engine speed

PAYNEY05 · Post by **PAYNEY05** » Tue Feb 22, 2011 9:33 pm

Hi Guys,
I am running a stock 138 red motor with a hr gearbox and have recently changed the diff ratio to 3.36(15inch wheels 195/55).On the weekend i put a tacho in and find at 100km/hr the motor is revving at about 4800rpm.At 110 it is about 5200rpm. Is this a normal figure i would expect? The speed is true as i used a gps.The car seems to be quite comfortable at 100.
Cheers,Payney

Trev · Post by **Trev** » Tue Feb 22, 2011 9:48 pm

It sound like the tacho is out

, at 4800 or 5200 rpm the grey would be screaming it's ring out.

What color speedo gear are you using?

PAYNEY05 · Post by **PAYNEY05** » Tue Feb 22, 2011 9:54 pm

Hi Trev,
Im not sure about the color of the speedo gear,but it is a 138 RED motor and the tacho is a smiths.
Cheers,Payney.

Devilrod · Post by **Devilrod** » Wed Feb 23, 2011 7:54 pm

An old Smiths? Is it a 6cyl one? They do sh!t themselves after a while and can be all over the place my brothers one did exactly that.

Those revs are way out....my guess tacho is not right.

Cal · Post by **Cal** » Wed Feb 23, 2011 9:32 pm

It does sound like the tacho is out, although a 195/55/15 is quite a small diametre tyre. It wouldn't be helping the cause.

Trev · Post by **Trev** » Thu Feb 24, 2011 4:22 am

That size wheel n tyers are only 2% different to the stockies, so thats no big deal.

Brodie · Post by **Brodie** » Sat Feb 26, 2011 3:42 am

Hey Payney,

In top gear (ratio 1-1), with a 3.36 diff ratio and 195/55/15 tyres (overall diameter 23.44 inches), your engine will be revving as follows

Engine Speed (RPM) - Speed (MPH)

8000 RPM - 166.03 MPH
7500 RPM - 155.66 MPH
7000 RPM - 145.28 MPH
6500 RPM - 134.90 MPH
6000 RPM - 124.53 MPH
5500 RPM - 114.15 MPH
5000 RPM - 103.77 MPH
4500 RPM - 93.39 MPH
4000 RPM - 83.02 MPH
3500 RPM - 72.64 MPH
3000 RPM - 62.26 MPH
2500 RPM - 51.89 MPH
2000 RPM - 41.51 MPH
1500 RPM - 31.13 MPH
1000 RPM - 20.75 MPH

So at 100kmh your engine is very close to 3000 rpm
and at 110kmh it is just under 3300 rpm.

Trev · Post by **Trev** » Sat Feb 26, 2011 8:10 am

Maybe your doing MPH not KPH Payney

.

Cal · Post by **Cal** » Sat Feb 26, 2011 10:56 am

Trev wrote:That size wheel n tyers are only 2% different to the stockies, so thats no big deal.

Not really, Trev...

Closest thing to a stock tyre.......185/80-13.

24.7 inches diameter. 818 revolutions per mile.

195/55-15

23.4 inches diameter. 860 revolutions per minute.

That's 1.3 inches difference in diameter, which is huge. Though it doesn't explain the wild tacho reading

Post by **Blacky** » Sat Feb 26, 2011 1:04 pm

This is a handy site - http://www.miata.net/garage/tirecalc.html

PAYNEY05 · Post by **PAYNEY05** » Sat Feb 26, 2011 9:42 pm

Thanks for the reply guys,
I am sure the smiths tacho is for 4 cylinders and doesnt seem to have any switching to 6 cylinder.Checked today with my meters which do rpm and found at 80kays the car is doing 2500rpm,100 is 3100,110 is 3450. These are far more realistic
figures and close to what Brodies chart says.
Thanks again for the advice everyone!!!

fbekholden.com

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