RPM at 60mph(100km) per hour

We drove the roadster for the first time and I believed that the rear end ratio was 3.1.I am using a Turbo 350 whcih has a final drive of 1:1.

I found a formula on the net that goes like this.Not sure what the 336 is in the calculation.

MPH X Gear Ratio X Final drive Ratio X 336 / Tire diameter = RPM
Examples:
60 X 3.73 X 1 X 336 / 26 = 2892.1846
60 X 4.08 X 1 X 336 / 26 = 3163.5692
So if my diff ratio is 3.1 with tyres at 29.3 inches should be around 2132rpm.
The tacho was reading 3500rpm at 60mph which I think is going to be unbearable for prolonged highway driving.I haven't checked but maybe the tacho is not reading correctly.It is a new engine,no hot cam,and was idling at 1000rpm which I thought was high.Maybe the tacho is not set correctly and maybe on 4 or 6 cylinder setting?The car is 200kms away and I can't check straight away on that.

 

Yep, I use an on line calculator and it comes up with 2140 RPM. I'd say your tach is on the fritz

 

well, even at part throttle there will be some converter slippage, but not 1400 rpm!

 

336 is a constant. It's derived from something, but I don't recall the whole explanation. If you really want to know more you can Google it.

I think your tach might be set for a 6 cylinder. If it was set to be with a 4 cylinder it would read twice as wrong, but it's almost exactly in the middle.

 

Who does only 60?

 

336 is a constant, its basically a mile in seconds divided by Pi.

 

That stuff or you got a 4.11 rear gear and/or your speedo is slightly off. Tachs arent that accurate anyways. Something has to be verified as correct. The constant i think is derived from a distance to time ratio. I made my own formula once and i used 60 mph and 60 seconds in a minute to get my revolutions per minute in one mile. I like math

 

forget the speedo or the tach, if you really need to know use a GPS for kph and surely your ears told you whether it was doing 3500 or 2140 rpm?? your tach or speedo can be out. But my ears usually tell me if a motor is revving too much.

 

I just drove 100 miles with a 2.93 rear end and 1:1 final drive and 27" tires and I was taching around 2200RPM and the formula figured out to 2188 so this is probably close to what you should be

 

Either your tach is way off or your ratio is a lot deeper than you think. I am running 4.30 gears, no OD, with 29.5 tires, and I tach 3000 at 60 mph and 3500 at 70 mph. It can only be one of those two things. You know, some tachs have settings on the back for 4-6-8 cylinders and if you have it on the wrong setting it will give way wrong readings.

Don

 

Yes I think the tach is set wrong.The auto electrician was coming back yesterday and he will check the setting.If I was up there(in Toowoomba) then I would do it myself but I am nearly 200kms away.