TR2/3/3A TR3A RPM vs MPH

[CraigLandrum]

I could probably figure this out, but I'm betting one of you guys knows this off the top of your head...

I trust my tach(checked electronically) but know my speedo is off after I cleaned and refurbished it. On a non-overdrive TR3A, with 165R15 tires, what are the tach numbers for some common speeds in 3rd and 4th gear? It would be useful to know the numbers for 25,35,45,55,60, and 70.

 

[Moseso]

Quick and easy check:
3000 RPM in 2nd = 30 MPH
3000 RPM in 4th = 60 MPH
Plus-or-minus not enough to be concerned about.

 

[Geo Hahn]

The handy 'Practical Hints' book aka the TR3A owners' manual has a table of RPM:MPH data in each gear and for each rear-end plus OD.

As noted above, there is a quick conversion that is close. Also (as on most sports cars I've owned) the tach and speedo needle are roughly parallel in 4th gear.

 

[GBRandy]

A borrowed Garmin Nuvi can really dial things in :wink:

 

[Andrew Mace]

Quick and easy check:
3000 RPM in 2nd = 30 MPH
3000 RPM in 4th = 60 MPH
Plus-or-minus not enough to be concerned about.

Exactly! Keep in mind that most current 165R15 tires are likely to be a bit smaller in diameter than the original Dunlop Gold Seal 5.90/6.00 x 15 bias ply tires on the car when new, but it's probably not enough of a difference to worry about given the overall "accuracy" of the speedo and tach. :wink:

 

[NickMorgan]

Quick and easy check:
3000 RPM in 2nd = 30 MPH
3000 RPM in 4th = 60 MPH
Plus-or-minus not enough to be concerned about.

Exactly! Keep in mind that most current 165R15 tires are likely to be a bit smaller in diameter than the original Dunlop Gold Seal 5.90/6.00 x 15 bias ply tires on the car when new, but it's probably not enough of a difference to worry about given the overall "accuracy" of the speedo and tach. :wink:

... and should make the same difference to both the speedo and rev counter, so won't matter!

 

[CraigLandrum]

Thanks for the rule of thumb guide - looks like I'm 5mph too low.

 

[Geo Hahn]

...165R15 tires are likely to be a bit smaller in diameter than the original ...but it's probably not enough of a difference to worry about given the overall "accuracy" of the speedo and tach.

... and should make the same difference to both the speedo and rev counter, so won't matter!

Huh? How does tire size affect the accuracy of the tach?

 

[NickMorgan]

Aren't we talking about what the speedometer reads at certain revs, rather than actual speed?

 

[BobbyD]

Here's a few calculators that let you input a bunch of variables like: tire size, diff ratio and tranny ratio to calculate MPH and RPMs:

https://www.fatboyraceworks.com/gears/

https://www.csgnetwork.com/rpmcalc.html

https://wahiduddin.net/calc/calc_speed_rpm.htm

https://www.mintylamb.co.uk/?page=tyre.htm

 

[martx-5]

That's correct, the speedo and tach readings will remain the same regardless of tire size. The speedo is registering the speed at the output shaft of the transmission. Anything that is changed after that will have NO AFFECT on the speedo reading. The <span style="font-weight: bold">actual speed </span>of the car will change with a difference in tire size or differential ratio, but the <span style="font-weight: bold">indicated</span> speed will remain the same.

BTW, my experience with TR's has fourth gear speedo readings slightly higher then double the tach. IE..tach at 3000 rpm is equal to about 62-63 mph...<span style="font-weight: bold">INDICATED.</span>

 

[Geo Hahn]