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Triumph TR6 - MPH questions

I've tuned my carbs, as best I can, and I think everything is running better. At 2,000 RPM, in 4th gear, I'm at about 38 MPH. What do you guys think is the right range? John B.
JL Bryan

Hi John

Well your Tranny and diff. and instruments are right on and your clutch isn't slipping.

In top gear the tranny is a 1:1 ratio. Rear end is a constant at 3.45:1 and you have 15 inch wheel/tire. So 2000 divided by 3.45 divided by 15 gives me 38.647342 mph. Good for cruise. Low on the powerband so I personaly would downshift from there for acceleration? Easier on the engine. Third at that speed gives about 2600 rpm.

John heres a sight you may not have seen with all kinds of info. Check the links to sol and VTR.

Bill Brayford

Bill-Sorry to disagee. Unless you have a PI car or have replaced the ring&pinion, the ratio should be 3.7. With stock size tires this gives about 21 mph/1000 rpm in 4th or 42 mph at 2 grand.
Berry Price

Bill and Berry, thanks. It sounds like everything is pretty much right on. I know that doesn't mean anything about the condition of the engine or anything, but it is comforting. John B.
JL Bryan

Guys, 2000/ 3.7=540.54. 540.54/15=36
2000/3.5=571.42. 571.42/15=38

Either one is right in the ballpark. John.
JL Bryan

Berry of course is right 3.7. I read the wrong column. Thanks for spotting.
Do not tell my wife! Still trying to live down the last time she found out I was wrong 23 years ago....:) Never forget do they?

Bill Brayford

Bill-Your secret is safe. A speedo that reads low might make a good defense for a speeding ticket.

Hi there! I have been following this board for some time but have not made any postings yet. Thaught that I start by correcting your math :)

To calculate road speed for a certain rpm, a known rear-end and known tire size goes like this:

rpm 2000
rear 3,7:1
Tire 205/70-15

=> tire diam = 38,1cm + (2*(20,5*0,7))= 66,8cm => tire circumferance = pi*66,8=209,86com (I'm metric, sorry...)

Drive shaft at 2000rpm => 2000/3,7=540,54rpm

=> cm/min = 540,54*209,86=113437 equals 68,06 km/h, wich, finally is 42,25 mph

Or am I just making things way too complicated?? Please tell me in that case.....




If your car had gone 34 mph at 2k in 4th what would we have done then?

Yet another question for the ages.

Jim (pondering the meaning of ratio)
Jim Deatsch

SR- Welcome aboard. You are right in your calculations, I just didn't want to beat the subject to death. Also, there is a speed calculator in the tech articles on the Buckeye
Berry Price-also signal red

OK Red the gloves are off...:)

First off you should get out more. There are so many wheel/tire profiles nowadays its scary. But most try to come in at a final circumferance?

Back when I was young before radials. Fairly simple. And they kept it that way. Making speedo ratios etc. Still do.

The question from John was 2000-38 mph in fourth.

If I'm buying a mechanical speedo cable for my hot rod build I look for and can order. Tranny type/rear end ratio/wheelsize.

Wheel size not tire size.

The ratio is designed close to what over all diameter will be. Not an engineer but even back when I was younger tire profiles changed a bit so the math was based on the wheel size for production cars? RPM/Final Drive/Wheel size.


Bill Brayford

Actually, the problem with "fine tuning" the calculation based on tire profile is in the right direction, but it assumes that the wheel is round. Oops. The tire has a flat footprint on the road so the circumferance used for the calculation is best based on the radius measured from the center of the wheel to the road.

Brent B

Ignore that last comment - somebody impersonating me gave out wrong information. Since one rotation of a tire moves it one circumference in length the inflated radius has no bearing on the speed calc. When I catch that stooge.....

Brent B

I think this is more than I really needed to know! Now, if my mpg picks up, I'll know I done good! It's really been fun listening to advice here and then trying to translate it to my car. Most of the little jobs are done, and I have to really work myself up to the bigger ones, like taking the dash apart to really check wiring, or pulling the head to check the valves and guides. Even my wife thinks its wonderful that I'm having this much fun doing things that, in the past, were best left to real mechanics or those 7th grade dropouts from my youth that could fix anything on a car! John.
JL Bryan

OK, I gotta jump in on this one. The Speed/rpm ratio ABSOLUTELY depends on the tire size, NOT the wheel size. Just look in the tire catalogs for the +1 and +2 tire selection options. +1 means that if you increase the wheel size by 1 inch, ie go from a 14" wheel to a 15" wheel, you have to reduce the tire size by 1" to compensate if you wish to retain the same speed/rpm ratio.

