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MG MGB Technical - MGB RPM at Speed

Hello,

First post, even if I have been dipping in and out of here for years...

I've just taken ownership of a 1978 B with the overdrive transmission and I suspect the speedo it under reading. I used to have a 73 GT (with OD) and I don't remember the engine being nearly so busy at speed.

What I'm getting it as follows -

In 4th gear (OD on) indicated 60MPH is about 3000 RPM. In my experience of other cars 4th gear with a ratio of 1:1 would give this reading, but surely the OD should knock about 500RPM off - giving 2500 RPM at 60MPH?

The rear typres are 175s which I'm given to understand give a 3% under reading, and then I suspect a slight inaccuracy in the speedo.

What do other B owners get RPM/MPH wise?

Look forward to hearing from you.

Regards, etc.

Benjamin
B Oldcorn

My car, 1970 BGT, on 185/65 14" tyres is doing 2700RPM at 63 MPH indicated in 4th overdrive.

It sits on about 2000 RPM in direct 4th at 35 MPH indicated.

John
J Tait

Autocar road test (1977) says 1000rpm is 17.90mph in top, 21.83mph in ODtop, so 60mph would be about 3,350 or 2,750rpm respectively.
Geoff Everitt

Thanks for the replies - keep them coming. Hopefully I'll get an average for RMP/MPH and know what to aim for.

2500 in 4th with OD - somewhere between 50 and 60 MPH?

Thanks again.

Regards, etc.
B Oldcorn

It's not really an opinion poll is it?

2,500rpm is 45 in top and 55 in ODtop as near as makes no odds.

Geoff Everitt

Factory figures with factory tires were 18 mph per 1,000 rpm in 4th gear. Overdrive was 21 mph per 1,000 rpm, again with the factory tires. Speedometers were, in that era, considered accurate if they read within 10% of the actual speed. (This was the US government standard for mechanical speedometers back in the day.) Larger diameter tires will add, slightly, to the indicated (speedometer/tachometer) speed as compared to the factory tires. A road trip with the wife/girl friend, a GPS and somewhere to stop for lunch should tell you all you need to know about your actual figures.

Les
Les Bengtson

While on the subject, without o/d, what will be the rpm to correspond to the statutory 30mph?, (could be of use to check whether the speedometer is any where near accurate)
Regards
Stuart
si robathan

30mph should be about 1,650 in 4th, 2,300 in 3rd

This page confirms 18 and 22mph/1000rpm http://www.mgcars.org.uk/MGB/mgbspec.html
Geoff Everitt

Les, isn't it unwise to take both your wife AND girlfriend on the same roadtrip? For a start where would they sit...

The suggestion of using a GPS though is an excellent one. Will definitely remove any doubt. I want to check my car against my GPS but I know the speedo must be within spec as that is something they check when a car is made road legal again here.

As far as I know it's illegal for a speedo to over read anywhere in the (civilised) world. They always have to err on the side of saying you're going faster than you actually are.

Simon
Simon Jansen

Also, how do you know your tachometer is accurate :)
Simon Jansen

Simon. First, I have a GT which allows me to take both the wife and the girl friend (black Lab) with me. Second, the tach can be calibrated against a dwell/tach to see how accurate it is. Usually, it is fairly accurate, but I have seen up to a 500 rpm error. Third, here in the US, the old standard (back when these cars were new) required that the speedometer be within 10% of the actual speed. Consumers Union, a testing organization, noted that one Chevrolet that they tested had a "ticket getter" speedometer which read almost ten mph under the actual speed. Things are easier today with GPS available, making an accurate speed check a very easy event.

Les
Les Bengtson

OK, I need to correct my post above. Did a check today and found the following for my car in direct 4th gear:

30 MPH (indicated) = 1650 RPM
35 MPH (indicated) = 1850 - 1900 RPM

John


J Tait

I wouldn't be surprised if Mr. Everitt's numbers of 17.9 in non O/D top and 21.83 in O/D are both more accurate and precise, but I can calculate 18 and 22 mph per thousand rpm a lot easier in my head while driving.

Of course, changing tires changes things, and 165/80 (or 78) being "standard", and 185/70 and 195/60 would nominally "identical" in diameter. A 175 tire would be slightly larger in diameter and would thus turn a little slower, so your 3% slower seems like a good figure.

I suppose the difference means that when your are supposed to be traveling at 60 mph and the police "give" you 5 mph, then with 175 tires, DON'T go over the 60 mph indicated.

As usual, I agree with Les on a number of matters. First, speedometers are not necessarily completely accurate. Secondly, a GPS unit would probably provide more accurate readings. But thirdly, just be careful out there regarding speed, try to let the other guy make that contribution to the local coffers! We can dance around how accurate our reading are - but the real relevant measure is the speed gun.
John Zajac

Speedos were not designed to be within 10%, but anywhere from 0% to 10% *over* reading. They should never be under-reading for obvious reasons.

"isn't it unwise to take both your wife AND girlfriend on the same roadtrip? For a start where would they sit..."

I think seating plan is the least of the potential problems ... http://www.everydayisbetterthanthenext.com/2004/12/30/upgrading-girlfriend-10-to-wife-10/
PaulH Solihull

I believe a larger-diameter tire will result in lower speedometer and tachometer readings, not higher as Les states. Imagine if the tires were large enough to travel from Cornwall to Scotland with one revolution. You wouldn't see much action on either instrument, even if you traveled the distance in a minute.

Excuse me, Les is not often corrected, and I'm off to buy a lottery ticket.

O.K., back now. Just wanted to add that I pay little attention to speedometer and tachometer readings in the MG, as I don't trust either device to be accurate within 10%. In fact, I've noticed that the tachometer reading varies with ambient temperature, all other things equal. -G.
Glenn G

Changing tyre size or even final-drive ratio will not alter the relationship between the tacho reading and the speedo reading, only the difference between true and indicated speed.
Dave O'Neill 2

"Larger-diameter tire will result in lower speedometer and tachometer readings"

'for a given road speed' I assumed Glenn meant, which is correct.

Very easy to use a 3rd-party tachometer (or two) to check the in-car tach. They have internal temperature compensation in the shape of a thermistor, but after 40 years these can lose their original characteristics.
PaulH Solihull

2500 rpm in o/d 4th 62 MPH indicated today. The speedo is a bit optimistic so the 25 MPH/1000 rpm is probably more like the 22.3 factory number. This is on 165* tyres, not low profile.
* size from memory, but bigger than OE radial fit. I think 155s were specced, they made the car very fussy, about 19.5/100o.
Stan Best

The GT had 165/80 as standard, which would under read by 2.7%, but your 24.8/1000 is over reading by more than 10% so 165s just bring it back into the correct tolerance. One would hope that all GTs had speedos that over read by at least 2.7% ...
PaulH Solihull

This thread was discussed between 22/07/2010 and 26/07/2010

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