mph to revs

[brian-nz]

Hi All

Having completed a 1324 mile trip to go to our TR Nationals, I noted that at 60mph 100kph) the rev counter was registering about 2700 revs in top overdrive

 

Can anyone advise if this is correct. Others cars seemed to be revving at 2500rpm.

I do not know what diff I have (3.7 or 4.1) but a PO has street raced the car some years ago.

 

Regards

Brian

Edited March 8, 2015 by brian-nz

[RogerH]

Hi Brian,

you have a lot of variable there.

Does the speedo record the correct speed.

likewise the tacho.

What marque of OD have you got - 'A' type has 22% gearing 'J' type has 28% gearing (I think).

Tyre diameter at the back

And the diff (as you say). If it was street raced then perhaps acceleration was the key issue so 4.1 would be useful.

 

Use a satnav to give correct road speed.

 

Just recently I had my tacho converted to electric.

I used to rely on 70mph (indicated) = 3000rpm

With the new electric tacho it is nearer 80mph = 3000rpm

 

60mph in 4th = 3000rpm

 

the above numbers are very apprx but show the problem.

 

Roger

[TR 2100]

My rule of thumb is 20mph/1000rpm in 4th

and 25mph/1000rpm in 4th o/d (3.7 axle).

 

A 4.1 axle (they are rare) would obviously

increase the revs proportionately.

TR6s have a 3.45 axle that gives very smooth

cruising at low revs in 4th overdrive.

 

AlanR

[mleadbeater]

If you buy a hand-held optical tachomater, around £20 on ebay, £12 for a used one at present, you can callibrate your cars tacho.

 

They work by attaching a piece of self-adheasive reflective tape to the rotating part, say the front pulley, you then aim the beam at the tape and get an instant reading of rpm. You will then know how far out the cars tacho is, and maybe mark some accurate reading points with a marker pen or whatever.

 

Then, as suggested, your satnav can give accurate speed readings for your tests.

 

To power your satnav you can buy ( ebay again ) at low cost a cigar-lighter style socket, wired under the dash to a suitable fused supply.

 

This is something I will do myself shortly to determine my cars approximate performance, without abuse ( of the car) as has been mentioned in my earlier posting.

 

Cheers

 

Mike

[ianc]

If fitting a cigar lighter socket for a SatNav, make sure that the connections are the right way around as nowadays everything on a car is negative earth, but TRs with dynamos will be positive earth (unless someone has converted to negative earth).

If the car is positive earth, you would need to mount the socket on a piece of insulation in order that the casing of the socket can be made negative.

Wouldn't want you to blow up a newly-acquired SatNav!

Ian Cornish

[Don H.]

As Roger notes, ya gotta check your data.

 

There are useful iPhone apps for accurate speedometer measurement. When I want to know speed precisely I use one called SpeedHUD. It has, as the name suggests, a heads-up display option where the windscreen reflects the speed in the driver's field of vision from the phone placed on the dash. That particluar option works better on cars with significant windscreen rake and a lot of dasboard suface in front of the wheel -- so not a realistic option in a TR3 -- but the speedometer is perfectly accurrate no matter where it's placed.

[BlueTR3A-5EKT]

You want to be going slightly quicker than the cotton heads in the Nissan and slower that the rep in the Audi

 

In NZ stay up with the empty logging trucks if you can....

 

Cheers

Peter W

 

Minty Lamb's Excellent ratio/speed/rpm calculator

http://www.mintylamb.co.uk/gearspeed/

Edited March 8, 2015 by BlueTR3A-5EKT