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MG Midget and Sprite Technical - speedo way off?

Last weekend I am pretty sure that my speedo was reading 80 MPH in 4th gear at 3500 rpm. I am hoping that I just forgot my reading glasses and I was really indicating 60 mph. I will check again. ;) I was thinking that my rear end gear ratio is 3.9. Was there more than one choice in 73? I thought that I read somewhere that some of them were 4.11 but I can't find where I read that.

I will report back on my speedo reading on monday. ;)

Rebecca
R Harvey

3.9 was the common, 3.7 is the rocking horse poo version and 4.11 are for the track I dont think many use them on the street
Prop

might have a tire shop look at the tires for proper size... then aagian it is a midget, maybe it was tring to impress you.


Prop
Prop

80mph at 3500revs in fourth is perfectly acceptable as a reading on the speedo - exactly why are you worrying?
rachmacb

3.9 would be the original for a 1973 1275 car.

If you had a 4.11 then that would be an even lower ratio and your speed wought to be well below that idicated, even a later 3.9 diff comes nowhere near.
Bob Turbo Midget England

Because the speedo drive is taken from the gearbox and not the road wheels, the relationship between the indicated speed and the engine revs does not change when you alter the final drive ratio. What does change is the difference between indicated speed and actual speed.
Dave O'Neill 2

I did not drive with a GPS yet but at the time that I thought that I was going 80 MPH I think that I was not actually going over 60 MPH. I am hoping that I read it wrong because when I glanced at it the numbers were in a different place than my other cars.

I had spent a lot of time calcualting what my speed would be in each gear (beacause I was wondering how high it would wind up in second gear during autocross).

Rebecca
R Harvey

Hey reb,

Its been along time scence Ive seen my speedo above .25 mile an hour, but Im thinking 60 is about right

Thanks Dave,

yeah i was in lala land...good catch.

Prop
Prop

I'm thinking you were doing 60, here's a shot i took of my gauges in my '73 before i swapped the transmission (so this was with the stock set up).

Speedo reads just a hair over 60, revs reading almost 4000.


Chris Edwards

I find that, if you're worried, the best thing to do is drive with either a dashboard ap on an iPhone, or GPS is the safest thing to do.

I always have one or the other on, especially going through urban areas, that way I know I'm under the speedlimits and won't get fined!
rachmacb

>>> 80mph at 3500revs in fourth is perfectly acceptable as a reading on the speedo - exactly why are you worrying? <<<

Wha?!?! My '78 Midget shows 4000rpm at about 70mph. So unless you have a 5-speed conversion, I doubt you were turning 3500 at 80. Chris's picture would seem to back this up.

IMHO,

-:G:-
Gryf Ketcherside

Yeah, my assumption was that there was a problem with the speedo or my brain (I won't find out until this weekend).

Rebecca
R Harvey

Had it on a nice gentle long early-morning run on a motorway - but hey ho - never get them normally, mostly due to the stupid cameras ... hence the need for GPS here nowadays :(
rachmacb

"perfectly acceptable"
Yes, and highly desirable, but very wishful thinking.

Speeds per 1000rpm for stockish rubber at different ratios are about 15.5/4.22 (pre 69); 16.8/3.9 (69-76 approx); 17.7/3.7 (later). There are other optional ratios, but they are rare, aka race car stuff.

There is a number on the speedo, 3 or 4 digits like 975 or 1040 or 1420, that is the turns (of the cable) per mile. That's what changes for different axles, wheels, etc. Speedos from other cars/MC like MGB fit too, but have different TPM. Speedos also have spec numbers like ****/** for the actual instrument including appearance etc., but TPM is what matters for the readings.

So, find out the axle ratio (stamped on the alloy diff case as tooth count ie 39/10 = 3.9) & correct TPM (Chris??), and check against yours.
It is possible for the speed to read high due to a fault, but it is usually obviously variable/flaky, and the odometer will still read as determined by TPM; check against mile posts.

For autox you probably want the 4.22 and matching speedo, but it is a bit buzzy for a road car.

