[TR] RPM speed calc

[TERRY SMITH]

For the longest time I was confused.  ...Okay, several decades for those who are critical about exactness....

I was running an OD in my '3 and the speedometer read 80 mph.  I forget the rpms.  3 or 3.5 thereabouts.  Yet traffic on Hwy 93 still passed me by, despite that they didn't when I was driving 80 mph in my Tacoma.  

Discovery came when I'd rigged a electrical charger in the'3 so my wife can take her phone, then used it for my GPS on a Craigslist trip out of town.  And lo and behold, when the car's speedometer said "80," the GPS rebutted saying "73" or thereabouts instead mph.  For whatever it means, I gather I have a 4 speed differential leftover from when I swapped the 4 speed for an OD.  

I've decided it doesn't matter all that much to me.  The real win is that the '59 TR3A is a now 61 year-old 4 cylinder car that can keep up with modern traffic.  If I wanted to run a true '80' mph, I could without much difficulty.  

Actual question here:  How many other cars from that vintage can keep up with traffic?  Talking your basic 4 cylinders here.  I'm particularly interested in the Porsche 356.  Didn't it come stock with a 1600 cc engine?

Terry Smith, '59 TR3A  TS 58667
New Hampshire

> On June 1, 2020 at 3:24 PM John Macartney <John.Macartney at Ukpips.org.uk> wrote:
> 
>     It’s important to remember that cars without overdrive had a higher axle ratio than those with it. As an example, the GT6+ had a 3.27 final drive for non overdrive and 3.89 for overdrive. Just to complicate things, it was entirely possible to order a 3.27 diff with overdrive on top! The factory never promoted this facility for fairly obvious reasons. My own GT6 mk 2 was supplied like that becxause I knew my way around the factory build books. Iit made an excellent cruiser with good fuel consumption but could not pull more than 4000 rpm, even downhill on a motorway. That said, refuelling was a little less frequent.
> 
>     Jonmac
> 
>     At the Battle of Waterloo, 1815:
>     Lord Uxbridge:  “By God, Sir - I’ve lost my leg”
>     Duke of Wellington: “By God, Sir - so you have!”
> 
> 
>     On 1 Jun 2020, at 19:41, Dave MacKay < dave at mdmackay.ca mailto:dave at mdmackay.ca > wrote:
> 
> 
>         > > 
> >         >Date: Sun, 31 May 2020 16:52:23 -0500
> > 
> >         >From: bill beecher <notakitcar at yahoo.com mailto:notakitcar at yahoo.com >
> > 
> >         >Subject: [TR] RPM speed calc
> > 
> >         > 
> > 
> >         >Came across this calculator with all the right input fields.  But, I wonder how accurate it really is given the following input.
> > 
> >         >TR3 with a 3.7 rear, OD at redline of 5,000rpm.  Vredestein 165R/15 tires, 25.4? diameter.   Should be moving along at 124mph!
> > 
> >         > 
> > 
> >         >https://wahiduddin.net/calc/calc_speed_rpm.htm
> > 
> >         > 
> > 
> >         >Anyone get a different answer?
> > 
> >         >Bill
> > 
> >          
> > 
> >         Here (attached) is a spreadsheet that I wrote to calculate speed versus rpm given different parameters. I wrote it to check my speedo after I installed an HVDA transmission. If the spreadsheet doesn’t make it through the list as an attachment, just e-mail me directly and I’ll send it to you.
> > 
> >          
> > 
> >         With a standard Triumph gearbox + overdrive, and the parameters you gave, I calculate speed at 5000 rpm in top gear (with overdrive) as being 124.5 mph. Note that, I think that the tire diameter is 25.25”, slightly less than you indicated.
> > 
> >          
> > 
> >         I hasten to add that calculating a speed and believing it’s achievable are two different things!
> > 
> >          
> > 
> >         Regards,
> > 
> >          
> > 
> >         Dave MacKay
> > 
> >         1960 TR3A s/n 68639L
> > 
> >         near Toronto, Canada
> > 
> >     > 
>         > >         <Triumph rpm to mph.xls>
> > 
> >     > 
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