Speedometer markings: OD and not OD that is the ?

[karls59tr]

I recall a thread mentioning the two different speedometers that were availble for the TR3A depending on whether it came witha an OD or not. Does anyone remember if these markings on the Jaeger speedo would indicate which application it was for:SN 6319/00(above odometer) and 1184(below odometer). Thx Karl

 

[Geo Hahn]

Just as the speedo doesn't 'know' what gear you're in... it also doesn't know if there is an overdrive.

All other things being equal the same speedo would be used for OD & non-OD.

Of course changes in tire size and diff ratio would matter.

The 4-digit number on the face of the gauge indicates the number of turns (the driveshaft?) does in a mile -- that would not change with an OD.

 

[Moseso]

Geo's got it. The two speedos were for the two different rear-end ratios available. The OD confusion would stem from the fact that the lower ratio rear (4.1:1) was only fitted to cars with OD.

The 1184 number you quote signifies 1184 turns/mile, and is the one for the higher (3.7:1) ratio rear-end.

 

[karls59tr]

Would you happen to know what the number on the speedo would be for the lower ratio rear(4.1:1)? I swapped in a Diff from an OD car into my Non OD car years ago.I believe the tach and speedo dial indicators should be straight up at 60mph for a Non OD car and mine are not ie.at 60mph the tach reads lower. Does this make sense?

 

[Moseso]

I don't know that number.

Re: your speedo - tach relationship, https://www.mintylamb.co.uk/gearspeed/ is a very cool calculator that will give you the exact figures, even taking your tire size into account. It doesn't have a "TR3" setting, but "TR4" is identical.

Approximations (close enough to keep one from getting speeding tickets) are:
(Not in Overdrive, with the 3.7 rear end and near-stock-size tires)

2nd gear: The speedo is the tach 1:1 -- 2000 RPM = 20 MPH, 3000 RPM = 30 MPH, etc.

3rd gear: tach x 1.5 = speedo -- 2000 RPM = 30 MPH, 3000 RPM = 45 MPH, etc.

4th gear: tach x 2 = speedo -- 2000 RPM = 40 MPH, 3000 RPM = 60 MPH, etc.

If you have the 4.1 rear end, RPMs would be about 10% higher at any given speed. There is a chart in the TR3 owners manual. It says:
10 MPH in 2nd = 1110 RPM (30 MPH = 3330 RPM)
10 MPH in 3rd = 731 RPM (30 MPH = 2193 RPM)
10 MPH in 4th = 552 RPM 40 MPH = 2208 RPM, 60 = 3312)

 

[karls59tr]

Thanks again. Cool calculator.

 

[HerronScott]

Karl,

Here are some of the model numbers which should be for 4.1:1 differentials which were used for TR2-3B cars.

SN6307/08
SN6307/10 with optional Michelin X tires
SN6319/02
SN6319/04 with optional Michelin X tires

Scott

 

[TR3driver]

Would you happen to know what the number on the speedo would be for the lower ratio rear(4.1:1)?

The calibration number would be around 1350.<div class="ubbcode-block"><div class="ubbcode-header">Quote:] I swapped in a Diff from an OD car into my Non OD car years ago.[/QUOTE]Most OD cars still had 3.7 diffs.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I believe the tach and speedo dial indicators should be straight up at 60mph for a Non OD car and mine are not ie.at 60mph the tach reads lower. Does this make sense? [/QUOTE]No. Swapping the diff will not affect the relationship between speedo and tach. (It only makes the speedo inaccurate, assuming it was accurate before.) Plus if you had fitted the speedo for a 4.1 diff, then the tach would read higher, not lower.

More likely IMO, your speedo is reading too high.

 

[HerronScott]

No. Swapping the diff will not affect the relationship between speedo and tach. (It only makes the speedo inaccurate, assuming it was accurate before.) Plus if you had fitted the speedo for a 4.1 diff, then the tach would read higher, not lower.

More likely IMO, your speedo is reading too high.

Good point Randall! This is where a GPS unit for testing the speedometer accuracy and a dwell/tach meter to test the tachometer accuracy would be helpful.

Scott