TR2/3/3A TR3 speedometer and wheel revs per mile

[sammyb]

I just looked at my speedo in my '60TR3 and the number on the speedometer indicating wheel revolutions per mile is 1184. From reading the Original TR2/3/3A, it indicates 1180 is the most common.

Anyone have an idea what 1184 would signify (what equipment? OD with different rear end ratio?) I would assume that this speedo is not original to the car and might explain why the speedo reads 75+mph at 60mph.

 

[Dave Russell]

Hi Sam,
I don't know what ratios are standard.
The speedometer marking is speedo cable revolutions per mile, not wheel revs per mile which would be more like 725. 1180 or 1184 is only a .3% difference which would put the speedo off by 0.2 mph at 60 mph. You need to look elsewhere for the source of the 15 mph error.

Mark off a distance of 52.8 feet. Roll the car exactly this distance & count the speedometer cable revolutions as closely as possible. (Attach a small tape flag to the inner cable end to make counting easier) Multiply the revolutions by 100 to get your actual cable revolutions per mile. If it doesn't come out to 1184, the speedo or the angle drive ratio is not correct for your car. If it does come out to 1184, the speedo is defective. A speedometer shop can recalibrate yours if necessary. They will ask you for your actual revs per mile, or find it for you.
D

 

[MGTF1250Dave]

Aloha Sammy,

I looked at the face of my speedometer in my '58 TR3 and the number under the needle is 1152. Is that number you were referring to on your speedometer?

The speedometer and odometer are two separate gauges in the same housing, so one can be accurate and the other faulty. As I recall, the odometer is directly connected to the speedometer cable, in that as it turns a gear provides a mechanical input to the odometer. The speedometer input is from a magnet rotated by the speedometer cable. the rotating magnet influences the movement of a metal disk that turns the speed indicating needle on the gauge face. There is no direct connection between the speedometer cable and the needle. I agree with Dave that the speedometer may be in need of calibration. If the outside diameter of your current tire is changed from the original road tire and wheel diameter that will cause a speedometer error, but would have to be quite dramatic to be 15 MPH off.

Buckeye Triumphs has this nice calculator,
https://www.buckeyetriumphs.org/technical/Calculator/Calculator.htm

It allows you to enter tire size and rear end ratio and gearbox ratios to determine RPM vs MPH and vice versa. This should allow you compare your tach to speedo for accuracy. Of course now you have to assume one of them is correct.

Safety Fast,
Dave

 

[sammyb]

Gauging by the flow of traffic over the weekend, my tach seems to be fine...in a TR3 in 4th gear you basically just double the tach to get MPH. At 3000 I was on pace with traffic in the 60MPH zone (on HWY 101, which notoriously has "scenic viewing drivers" going slightly under the limit.) At 3200 I was pulling ahead of many cars, but being passed by others.

 

[Geo Hahn]

[ QUOTE ]
I just looked at my speedo in my '60TR3 and the number on the speedometer indicating wheel revolutions per mile is 1184...

[/ QUOTE ]

FWIW: The speedo on my 64 non-OD TR4 is original and is an 1184.

 

[MGTF1250Dave]

Aloha Sam,

It would seem your tach and speedo are reasonable accurate based on 3000 RPM is about 60 MPH.

The tach and speedometer in the TR3 and TR4 are the same in appearance with the exception of the gage glass (TR3 convex - TR4 flat) so I'm sure many have been moved about between cars. It is fairly simple to swap the gage glass to give the correct look.

If you do the test Dave Russell describes to test the odometer you see if you getting accurate mileage.

Safety Fast,
Dave

 

[donbmw]

My 1959 TR3 is off the same amount. I have followed my wifes car at 60 mph and the speedo will be around 70 to 75 mph. I have used a GPS and get the same difference when the GPS is 60 mph. The tach reads 3000 rpm when the speedo is 60 mph. The tach matches with the reading of the speedo but you are not going that speed.