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MG MGB GT V8 Factory Originals Technical - Speedo calibration (... longshot)?

I need to have my B speedo calibrated to work in a V8. I'm running a fairly common setup, so I wonder if just by chance one of you would have the revs. per mile that the speedo should be set to:

- Rover 5-speed gearbox
- MGC 3.07 rear end
- 195/60R15 tires

I will be stunned if anyone actually has this info, but I thought I'd ask!
Harry

Harry,

This is from an old issue of the V8 newsletter. It may be of help. Engine RPM=(V x T x R x N)/60

V=MPH
T=tire revs/mile
Figured as - 5280/(tire dia. in inches x .2618)
Eg. 22"dia tire: 5280/(22x.2618)= 917 revs/mile
R=rear end ratio
N=transmission ratio

Eg. engine rpm=(60mph x 917 tire revs x 3.9 rear end ratio x .82 trans ratio)/60 = 175954/60 = 2933 rpm.
I think the SDI trans final drive is .83.

HTH,

Mark
Mark Marchbanks

Mark,

OK, I think I get some of what you're saying ... but this is just for a speedo, not a tach. I did a quick search; at the site

http://pw1.netcom.com/~sgalaba/tiresize.htm

there's a calculator that tells me the 195/60R15 tires turn 832.96 times per mile. So the driveshaft must turn 3.07x that amount, or 2557.2 times per mile. Does the speedo cable just rotate at the same speed as the driveshaft? (I was told by the speedo shop that if I can just tell them how many times the cable rotates per mile, they can dial me right in.)
Harry

The easiest way to figure turns per mile is to disconnect the speedo cable at the speedo end, 'flag' the inner cable with a piece of tape, jack up the rear of the car, and turn the wheels 10 times. Your faithful assistant in the car counts how many times the speedo cable turns for each 10 turns of the rear wheels.

Note; make sure both rear wheels turn forwards at the same time- due to diff action, its possible to turn one wheel and have the other go backwards.

Anyway, now you know the speedo cable turns 'X' number of times for every wheel rotation, it's easy to use the figures you have above regarding wheel rotations per mile to calculate speedo cable rotations per mile.

HTH
G
Growler

Harry, The Rover 5sp. did it come out of a TR-8 or Rover SDI or TR-7 ? The Rover, TR-8 will use a 1,000 ct. speedo that is found in a 73 to 80 M.G.B. The TR-7 trans uses a diffrent set of speedo drive gears in the trans. & it will take a speedo with a 800 ct. that has to be reworked , I might have what you need sitting on the shelf at the shop & then there is 5 drive geas that can be changed to fine tune the speedo for tire size changes. 302-734-1243 so now your STUNNED with info & parts to boot
Glenn Towery

Harry,

I have same set up as you except 3.08 rear axle. The amswer is 28mph per 1000revs.

Paul
Paul Wiley

Sounds like you guys have this stuff down. How can I tell which model of Rover 5-speed box I have on board? I got a bad speeding ticket last week but when I noted to the officer that my speedo wasn't even close he didn't even flinch (20 mph over the limit is a reckless driving offense here)!
Harry Peeters

Harry,

I probably have typical B Speedo that has a mind of its own. I therefore use rev counter at 30mph per 1000revs which keeps me within speed limits.

Paul
PAUL WILEY

Growler has got the right idea-but you have to be a bit more accurate.

First remove the speedo and support the cable end with a piece of card with a key hole slot-mark out the card into quarters (like crosshairs) and make a cardboard pointer with a hole to press onto the square cable end .

Drive to a flat car park with an assistant,tape measure and some chalk. Set the pointer up at having driven the the car forward to rest -so that the slop is taken out of the drive train. Have your helper make a mark on the rear tyrewall and the ground-then drive the car forward whilst he/she notes the exact no of turns as the tyrewall mark reaches the ground. You will also be noting the exact no of turns on your speedo pointer-hence you need a quiet car park! Six complete turns of the road wheels should do it. As the car comes to rest on the sixth turn mark the ground next to the tyrewall mark and measure between the two. This method takes into account the reduction in rolling radius cuased by tyrewall compression. Having noted the no of cable turns and distance travelled it should be possible to get a good speedo repair shop to re-calibrate the speedo. Here in the UK I packed my old speedo off to Speedy Cables and it came back looking spanking new-replated -overhauled,etc and recalibrated all for £33.20 as I recall.

Hope this helps

Regards

John Bourke
John Bourke

I have used exactly the same mthod that John describes. However, I would just add that it is advisable to repeat the process at least three times until you can be sure of the results you get. The slop and sometimes wind up in the speedo cable can throw a speanner into the works.

Rog
Roger Parker

This thread was discussed between 29/11/2000 and 03/12/2000

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