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MG MGA - 5-Speed and 3.9 Diff Speeds/RPM

I am up and running after my master cylinder failure. Whilst I was waiting for a decent new set of seals from Bob West I took the opportunity to swap over the differential to 3.9.

Just got back on the road an hour ago - beautiful. clutch and brakes rock solid again and the new gearing feels excellent.

Now for the question. I have not got round to recalibrating the speedo yet, so in the meantime can someone give me a few top gear (5-gear Ford Type 9) speed/RPM figures to keep me out of trouble with speed cameras. Many thanks. Oh yes, standard tyres.

Steve
Steve Gyles

Steve,

If memory serves (and that get trickier every year), each 1,000 rpm is about 22MPH. I'll check my file and verify.

Cheers,
Joe Cook

Darn memory!
with a 24.9" tire, it works out to about 19 mph per 1k rpms.

Cheers,
Joe Cook

If the std MGA is 17mph/ 1krpm, (I think this is the case) then the 3.9 conversion with a type 9 box (.82 5th gear) should net you 22.85 mph / 1krpm.

The tranny gear data comes from:
http://www.hi-gearengineering.co.uk/frames.htm
and simple math of 4.3/3.9 for the differential.

Joe, I think your original answer of 22 is very close.

Chuck
Chuck Schaefer

Thanks guys for your quick responses. With my current readings just now, 2400 was giving me 50mph so it would appear I am underreading by just under 5mph. That is probably good enough to be going on with. Many thanks.

Steve
Steve Gyles

Best tests - temporarily fit a GPS with speed readout, or drive behind someone who can show three fingers @ thirty, 4 @ forty, 5 @ fifty, 6 @ sixty, 7 @ seventy, and then ask the cop what his radar was reading!
P. Tilbury

Missed the part about the 5-speed. Add bad eyes to bad memory.
Joe Cook

A hand held GPS is the best and most accurate solution. Your tach is probably a bit off so doing the math and trusting the tach could be a mistake. Just make temporary marks on the speedo with a sharpie as you hold a steady speed with the GPS.
Using a gps I found my speedo starts out reading 5mph high at 30 becomes accurate at about 60 then reads lower than it should at higher speeds. Remember these old gauges are not totally linear.
R J Brown

Hi Steve, Try one of the road signs that show your actual speed as you go through them. There is one on the A59 as you approach Rufford village. The speed limit is 40 but it should give you an accurate measure to calculate your rpm figure - a bit cheaper than a gps - unless you've got one already- or as our Canadian friend has said you could keep on going through up to 70 and risk plod with his radar - cheers Cam
Cam Cunningham

Cam

Thanks. Chuck came up with the right figures. I now have a bit of paper with all the important RPMs stuck on my false radio grill. It will be more than adequate until I sort out the speedometer.

Steve
Steve Gyles

Hi Steve

Drive through a 30 mph speed camera lots of times increasing your speed by 5 mph indicated, you'll find out in a couple of weeks how fast you were going, stick something different on your rear ( undamaged ) bumper each time so you can relate to each offence.


Sound advice always available

Terry
Terry Drinkwater

This thread was discussed between 18/08/2006 and 19/08/2006

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