rpm v mph

Just taken delivery of 1989 328GTS. Tach appears to read approx 50% of actual. Can anyone provide an accurate rpm/mph reference point in 5th?
Is it an easy fix?

 

Several years ago I had an ignition misfire on my 1987 328 that just changing spark plugs didn't fix. In the course of diagnosis I started measuring plug wire resistances and found them to be all over the board from near infinite to complete open circuit, so I decided to replace the carbon core wires with copper core silicone-insulation wires. The actual wires run inside the red covers, so you can't see that they are blue! My wife didn't like me working with distributor caps and wiring on the kitchen table in the evening, but it was not that hard a job. The result was great, but now the tachometer reading was 1/2 actual! I don't have an explanation for this, but it is true. I have learned to live with 4,000 RPM being redline. Regarding speed vs RPM, with everything operating normally in 5th gear the 160MPH top speed occurs at 8,000 RPM. The relationship between engine RPM and road speed is linear in top gear (in any given gear). There might be some tiny increase in tire circumference as speed increases, but with steel-belted radial tires it is insignificant.

 

Thanks Tom, it appears to be where mine is operating.

 

Do you have the owner's manual? The answer is on page 18 under "Performances". In each gear, it shows the speed at 1000 RPM. For 3000 RPM in 5th, for instance, multiply the 5th gear figure of 19.8 by 3. = 59.6. Tom's statement above is correct - at 8,000 RPM in 5th it would be just a bit below 160 mph.

 

If you don't have the manual, I've attached the pdf.

 

Thanks Dave!

 

Hi there. My tach had the same situation and here’s what I did about it. Your tach should be reading 4000 rpm at 80 mph in 4th. If you have a look on the forum you’ll see that Birdman I believe had an article on recalibrating your tach. Mine is an 88 328gts for reference. Anyway the post describes how to take the tach out of your instrument cluster and then take it apart and there is a rheostat screw that you note the position of and then simply rotate it back and forth about ten times, leave it in the position you found it and reassemble the tach and put it back an the instrument cluster. I figured if I was going to have to take it out anyway to have it recalibrated I might as well give that fix a try. After I did, my tach reading was only off about 100rpm at the 4K at 80mph in 4th gear which was good enough for me! So look for that post and if youre up for giving it a try rather than sending it out, go for it!

 

I had the same problem with mine.
This is the rheostat, as mentioned above.
Mark the position of the wiper arm to the track, , spray it with contact cleaner, rotate the wiper arm across the length of its track several times then put it back to its original position.
Reinstall the tacho. Job done!

 

All I know is that in my '83 QV there is a point in 5th gear when both needles are straight up 5k rpm and 90 mph because I see it all the time