Speed verse RPM

I own a Euro spec 308. 1979 carbed car. Dry sump. When im at 3,000 in fifth im at 115/6 kilometers an hour. Curious if the gearing on this car has been altered.

 

Stock tires?

 

If you've got the stock tire size, your speedometer is probably a little optimistic (auto manufacturers tend to have their speedometers read somewhere between correctly and too fast as they don't want any to read too slow). Your 155/78 OM indicates that you'd be at ~105 kph at 3000 RPM -- see Section 1 "Performances".

 

Yes stock tire height.

 

Maybe I'm missing something here but tire size is not going to affect the relationship between tach and speed sensor although it would affect your actual speed as measured by an external device such as radar or gps.

 

Good point!

 

Mine is real easy, road side radar (those SLOW DOWN radar boxes) confirmed 20 MPH/ 1000 RPM in top gear. Modern 17" wheels /tires, but stock tire diameter.
3K RPM=60 MPH, 4K RPM=80 MPH. 6K RPM=Fun.

FWIW, my 1979 US car came with a Metric only speedo/ODO, so i use the tach as my speedo in top gear, saves having to do KM/MPH conversions.

Doug

 

I was checking to see if the gears in the transmission on my euro car were either changed or different than the US model. Easy enough to calculate Km to MPH.
But Doug you quick explanation is helpful.

 

The carbed euro 308 have a "taller" set of transfer gears (27 driving/29 driven) versus the US 308 (27 driving/30 driven) IIRC (and the CH version is different than both in several ways IIRC). See the gear data in the OMs Section 1 -- they do a pretty good job giving the tooth counts of all the gears (transfer, gearbox, and diff).

 

yrs ago I wrote an xcel spreed sheet to calc the various 'flavours' of speed and gears. It's been some time and I think the attached speeds are based on stock tire sizes. either way it's very close. there's a slew of gear ratios thru the yrs between models and country of delivery. All that aside, the speedo uses impulses to calc the velocity so there's no direct gear ratio internally to insure or check accuracy. the impulse signal is driven by the secondary shaft before the differential output, 4 impulses/rev. I wrote an equation for it once as well for a build yrs ago... I'd have to dig it up, interestingly it results in non-whole numbers due to the gear ratios.

data taken from the WSM's..
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Well, since the US cars were gutted HP wise, it makes sense they put lower gears in to get back a bit of lost acceleration.

Doug

 

smg2,

Help a brother out...what does CH indicate in your table?

 

Swiss
CH stands for Confœderatio Helvetica, or Helvetic Confederation in English.

 

Ah, thanks. Interesting that market has different gearing, especially as it's shallower than the others. Something to do with local regs?