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rpm vs mph

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NutmegCT

NutmegCT

Great Pumpkin
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I've noticed in a few "picture through my windshield" photos here that the tach and speedo are often at the same "angle" when in top gear.

Mine aren't.

If my speedo is at 12 noon (60mph), my tach is around 1:00 (3500). Seems my rpms are faster than they should be for a given speed. (four speed, no o/d)

Is there a "standard" correlation?

There are lots of variables possible (ratios, replacement parts like differential and gearbox, wheel and tire size, etc.) but as far as I know I've got original parts other than my Minator 15x5.5 wheels and Kumho 165/80-15 tires. Had the same rpm-mph issue with my original wheels and the Kumhos. Tach doesn't wander or stick, nor does speedo. Clocking my speedo with a GPS shows the speedo is about 3mph slower than true.

Would the modern wheels and/or tires cause this?

Any ideas?

Thanks.
Tom
 
Tom-

Here's what mine looks like. No direct ratio it appears.

recordMiles55.jpg
[/img]
 
A non overdrive TR6 with a standard rear should read 2:1 for speed to RPM. 60 equates to 3000.

That being said on the new wedge when I get over 50 the speedo bounces around. Anybody out there with the 3.38 (?) rear know the ratio for 5th? Just let me know at 2500 and 3000
 
The ratio of mph to rpm in each gear (OD & w/o OD) are in your manuals such as your 'Practical Hints', page 7.

A TR3A with the the 3.7 diff in top gear no OD turns 501 rpm for each 10 mph, e.g. 60 mph = 3006 rpm.

A car originally equipped with Michelin X would have slightly lower rpm at that speed and would have had (I believe) a slightly different speedo calibration number.

Since you have confirmed your speedo with a GPS, most of your difference from the 60=3000 benchmark would seem to be either --

You have the 4.1 rear end (possible but unlikely) or...

Tires that are a smaller diameter than the originals (think they would have to be quite a bit squattier) or...

Your tach is inaccurate (my guess).
 
Tire size and rear end will not have anything to do with what your speedometer says, since the speedo is linked to the tranny.

I have been guaging my speed based on my tach for quite a while, not having the proper linkage between my Triumph speedo and my Toyota tranny.

But then, it wouldn't be a proper British car if the wasn't something left to fix on it now would it?
 
gsalt57tr3 said:
Tire size and rear end will not have anything to do with what your speedometer says, since the speedo is linked to the tranny.
Click to expand...

Tire size and rear end will affect the relationship between the speedo and the tach... which is what the original question was about.
 
Tire size and rear ratio will effect the relationship between road speed and spedo reading. That is why different size tires or a different rear ratio will change the road speed indicated in relation to the actual road speed.

Tire size and rear ratio will not effect the spedo to tach ratio. But a different transmission might.
 
tomshobby said:
Tire size and rear ratio will not effect the spedo to tach ratio. But a different transmission might.
Click to expand...
/bcforum/images/%%GRAEMLIN_URL%%/iagree.gif Also, for the 3 (and through early TR6) there are different speedo heads that will give different speedo:tach ratios. Later TR6 used different driven gears in the speedo drive (in the gearbox), which as I recall will fit the earlier standard gearboxes.

Having the speedo agree with GPS is no guarantee the speedo head is not the cause of the difference. For example, if the speedo reads 7% low but you've put 7% taller tires on (or a 3.45 axle); then the speedo reading will still match the road speed but not the tach reading.

But I agree, the most likely problem is an inaccurate tach. Reading 500 rpm high @ 3000 isn't unusual.

BTW, the optional Michelin radials were shorter than the bias-ply Dunlops, not taller. If you have the speedo calibration for a 3.7 axle & Michelins, you would expect to see 3090 rpm @ 60 mph indicated.
 
Interesting ! The chart I posted came from the 2nd edition, while I see Geo's chart in the 6th edition.

On the previous page, the 2nd edition lists tyre size simply as 5.50-15 while the 6th shows it as 5.50/5.90-15. Looks like it must have been the radials that increased in size, rather than the bias ply (as Piggott implies).

Tachs were always 2:1 (indicated rpm is twice cable rpm), but there is a calibration number on the speedo face that gives cable turns per mile. Should be around 1150-1180 for a 3.7 axle.
 
Tire sizes, axle ratios, etc. have no relationship to MPH or to RPM it's all junk. It only depends on the size of your tail-lights.
 
/bcforum/images/%%GRAEMLIN_URL%%/lol.gif /bcforum/images/%%GRAEMLIN_URL%%/lol.gif
 
DNK said:
A non overdrive TR6 with a standard rear should read 2:1 for speed to RPM. 60 equates to 3000.

That being said on the new wedge when I get over 50 the speedo bounces around. Anybody out there with the 3.38 (?) rear know the ratio for 5th? Just let me know at 2500 and 3000
Click to expand...

If you mean 3.08 (standard TR8 rear diff), then 3,000rpm in fifth gear is about 82mph for me. But my tires are slightly larger in diameter. /bcforum/images/%%GRAEMLIN_URL%%/driving.gif
 
tomshobby said:
Because a 4 cycle crank turns twice for every rotation of the dizzy.
Click to expand...
Except that there is no guarantee that the tach drive cable turns at distributor rpm. After all, the speedo cable doesn't turn at driveshaft rpm ...
 
Go to this site and download the second program. https://www.landiss.com/mphvrpm.htm. Plug in the numbers for your car and it will give you almost exact figures. Play with the numbers and you will see the coralation between rear end gear raito, tire size, wheel dia, speed and tach readings. It is an excel spreedsheet so you have to have MS Excel or Sun Microsystems Office suite. Sun is a free download if you need it.
 
Great info, now if I can only get all the info I need for it.
 
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