RPM at a true 60 MPH

[tdskip]

IMG_6247.jpeg

2620 rpm @ 60 mph
3060 rpm @ 70 mph
3495 rpm @ 80 mph
3935 rpm @ 90 mph

3.94 diff ratio (effective) - uncompromised acceleration

That sounds perfect

[TBG]

1owner69

I don't understand your figures! Those quoted are almost a dead match for my 3-1 diff. and you are running

nearly a 4-1. Can you explain where your high ratio comes from please?

My figures 4 speed box an 3-1 diff

2620 rpm @ 60 mph 2700@60 mph
3060 rpm @ 70 mph 3100@70 mph
3495 rpm @ 80 mph 3500@80 mph
3935 rpm @ 90 mph 4000@90 mph and 110 mph @ 5000

3.94 diff ratio (effective)

[richardcox_lotus]

Handy spreadsheet in this link:

https://lotuselan.net/forums/viewtopic.php?f=37&t=17000&p=87921&hilit=+Gearing#p87921

Regards
Richard

[TBG]

Thanks for the link but I have followed the instructions and it comes out as a jumble of stuff.

[1owner69Elan]

1owner69

I don't understand your figures! Those quoted are almost a dead match for my 3-1 diff. and you are running

nearly a 4-1. Can you explain where your high ratio comes from please?

My figures 4 speed box an 3-1 diff

2620 rpm @ 60 mph 2700@60 mph
3060 rpm @ 70 mph 3100@70 mph
3495 rpm @ 80 mph 3500@80 mph
3935 rpm @ 90 mph 4000@90 mph and 110 mph @ 5000

3.94 diff ratio (effective)

TBG: My 5th gear ratio is 0.75

Thus, your overall top gear ratio is 3.0 (1 x 3.0) vs my 2.955 (0.75 x 3.94). So, as you note our top gear (4th vs 5th) figures for rpm/speed are very close.

The difference comes in the lower gears. The comparison table shows the overall ratios for the 3.0 (semi-close: 2.97/2.01/1.39/1.0) vs the 5-speed in my car (close: 2.66/1.75/1.26/1.0/0.75):

As shown, there will be less torque at the wheels in every gear 1-4 with the 3.0 diff.

For driving on backroads the most important gears are 2 and 3 (and sometimes 4). If you gear down to 2nd or 3rd for a turn and then want to accelerate out of the turn - there will be less available torque with the 3.0. If you have a high spec engine then one could argue you won't notice it. But, frankly I wouldn't want to see my 143 lb-ft reduced to 125 or 120 lb-ft. Or consider a Sprint Big-valve being reduced from 113 to 99 or 95 lb-ft, less than a standard Elan.

I find that I only use the 5th "overdrive" on the freeway as a cruising gear. For backroad driving, it is really not used and having the "full available" torque on hand in the lower gears (1-4) is desirable.

Bottom line, there is no "free lunch" with going to a taller diff ratio like the 3.0. You can achieve a nice, relaxed high speed cruising rpm. But, there is always some kind of compromise: In this case, less "at the wheels" torque in lower gears. Whether the compromise is important is a matter of personal preference and the type of driving you do.

In any case, there is no question that the conversion to the 3.0 CWP is certainly less expensive and much less effort than going the 5-speed route.

[TBG]

Iowner69 - thank you very much for that clear explanation. Very interesting it is as well. I am lucky in that I have a 1700cc motor with QED 420 cams and I have to say that second and third gears are an absolute hoot, but top is still very much in use on Somerset roads. Some owners have been worried that I would have trouble starting off on steep gradients but this definitely not the case. All in all I am thrilled with the results from the risk I took - and for only about £500 - as a bonus as the diff has "run in" the over run whine has gone!! Happy bunny down here in the South West.

[1owner69Elan]

I am sure the local authorities here would cite me for having my young grandchild riding on someone's lap in my Elan, as shown in your photo. Let alone grandma not biting my head off!

But, driving in an Elan, given its less than substantial safety profile, is an act of faith for anyone.

[TBG]

OOOPS !! He is 19 today so he has survived!!

[1owner69Elan]

Reminds me of riding with my brother in the "back" (jumpseat) of a new 1958 TR3, with my parents in the front, for a trip to Disneyland. Over 400 miles each way with no seatbelts or anything, front or back.

I am glad we all survived.

Different times. Blissful ignorance.

[TBG]

Seatbelts? I actually fell out of this one whilst driving it....................note USA

[tdskip]

OK - got the digital tach connected and my indicated 5000 RPM is actually around 3800 RPM.

I'd go cross-calibrate that with a GPS driven speedometer but my handbrake is stuck on. So it works, which is nifty, but need to figure out how to get to release. Maybe it is time for a pint?

[tdskip]

Jiggled it and handbrake released so here is the GPS data

Tach | Actual RPM | Speed
4500 | 3400 | 60
5000 | 3800 | 66

[1owner69Elan]

Jiggled it and handbrake released so here is the GPS data

Tach | Actual RPM | Speed
4500 | 3400 | 60
5000 | 3800 | 66

From my calculations, pretty close to your actual observed:

3400 rpm = 61.0 mph
3800 rpm = 68.1 mph

[tdskip]

Jiggled it and handbrake released so here is the GPS data

Tach | Actual RPM | Speed
4500 | 3400 | 60
5000 | 3800 | 66

From my calculations, pretty close to your actual observed:

3400 rpm = 61.0 mph
3800 rpm = 68.1 mph

I’d say so!