Think about it! For each revolution of the wheel/tire, the car covers a distance equal to the circumference of the TIRE, not the circumferance of the wheel. A 185/70/15 tire will fit the same wheel as a 185/60/15 tire, but the circumference of the two tires is NOT the same. Therefore, the distance covered by the two tires per each revolution will not be the same.

Take a look at the speed/rpm spreadsheet mentioned above on the Buckeye triumph site , download it and play with it a bit (it's an Excel spreadsheet), and you'll see what I mean.

A quick and dirty formula for calculating mph/rpm is:

2.975 X (tire diameter in inches)/rear end ratio

This will give you mph for each 1000 rpm. With the stock tires, 2000rpm should give 41mph. If you look in the TR6 owner's manual, you'll find a figure very near that value.

Bill, you owe Red an apology. I don't know where you got that formula from, but it is completely wrong.

Red, Thanks for your input. Don't be put off, feel free to contribute to this group.
Dan Masters

Brent B,

You are correct that the "flat spot" has no bearing on the speed, but it does impact the "effective" rear end ratio as it pertains to acceleration. Let some air out of the tires, and you have a shorter "lever arm" to work with. Increase the tire pressure, and you have a longer arm. A shorter lever arm will have the same effect as a lower (higher numerically) rear end ratio.

The flat spot does or does not make a difference, depending on what you are talking about, thus the confusion on this.

Take a look at the rear tires on a dragster sometime. At high speed, the tires are much (MUCH) larger in diameter than they are at low speed. This gives the best of both worlds for the dragsters - a low ratio for maximum acceration off the line, and a high ratio for maximum speed through the traps. We get a little bit of that with our tires, but is is not enough to make a real difference at the speeds we travel.
Dan Masters

Not to mention drastically lowered wind resistance at speed.

See? Dan's not just another pretty electrical face! He knows tires too. <G>

Jim Deatsch


I'mnot sure I get the lever-arm concept you refer to. The larger "flat spot" on dragster tires would have a significant traction advantage at low speed off the line where needed. At high speed where the traction is probably not as important would'nt centrifugal force throw the tread out so tire circumference is actually larger?

Brent B

Why don't we let the goose figure it out?
Doug Campbell

Brent B wrote:

"...centrifugal force throw the tread out so tire circumference is actually larger?"

Exactly! As for "lever arm," consider the "point" where the tire makes contact with the ground as the end of a lever, with the other end at the center of the wheel. The longer that lever, the less force it takes on the ground end to stop the tire from rotating. Or, expressed another way, the more force it takes on the tire to rotate. The shorter the arm, the less force it takes to rotate.

The lower the rear axle ratio (higher numerically), the less force it takes to rotate the tire, provided for quicker acceleration. Gears are, after all, nothing more than levers. When considering the effects of the rear axle ratio, you must take into account the total picture - gear ratio AND tire size. Coming off the line, a dragster will have a very low effective gear ratio (very high numerically), but at the end of the strip, when the dragster is hitting 300+ mph, the added diameter of the tire will produce a very high effective gear ratio. It would be like shifting from 1st to fifth, without having to shift gears, and would be a smooth, continous gear change.

The larger flat spot on the tire at takeoff would indeed help with the traction. The smaller footprint at high speeds would also be helpful in eliminating drag.
Dan Masters

Especially when you consider that at that point the dragster (and the F/C) are both in clutch lockup and have gone direct.

It DO make a difference, yep, it do.

The goose has known that for years. After all, he flies with his gear 'up'.

Jim Deatsch

I just wanted to know if my mph at 2,000 RPM was within reason! Gooses? We don't need no stinking gooses down here! John.
JL Bryan

Dear John,
Geees know look what you started...:)

First off if Signal Red is not an alias of Berry's I do apologise. Please note I try not to offend anyone more than once in any one day that way they they get over it faster :)

Taking one beating on at a time here.

Signal Red: Your math is good. Very precise. But if the guy in the next lane is blowing your doors off? Who cares? OOPs that's twice.

Bill: You forgot to point out that. Yes the "Engineers" calculated the rolling stock "recomended" tire size when figuring out what goes on that 15" wheel. Nope its not precise. Grade seven stuff "Need speedo drive gear for my TH350" Partsdude "whats the rear end ratio. 7dude "3.70" Partsdude "whats your rolling stock" 7Dude "15s" Heres your gear! 7dude knows the engineers are there for him? Doesn't have to worry about that as he rebuilds his Turbo..:)

Dan: see Bill and Red. Your reference to +- on tire and wheel size is exactly what I was refering to. In production they don't know whats mounted or how hard its inflated. My calc fits that to speedo. Over the counter Speedo gears are based on final ratio and wheel size. Not tire size. 18" wheels on a Civic with Skinnys may be out of the Zone but I'm sure someone is cutting new helicals. Junk anyway.

