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Does my tachometer show inflated rpm numbers?

 

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Question:

A few months ago I swapped the instrument cluster in my 1992 Ford Festiva for one that has the tachometer included. My Ford Festiva has a manual 5 speed transmission wile the donor vehicle was a 1989 model with an automatic transmission. The tachometer seems to show too high rpm numbers now e.g. in 5th gear:

3050 rpm at 55 mph
3250 rpm at 60 mph
3500 rpm at 65 mph

Answer:

Your numbers are indeed about 750 - 800 rpm too high. For a given rpm, the speedo is going to read the same no matter what size your tires are. Remember, it is calibrated is based on the assumption that you have stock tires -- that's the way it was delivered from the factory -- so it gives you back what it thinks is the correct (real world) speed for those tires.

That said, if your speedo is properly calibrated, you will have reasonably accurate readings if your tires are 145/80-12, 165/70-12, or 175/60-13. All other sizes induce error.

Here is what other Festiva owners measured:

MPH  RPM
==========

25 1040
35 1450
45 1870
50 2080
55 2290
60 2500
65 2700
70 2910
75 3120
85 3540
95 3950
105 4370
115 4780
125 5200


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Comments:

2005-11-07, 16:48:47
anonymous from Canada  
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