FRM
Fletcher R Millmore

LOL - was a fantastic drive - after months of working on her and getting everything just about as good as you could ever hope - perfect atmospherics, blah blah blah - ruddy thing came home on a recovery vehicle having blown the wiper motor the next morning and fused everything ...!!! Oh well - can't have perfection too much you might get used to it!!!!!

On the plus side - found the most wonderful, helpful, knowledgeable, kind mechanics one could ever hope to find in MG world!!!!
rachmacb

I think that I know what my problem is. I have the wrong speedo in my car. It is a 120 MPH speedometer. Probably I have the correct speedometer drive gear and the expected 3.90 rear end. The speedo numbers where the pointer is pointing are not marked correctly (if the correct speedo was installed I think that it would be indicating correctly).

I think that I just need to replace it.

Rebecca
R Harvey

Doesn't matter what the max reading is - it's all the TPM number. There could be ten speedos with the same max but different TPM, or ten with different max and the same TPM.
Maybe somebody will post the correct numbers for a 73; might help if you posted the ones on your speedo. And it's not hard to crawl under and check the axle ratio. AFAIK there is only one drive gear.

FRM
Fletcher R Millmore

As what I've got one (1973) I will say allowing for gauge errors that at 60 mph you'll be showing 3,600(ish) rpm

At 80mph you'll be showing 4,800(ish) rpm

With a standard Midget there's no point having a speedo above 100 mph as they topped out around 93 mph when new but if you want a 120 mph for future developement (or ego in some cases) then that's fine

I have a 100 mph speedo in my car it'll be interesting to see if the car will get to to 100 mph on the rolling road now as it topped out at 93.8 mph according to the RR guages last time before new bits
Nigel Atkins

I'm so sorry to have my engine so very well tuned when it was running well that I was able to get perfection - obviously, I do NOT take photos of my gauges when I'm driving at 80mph on a motorway - so I do not have any proof - bar my word - which is NORMALLY good enough for anyone.

I DID speak to my friend who taught me loads of what I know - and he confirmed that I am correct and not making it up.

However, I am not going to continue having to justify my tuning and car.
rachmacb

Stick wrong end of someone (is it me?)

Rach, I was talking about a 1973 (1275) Midget as that's what Rebecca has

A standard 1973, British I better add, 1275 topped out around 93 mph when new so no need for 120 mph speedo now unless you want one or need one later

Someone been out in the sun too long :)
Nigel Atkins

It topped out on a rolling road at 93 MPH??
Something seriously wrong with that Midget? with no wind resistance (the main restriction on top speed) to only have a top speed (I assume thats what top out means?) of 93MPH would suggest that the thing was very poorly.
Robert (Bob) Midget Turbo

<<<I DID speak to my friend who taught me loads of what I know - and he confirmed that I am correct and not making it up.

However, I am not going to continue having to justify my tuning and car.>>>


Am I missing something here? Making what up? I've read through the entire thread and I'm struggling to see what the issue is.
Dave O'Neill 2

Car toped out at 93 mph on the rolling road as the car was new to me then and the RR was to tune and sort needles not for getting top speed or figures on paper

Plus the RR guy probably didn't want to push too hard an engine he and I didn't know

Maximun power was at 85 mph, there was a huge lost of power between engine and wheels (tubed tyres), and somthing else I forget

I was happy with 94 mph - until now, my illusions have been shattered, I'm quietly sobbing :(
Nigel Atkins

Of course, rev counters can be wrong also! If you are on a rolling road it is worthwhile checking the rev counter dial against the RR equipment reading
Guy Weller

Yes good point, my rev counter is about 1-200 rpm out according to the gun I held - unless the guns vary too or it depends where the gun is clipped - some many possible variables

Gauges probably start out with a acceptable degree of inacurracy then you have to allow for wear and tear on the guages and what supply them with their readings

The figures I gave in my last post were from the very large anologue dials of the RR - now of course that machine could be inaccurate too

A degree of tolerance is need in Midget measurements - and in Forums

oh, Bob at the RR my Midget had to contend with a lot of wind resistance from the huge fan they put in front of it so that restricted the top speed



steady now, I was joking
Nigel Atkins

This thread was discussed between 24/06/2010 and 28/06/2010

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