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MG MGB Technical - 0 to 60

hello whys does my car go like the wind untill i get in to third gear then there seems to be a massive dead spot and the thrill as gone.ive noticed the car gets to 80 quickly then its a very slow creep up to 95 mph then thats about the max it will go .i dont think these cars can get past car is just short of 110 bhp at the wheels but its not changed the performance after 80 mph why

They will go over 100. I've been there many times. You don't state the year of your car but if it's an early 3-sync gearbox then there is a large jump between the first two gears, which are rather low, and third. That would account for the feeling of a loss of power.
Steve Simmons

Welcome to the wonderful world of wind resistance which increases at the square of the speed. It takes four times the power to overcome the drag at only twice the original speed, so increases in speed at the top end come very slowly unless you have tons of horsepower on tap. A well tuned early B can run the tonne, but it takes a while to get there and you need to reduce drag in everyway you can, from increasing tire pressure, having the hood up and windows raised.
By the way, the police will be round to collect your lisence soon, you've just admitted to a serious moving violation. ;-)
Bill Young

who said i did it on the public road

My '64 with its 3 bearing engine used to run out of revs before I swapped for an overdrive box in, the good news is it still does... at an indicated 117.
I daren't do it again... I like my licence too much.

My rubber bumper GT would go over 100, but it took agesssssssssssssssss...............

What year is your car Daren?
John Prewer


the RWP you quoted should be good for 115+ mph. I can only imagine that a OD box is fitted to your car with permanent engaged OD in 3rd and 4th.



hello its a late 18 v engine it as a peter burgess fast road head 285 cam ,balaced crank, four branch manifold and ss exhaust,bored 1860 just under 110bhp at the wheels.i thought that there would be more power at higher speeds .my car as got at least 35bhp on any standard b engine

Mine's a similar spec (big valve head, HS6s and 717 cam) but less power and I can easily hit the limiter (6000rpm) in top gear on the long straight at Castle Combe - flying lap of course! I'm currently changing the cam to a 285 and raising CR to try to remove the same feeling of running out of steam in third and fourth. Standard top speed is what 105mph and with say 60odd bhp at the wheels?
Neil Lock

It's an MG. It's like bragging about how fast a moped goes.

if you want to talk about speed anon i can give you the spec on my mg rv8

100+ MPH moped. That's a pretty fast moped!

I clocked nearly 120MPH in my '65 B once, but I'm pretty sure the speedo was optimistic by a few percent. So it was most likely closer to 110. I think my guardian angel flew out of the car and got run over.
Steve Simmons


mine is also bored 1868 with a Piper 285/2 and 11.5 CR. Balanced and lightened crankgear and valve train, ralley head, HIF 6 etc. Rear axle is a 3.9 ratio, tyres are 175/80*14. Top in OD 4th. with works hardtop is 208 Km/h, and 184 with hood down.

As the setup of the engine is not very different of yours, comparable speeds should be achieved.
If tere is a OD permanent engaged in 3rd and 4th, you will have problems to reve out in 3rd and 4th. If the rear axle had been changed to a 3.7 ratio (MG-C and MG-B Auto), it might be the same situation.
Take care not to compare the figures with a V8! A 18V engined B needs fighting for excalleration an speed. When I compare it to my GT V8, it is a joke but much more fun to drive.



The standard car in good condition has a top speed of 108mph.

With 90 bhp at the wheels it will reach 120 mph, with 110 bhp at the wheels it will reach 128mph.

Either your speedo is out or your engine isn't producing the power, did the power figure come from Peter's rolling road?
Chris at Octarine Services

Like John Prewer I have a 64 3 brg MGB but non overdrive...... In an mgb roadster an indicated 117mph must feel like flying!! Although my car drives superbly and the suspension and steering are fine with new tyres fitted, I get some steering wheel shake and vibration which is worst at 70mph but puts me off going above 80mph.

Does anyone else have a vibration problem with wire wheels? I realise it is difficult to have wire wheels balanced correctly. Robert
R J Collier

yes chris, it was tested on petes rolling road .he fitted the head and cam the year before.i had the balancing done at farndon motors,rebore at bates and the rebuild at a local garage.i then took it back to peters place and it made a good 105 bhp.i got to 80 quickly then it seemed not to want to go alot faster may be it would get to 95 but it would take forever.i dont know if it would make alot of difference but i had the top down

i tested the speedo today ,when traveling through a local village it went dead on 30 mph and the sign said 34 mph ,would that make a big difference

The wheels on my '67 roadster felt like they were square when I got the car and passing 70 was as traumatic an attempt on the sound barrier. New wires transformed the car.
Steve Postins

If you're saying your speedo read 30mph and the radar showed 34, then you're more the 10% off. Assuming things are linear, then 90 on the speedo would be 102 actual. Why don't you try going through the village at 90?
Derek Nicholson

Derek, that does not work. The person who designed the signs saw you, and everyone else, coming. They all have a cap on the speed they will display. So even though it is 2 digits it does not go to 99! I will leave you to guess how I found out.
David Witham

An MGB with 100+ bhp at the rear wheels will do close to 120 mph in Top O/D. If not, the engine doesn´t deliver the goods or you are driving with engaged handbrakes!
0-60 with 110 bhp should be in the 10sec-region, maybe less, depending on the wheel sizes fitted.

My car with 106 bhp @ 5500 rpm (rear wheel power measured on a rolling road) will just hit 120 mph with the top down on the autobahn, although it takes a while.

