what engine revs at 60 mph

G,day to All , Can someone tell me what engine revs they are doing at 60 mph with auto transmission with 3.54 diff & also what revs at 60 mph with auto and 3.9 diff. That would be very helpfull, Thanks Jimbo :?: :?:

Has'nt anyone got any info on this :?: :?: :?: Thanks Jimbo

Hi Jim,

Love to give you an accurate answer but the 260's tacho is u/s.

From memory it was high about 3k at 100kph.

You'll should have noticed a 3-400rpm change when you swapped out the auto. Although that might change according to the diff.

MOM

G,day Mike, I always thought I had a 3.54 diff in my 72 car & have thought so for the last 23 years I have had the car. When I swapped from the auto to manual gearbox a while back ,I was pleasently supprised how well it went, but was told it would go a lot better with a 3.9 diff under it.

So put a few $s aside and finaly get to put the 3.9 under it, BUT supprise supprise Daniel Moore discovers it already has a 3.9 diff in the car.

OK, car with auto & 3.9 ....60 mph(100kph) 3000 revs

car with 79 280zx box & 3.9... 60mph 2350-80 revs approx!!!

Allright ,what would an auto with 3.54 diff rev at at 100kph,

what would an 79 280zx box with 3.54 diff rev at 100kph can anyone solve the query??? thanks ....Jimbo.

Mike , got the package, thanks. Hope you got mine. Are you going to the Wangaratta Rally ? Also have been ringing work , Have you been away?Will talk soon.. :wink: :wink:

3.54/3.9 * 2380 ~= 2160rpm.

I find that somewhat hard to believe. I have a 3.54 and a 0.76 5th and with 195/50R15s I am doing ~2700rpm at 100kph.

Dave

3.54/3.9 * 2380 ~= 2160rpm.

 

I find that somewhat hard to believe. I have a 3.54 and a 0.76 5th and with 195/50R15s I am doing ~2700rpm at 100kph.

 

Dave

G,day Dave,

I am confused too, But the car was doing 3000 rpm @ 100kph with auto transmission , then swapped to 79 280zx box & am doing 2380 rpm @ 100kph in 5th without touching anything else!! same diff which I thought was a 3.54, BUT is a 3.9... Same tyres 195 70 14.

I never mentioned 2160rpm

I do appreciate your knowledge..Thanks...Jimbo...

3.54/3.9 * 2380 ~= 2160rpm.

This confuses me... what exactly are you trying to work out? The change in rpm?

Jimbo - re your difference in RPM, dave is running a smaller tyre which gives an 8.4% difference in speedo reading (ie. when his speedo says 100kph, he is actually traveling at 91.6 Kph). I assume this is for better acceleration on the track, but I don't know if he has corrected for that when quoting here. I think he may have, by the figures down further. 60 Mph is also not 100kph, it is actually 96.5 Kph (62 Mph is within .3 of 100kph). Add to this the fact that both your tachos may be reading off, in different directions even, and you have enough small differences to make a huge difference in end result.

I just had a play with the calculator on this site (http://www.spannerfodder.com/the_works_escorts-ratios.php), and either your tacho is reading at LEAST 250 RPM slow, your speedo is reading high by about 5%, or you have a 3.54 diff.

Target speed of 100kph, with 3.9 diff, .76 5th gear, and 628mm rolling diameter for your tyre (I'm pretty I've got that right) gives on paper 2517 RPM. Thats before you factor in wind and rolling resistances, which would add quite a few RPM at that speed.....

If you had a 3.54 diff, and no other changes, your RPM on paper is 2285.

With a 3.54 and auto, you would get 3000 RPM on paper (auto apparently is 1:1 in top gear.

With a 3.9 and auto, should have been getting 3300 RPM, plus a bit for resistances.

For comparision, Daves specs - (target of 100kph, 3.54, 0.76 5th and rolling diameter of 576mm gives 2491 RPM.... And I imagine he has a large aftermarket tacho screwed onto the dash somewhere for his trackwork, which should be pretty accurate, so I reckon on about 200 RPM should cover wind and frictional resistances.... although his speedo might be out a bit.

Note: if he hasn't corrected for speedo inaccuracy due to tyres, you get about 450 RPM in resistance, assuming his tach is accurate.

So I'd take a guess that both your tacho reads a lot low (especially if its the standard inductive pickup 240z tacho), and your speedo reads a touch high.... Unless your speedo drive wasn't changed to match the diff, but I think you'd probably notice that, its about a 10km/h difference in reading you'd get (low).

G,day Zeddophile,

thanks for your time. You are right, I should have stuck to the 60 mph readings as thats what my speedo is in.As I have said I always assumed I had a 3.54 Diff in the car with the auto for the last 23 years.

Rob at zworkz did the manual box swap but didnt touch the diff (didnt have the pennies) Anyway car with auto 60 mph--3000rpm

car with 79 5 speed 60mph 2350-80 rpm on the same rims & tyres 195x70x14 5 1/2 inch wide .

Some months later I wanted to change the diff to a 3.9 But was told it already had one in it. could this info be wrong, Hence my questions!!!

Thanks for your time.....Jimbo..... :!: :!:

As i said at the end of the post, there are three options, which I'll list in order of likelihood:

1. Your tacho reads out. Highly probable, if it has the original 240z inductive pickup style tacho, they have a reputation for being pretty inaccurate.

2. Your speedo is reading wrong. Only way to really check that is by befriending a cop car, or go through one of those 'advisory' speed check points where they display your speed on an electronic sign (ie. western highway, hume highway). I don't like them much though, because on the little ones they put in backstreets in melbourne, I've got different readings, on the same day and same time, two streets away from each other. Thats on two seperate occasions (about 8 months apart), suburbs were about 20km away from each other, and i had a different car. I don't think they are calibrated to any standard at all.

3. Your mechanic was incorrect, and your car does in fact have a 3.54. Now, I believe you were told that by Daniel Moore, and I believe thats the guy in Burwood, who has a good reputation, so I would be hesitant to second guess him. Its not hard to tell, the ratio should be stamped on the crownwheel, so if he's had the rear cover off your diff, he should be right on.

I'd start by sticking an accurate tacho on the car. Some tune-up machines will read engine rpms straight off the inductive pulses of the spark plug leads, and a dwell tachometer will read off the coil. If you ask a workshop nicely, they may be able to check it for you. Otherwise, you'll need to buy/borrow an decent brand aftermarket tacho, and just temporarily wire it up, so you can compare your tacho to it.

When you know that, you will have two options left if it is accurate - speedo and diff. However, I'd put money on the tacho reading low.

Thanks Zeddohpile,

I will do a bit more checking up on the subject & report back in later on... thanks for your time Mate....