1960 Speedometer

[Nicholas L.]

My speedometer reads about 10 MPH lower than I am actually going. When I am going 60 MPH the speedometer shows 50 MPH.
Any suggestions?

[Leif A.]

Re: 1960 Sreedometer

Nicolas,

How long has it been doing this? We need to know what transmission and rear end gear ratio you have to further assist. Also, what size tires.

Leif

[Nicholas L.]

Re: 1960 Sreedometer

The transmission is the original Borg Warner T10. Rear end gear is 3.70. This problem has been going on for about two years.

[Edward M.]

Re: 1960 Sreedometer

Did anything charge / happen to the car two years ago. Did you change transmission, read end gear, speedometer head, tires, anything? We need more info.

[Nicholas L.]

Re: 1960 Sreedometer

I did buy radial tires. The metric equivalent to 6.70 X 15.

[Edward M.]

Re: 1960 Sreedometer

What size tires are on the car now? What size tires did you take off of the car when you replaced them? Is this about the time the speedo error started?

[Nicholas L.]

Re: 1960 Sreedometer

I took off 6.70X15 bias ply and replaced with I believe 205X15 radial. About the time the problem occurred. Did I answer my question. Is it due to the tires?

[Robert K.]

Re: 1960 Sreedometer

I doubt it is all tires. A 6.70x15 tire has a diameter about 27.5. In order to slow your speedometer to 50 mph when doing 60 mph, you would have had to increase the tire size to 33"+. That is a big change, even for Jeeps.

When did the car get the 3.70? Any chance you have a 3.08 gear? Going from a 3.70 gear to a 3.08 gear will cause your speedo to only measure 50 mph when you are going 60.

How many RPMs are you turning at 60? It should be something near 2800 with a 3.70. If it is 2300 then you have a 3.08 gear in the rear and a 3.70 gear in the trans which would account for the reduction in mph.

[Edward M.]

Re: 1960 Sreedometer

Robert has hit the nail on the head. This really sounds like a rear gear change. The only other possibility that I can think of is a speedometer gear change in the transmission. Did any of those two things occur?

[Nicholas L.]

Re: 1960 Sreedometer

The rear axel code is for 3.70. At 60 MPH the rpm is about 3000. Don't know if any changes were made to transmission or rear gear.

[Robert K.]

Re: 1960 Sreedometer

It sounds like a 3.70. Therefore, your speedometer must be having issues.

[Edward M.]

Re: 1960 Sreedometer

Here is a chart that might help... This chart is based on 60 mph speed.

Rear Axle Gear Ratio	Tire Diameter
	24"	26"	28"	30"	32"	34"	36"	38"	40"	42"	44"
2.18	1831	1690	1570	1465	1373	1293	1221	1157	1099	1046	999
2.50	2100	1938	1800	1680	1575	1482	1400	1326	1290	1200	1145
2.74	2301	2124	1973	1841	1726	1625	1534	1545	1381	1315	1255
3.08	2587	2388	2218	2070	1940	1826	1725	1634	1552	1478	1411
3.23	2713	2504	2326	2170	2035	1915	1809	1714	1628	1550	1480
3.50	2940	2714	2520	2352	2205	2075	1960	1857	1764	1680	1604
3.73	3133	2892	2686	2507	2349	2212	2089	1979	1880	1790	1709
3.90	3276	3024	2808	2621	2457	2312	2184	2069	1966	1872	1787
4.10	3444	3179	2952	2755	2583	2431	2296	2175	2066	1968	1879
4.56	3830	3536	3283	3064	2873	2704	2554	2419	2298	2189	2089
4.88	4099	3784	2513	3279	3074	2894	2733	2589	2460	2236	2139

[Edward M.]

Re: 1960 Speedometer

I would take a look at the ends of the speedometer cable that is inside the housing, and also the speedometer gear in the transmission. I had a 74 Vette a while back that had an erratic speedometer. The input end of the speedometer gear in the transmission was worn to the point where it was slipping at higher speeds. New speedometer gear, problem solved.

[Nicholas L.]

Re: 1960 Speedometer

Thanks. Will check that.