Speedometer and Tachometer calibration

[Barrman]

2,300 rpm is 45 mph on my M35A2. That is what the gauges indicate at least. The speed is correct. Verified by a normal vehicle chase truck and gps. I can think of several reasons for the tach to be slow, but none that make sense to me for the tach to be 200-300 rpm high.

Any suggestions or places to look?

Thanks

 

[cranetruck]

If you know the gear ratios and wheel diameter, then you should be able to determine exactly what the tach should read. That should be your starting point....

 

[Recovry4x4]

Tach is cable driven. Don't know if there are different adapters. Where is 4th and 5th gear on your tranny as far as shift pattern?

 

[Barrman]

It is the OD transmission with the normal 1000 rpm drop between gears 1-4 and a 500 rpm drop going into 5th. 3rd and 4th are next to each other.

9.00-20 tires all around. 45 mph according to the math should be 2100 rpm. I can compensate in my mind while driving. I just don't like the idea of indicating 2700 or 2800 rpm at 55 mph thinking it is wrong, but not knowing for sure it is wrong. If that makes sense.

 

[jwaller]

sounds like a bad tach. egay has them cheap as do I.

 

[kurtkds]

Has anybody looked into a speedometer adjustment for running 11" vs the 9" tires? It seems like my spedo ready about 15% fast vs gps speed. So when I'm driving my tach will be at 2500 and that will give me 55 mph on the gps but the speedometer will be WAY past 60mph. I would think there would be a gear set change or something when the Deuce had a tire size change. I've done some searching but haven't had much luck.

I've currently got 9" X 20"'s all the around.

 

[Barrman]

There is a gear box attached to the t-case. The speedometer cable goes into it. Different boxed were used for trucks with different tires. That is the theory at least. I know the search feature isn't in full health today, but Cranetruck posted the different numbers for the different speedometer cable adaptor boxes not to long ago.

 

[BKubu]

BARRMAN is correct...there are different gear adaptors that attach to the transfer case that compensate for tires of varying sizes...at least this is the case for the 5 tons and I would assume it is the same for the deuces.

 

[kurtkds]

I'll do a search for that once things settle down around here...

 

[Stretch44875]

A weak return spring in your 40 year old tach would cause it to read high.

 

[Barrman]

I didn't know they had return springs. I swapped it out for another and all is well between what the two gauges read at speed now.

I had an airplane mechanical tach that read 200 rpm when the engine was off. Turned out it was 200 rpm fast running as well. This M35 tach was at 0 with the engine off.

 

[tracy]

bad tach. my guess,,,