RPM to MPH (trans and diff calc) question

[gearmiester]

Alright I am a newbie to this forum and to the Z car world. I have recently purchased a 1978 280Z (always wanted one) and have a question about speed.

 

At present I have yet to confirm that everything in the drivetrain is still stock but from all the research done in this forum and other tech pages I believe that right now this is what I am running.

 

LS28 engine with fact headers, Cold Air intake, 2-1/2" exhaust straight to mangaflow (no cat)

4 speed trans with following ratios

1st 3.321

2nd 2.077

3rd 1.308

4th 1.00

 

3.54 diff

 

205/50r16 all around

 

now according to two different RPM to MPH online calculators I should only be seeing 80mph in 4th at 4000 rpm. However the speedo says 90. She cruises very nice here no vibration, no squirlieness. And from 70mph @3100 rpm to 90pmh it feels only like a heartbeat away. I have only had her to 100mph at 4800rpm (sorry bubblegummers) and she still handles like a dream during lane changes.

 

So the question is can I really be 10mph off on the speedo and I am really going slower?

 

here is the beast

 

 

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[BluDestiny]

for starters your tire size is smaller than stock, so that accounts for a 2.7% difference according to this tire size calc.

 

http://www.miata.net/garage/tirecalc.html

 

stock tires are 195/70-14

 

The rest could be from a swapped diff or maybe the guage is just old and reading high.

[gearmiester]

Thanks for that info.

 

While I have been researching I found that out as well though I had the incorrect stock tire size.

 

Seems though that if change the sidewall by mere 5mm to 55 it will be pretty much dead nut for stock. But that only means that I'll have to either check my speedo or the speedo counter. I'll look again at the rear end and positively id the diff.

[Driven5]

So the question is can I really be 10mph off on the speedo and I am really going slower?

Absolutely. It's not terribly uncommon for even completely stock older vehicles' speedometers to be off by easily 5mph, generally reading faster than you're really going. Even today many (most?) will read slightly faster than you're really going, as due to manufacturing variances there is a potential range of accuracy for the speedometers coming off the assembly line and it legally protects themanufacturer from liability if any of their customers were to be caught breaking the law (speeding) when the speedometer says they're not. Add in any gearing or tire size differences from stock without correcting/calibrating the speedometer, and it's entirely possible to be off by 10mph or more.

 

Seems though that if change the sidewall by mere 5mm to 55 it will be pretty much dead nut for stock.

Just to clarify the 50 and 55 in the tire size designation do not represent millimeters of sidewall height, but rather the 'aspect ratio' or more simply the percentage of tire width that the sidewall is tall. But yes, changing to a 205/55-16 will bring you closer to stock tire size, and actually be slightly taller than stock now, as it adds ~10mm to the sidewall height.

[JMortensen]

Most 30 or 40 year old Datsun gauges provide nothing more than a vague suggestion as to what is happening. The right size tire helps of course, but even when you have the right size tire, you might still be off, and don't trust the tach either. If your tach is as accurate as mine was, it's only real function is to provide a very dim green light so that you don't have to be afraid of the dark when you drive at night...

[gearmiester]

So to ensure acurracy do i need to replace both the speedo gear and gauge? Or with new gear mesh on the gear will I not need to replace the gauge?

 

The RPM gauge seems pretty accurate though as it does not bounce or lag at all.

 

thanks for all the input. I am currently replacing the back tires to 205-55R16's I'll keep the 205 50's on the front. We'll see if there is a noticeable improvement in speed accuracy.