For standard size wheels and tyres the workshop manual give a speed of 22 mph per 1000 rpm in overdrive top. So in Joern's case his max power rpm and his max speed are well matched.

If your effective gearing does not suite the power delivery charicteristics of your engine you may not get the max speed increase you hope for.
David Witham


I once had a... shame of it... Vauxhall Viva, my first car, with a propshaft out of balance so it shook like the proverbial over 55mph. It was terrific as a first car because it kept my speed down.

Are you sure your problem is the wheels?

117 (indicated!) feels fine, the car has a 3/4 bar and stiffer front springs & shocks, and 185/70 tyres on steel wheels. My old Rubber bumper car felt deadly at 80 until I stiffened up the front end.

Its made me login again! I am in Yorkshire, honest.
JW Prewer

First, I wouldn't believe ANY MG speedometer. We've checked many of them and they are usually well off the mark - 10-15% in some cases.

You will get a closer measurement by using the tachometer reading (they seem to be more accurate) and working it out from your tire data.

A good stock MGB should do about 105 MPH, which is also about what my MGA Mk 2 coupe will do.

And MGC (or MGA Twincam) will do around 118 MPH.

If your tuned car isn't making the ton, you should look for some problem - perhaps ignition timing, but that's just a guess.

(MGC - 130 mph, MGA - 132 MPH, Jensen CV-8 153 MPH, Lamborghini - who knows - I haven't wanted to risk my license that badly...)
Bill Spohn

jw prewer when you say stiffen up the front end what bushes did you change and were they the sofest of the nylon bushes

Steve P
Thanks for the suggestion, new wheels may well be the cure, the thing is the wife only allows me to go 60mph (which is fine with the top down!)
I think it is the wheels as the vibration is on the steering not the car. Springs are new, V8 bushes, Kingpins steering fine etc. My first car was a Viva too!. It would do about 80mph flat... out everything vibrated especially the bonnet!
R J Collier

"I wouldn't believe ANY MG speedometer"

Sounds like you need your speedo calibrated! With the proper calibration, it will be as accurate as any speedometer can be. More accurate than most new cars even. Without calibration and on non-standard tires, it would be easy to be off by 10% or more.
Steve Simmons

I'll second that, my V8's speedo was calibrated by Speedy Cables and is spot on at all speeds as verified by the sat nav.


Get, or borrow a sat nav and check the speedo, if Peter tuned the engine and it is showing 105bhp at the rear wheels on his RR then you should reach 126mph in theory!

Running out of breath at 80mph doesn't sound right at all!
Chris at Octarine Services

My car will spin the tacho round to the orange in o/d top. I have hit 100 on the pit straight at Silversone , 3 passengers ( so yes the person jammed onto the battery shelf had no restraints, but it wasn't his day to die!) and the roof down ( = more drag). My car has the ST front spoiler which has to help at those speeds as well. If you disengage 0/D and nail it at motorway seppeds it really flies. Is the centrfigal advance working OK? Is the vacumn capsule OK? Do you have electronic ignition and can you easily go back to the points to see if that is failing to deliver the spark? Your car should be great fun to drive, and able to surprise a lot of more modern machinery.
ps, fuel pump OK?
Stan Best


I use V8 bushes, stiffer front shocks done with thicker fluid, Chrome bumper GT front springs (I don't want it too hard) and negative camber wishbones for chuckability. I keep the back end standard, as with leaf springs it needs to stay soft.

I take my advice from Malcolm Beer (Beer of Houghton, 01480 462034) MG & V8 specialists for decades. Malcolm regularly won the Car Club BCV8 championship, and now his son is doing the same!


The early MGB speedo was consistently rated as accurate in early road tests, I've checked mine against several modern cars over the years & it seems to read slighly faster than them (e.g a trifle more accurate as then all under-read). It is definitely over reading at 117mph, but its got to be 110ish, and I always said an INDICATED 117!

Question: is the speed indicated on a Sat Nav accurate? I have a Tom Tom Navigator 5 and a TomTom GPS and it shows the cars speed. I must try it in the MGB.

John Prewer


Provided you are clear of tall buildings and trees close to the road, then the sat nav is spot on.
Chris at Octarine Services

Exactly, I have a GPS that tells me the speed and the MG (at least mine) will do 100mph +. But then you need to ask - why? It is not a great high speed car. It is a great small run around car. It has the sensation of speed without the tickets. I can lay rubber without huge accelleration too (small narrow tires)
Frank Baker

This summer I went out on our local lake with a friend with a restored 1928 speed boat. He had replaced the engine with a mildly modified 350 chevy v8. It had the stock guages - no speedo - a beautiful work of art. I asked how fast we were going. He looked down at his leg and said 32 mph.

The contrast between the old and the new gps unit on his lap.

Barry Parkinson

Once, in my foolish youth, I had my '65 with the cable driven tach sitting at 6200 rpm in 4th gear, down a long hill/ up...........

So with 17.9 mph in top gear with the original size 165X14 Dunlop radials tires, and the 3.91 final drive = 110.98 mph ?


Don't know if this was a 3 main or 5........

In between years.

Hey in 1966-7 that was purdy good.........


DCM McCullough

On our motorways there are numbered posts every 100 metres. I have used these and a watch to check my speedo in a number of cars.

Rather than try and read the posts numbers while doing the test I find 2 landmarks and note the numbers on the posts nearest to them. Then I drive at constant speed between them measuring the time taken. After that it is just a bit of simple maths.

It goes without saying that this only works up to 70mph! The M45 is good for this.
David Witham

This thread was discussed between 21/02/2007 and 26/02/2007

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