans** » 15 Feb 2008 17:48

That indeed should make it clearer. but ...

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jeffremj

...a TR7 gearbox driven gear with a 3.9 axle could use 22 teeth (seems to be closer to reality in my TR716VSprint)<hr height="1" noshade id="quote"></blockquote id="quote">

... I want to add that this (also) depends greatly on the rim/tire combination you are using.
In both my cars the speedo is almost spot on.
DHC has 3.45:1 diff with 15"x195/55 tires, it uses nr. 219002 (green/21 teeth);
FHC Sprint has 3.90:1 diff and stands on 15"x205/50 tires, it uses nr. 219004 (black/23 teeth)

<center>

1981 TR7 FHC Sprint (better known as 't Kreng)
1981 TR7 DHC (not very well known yet, but back on the road)
Also a 1980 TR7 DHC, 1980 TR7 DHC FI, 1981 TR7 FHC
http://tr7beans.blogspot.com/</center>

jeffremj · Postby **jeffremj** » 15 Feb 2008 18:36

My info stands as a base line. As you say, non-standard rolling radius will have an affect. There is also a speedo affect - they are not perfectly accurate.

RJS · Postby **RJS** » 15 Feb 2008 22:45

I did go with a slightly larger tire. The internet calculator I used said it would give about 2% speedo error. At this point I would love to have only a 2% error.

Rob

john 215 · Postby **john 215** » 15 Feb 2008 23:32

Hi,
When i changed to a 3.08 rear end, had my speedo itself recalibrated by these people-
http://www.speedycables.com/
Carried out a first class job.You had to let them know how many time the speedo cable turns over a complete turn of a rear wheel along the ground. That way different size wheels/tyres are factored in
Cheers John.

LIVE LIFE A QUARTER OF A MILE AT A TIME!
1979 3.5 FHC(STATUS PENDING!!)

1982 2.0 DHC Soon to be a 4.6 fire breather!!
Read My Blog http://www.waringstowntr7s.co.uk/blogs/ ... hp/John215

jeffremj · Postby **jeffremj** » 15 Feb 2008 23:44

<blockquote id="quote">quote:<hr height="1" noshade id="quote">..... had my speedo itself recalibrated
<hr height="1" noshade id="quote"></blockquote id="quote">Whilst the maths say that a 20 tooth gear will give a 5% under-read on a TR7 with a 3.08:1 axle, in my experience with 205/50x15 tyres, the under read is less than 5%. My car is doing 72mph (GPS) when the speedo shows 70. To me, this is satisfactory. YMMV [;)]

Workshop Help · Postby **Workshop Help** » 16 Feb 2008 00:48

Boys, Boys, Boys, I think we have enough examples of cars out here in the hinterlands to establish a baseline for the various rear end ratios and speedometer readings. To keep it on a level playing field, if we may have your tach reading at 60 mph in forth gear, please. That's right...forth gear at 60 mph.

Note your rear end ratio and, if you know for certain, the color of your speedo gear. Also, note your tire size/height. With this infomation we can ascertain just which speedo gear is the appropriate one for a given rear end ratio and then juggle it for non stock wheel/tire sizes.

Thank you,

Mildred Hargis

jeffremj · Postby **jeffremj** » 16 Feb 2008 10:34

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Note your rear end ratio and, if you know for certain, the color of your speedo gear. Also, note your tire size/height. With this infomation we can ascertain just which speedo gear is the appropriate one for a given rear end ratio and then juggle it for non stock wheel/tire sizes.<hr height="1" noshade id="quote"></blockquote id="quote">No you can't! Your method ignores the INTERNAL drive gear! My definitive posting sets the baseline.

Anonymous

Speedo Calibration

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