Drag raced 3 cars not at same time some 35 to 40 years ago.
Mickey Ts where about all available then.
15 psi screwed to the rims. Heat them up on your burn pass. If you and tower timing worked well with no delays. You got what we called a ripple launch. Soft tires. At the end of a 1/4 your tires were hard. Always thought that was from the air heating in the tire after burn and hopefully a bit of slip grip going down the track? To my knowledge "Smokem If You Gottem" started then?

Never had a speedo in any. Getting out quick at the lights/hanging on/and redline/ that was about it. Yes I'm aware things have changed. Ran for fun not profit.

As for the grade seven business. 2 of the wealthiest men I know are both grade school dropouts. One made a max of $6.50 an hour at retirement working in a shirt plant. Owned a large part of the city realty but enjoyed the work and people. None of his co-workers were ever aware. The other is a tradesman always found a way to make what he had work. Owns the other half as we say. Both are multi-millionairs but have problems with big words? What they needed to know they know! Nothing fancy. Both like old cars hence our friendship. I'm not smart or rich.

30 years ago visiting Paul the latter fellow for a pint. His brand new 1/8th mile ashphalt driveway was more than our buddy Sterling could resist. With many bellows of you SOBs wouldn't. A 1968 427 Shelby Cobra line brake launched up his fresh driveway. The driveway has been re-done a number of times. But not that area, ruts are still there. A Trophy. Thats FPM (fun per mile) the most important part. GPM is for your day to day driver.

In 60's thinking back to the 60's "FPM" is the only one that counts. Sterling had a major stroke 5 years ago and about the only thing that makes him real happy is seeing those ruts in Pauls driveway. We still get together for a pint.

Bill Brayford

Wow... I could never have guessed that this thread would go on forever... But I don't think I have any further things to contribute with, at least not regarding the original question.

A few things:

I have nothing to do with "Berry", no connection what so ever.

I was not directing my first posting to anyone in particular, I just did not understand the rpm/rear-end/wheel math, so I posted another way of calculating it.

And as far as blowing doors off goes, I don't think you need a speedo at all if you have someone in the next line... But that wasn't the question here, was it :)

Take care,


Rick Crawford

I waited until this tread was almost over to ask three questions:

1) Can the six motor actually rev as far as 2000 rpm?

2) Can these cars actually exceed 40 mph?

3) If (1) and (2) are possible, is down hill with a wind at your back a requirement?

TR6 currently sitting at 0 rpm and 0 mph in the garage.
R.C. Blair

Couldn't we just put a Spitfire emblem on JL's car, the mph vs rpm would be close to Bill's calclations and there would not be any ruffled feathers (goose?).
Berry-not an alias for signal red

Hi Red
Welcome :) Sorry for the mix up. When I saw Berry's one sign off with "Berry- also Signal Red" I figured he was having some fun with me.

Actualy I answered Johns question in my first line of first reply. "Well your Tranny and diff. and instruments are right on and your clutch isn't slipping" Trying to say it won't tell you much about the engines condition or power. Just that its turning @ 2000. So will my John Deere.

Then I had to go and open my big mouth with a quick example of ratio. Not well researched granted and in tramped every Mathematician and Engineer from here to eternity. Some days just go like that don't they....:)

Again welcome
Bill Brayford

2 x pi x r = circumference
where r is the rolling radius of the tyre that allows for it being squashed over the 'footprint'.
So where does the rubber go that would turn at a circumference = 2 x pi x R, where R is the unloaded radius (e.g. at the top of the tyre)?

Just kidding.

P H Cobbold

Whats the big deal, everyone down here knows that pie are round, cornbread are square.......

pi= 3.14159265, but then pi are round. Thats an old math joke having to do with pi(r) squared. Can we kill this thread? It is long overdue for a decent, compassionate death. May the goose be with you! John.
JL Bryan

A couple of things that were missed in the thread so
1. The owner's manual ( TR250 sure, TR6? ) gives road speed data in all gears + overdrive; rpm at 10 mph and 10 kph; just ratio up or down to find the exact rpm correlating to the mph/kph you want.

2. You can time a measured mile ( as marked on interstate highways ) at 60 mph indicated; it should take exactly 60 seconds. Adjust your speed to meet that and record your speedo reading; the discrepancy represents speedo error. Record your tach reading at
the same time and compare to the road speed data ( 6X the 10 mph value for rpm ) and the discrepancy represents your tach error.

* Note: all the foregoing applies to stock tires! It will be necessary to correct by the factor of diameter difference if non-original sizes are used.
Tom Fremont

This thread was discussed between 09/11/2003 and 25/11/2003

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