I was wondering if this is about correct rpm's for a 5 spd 2.5. I am turning about 1600 rpm for an indicated 55 MPH. When I bought the car there was a 120 MPH speedometer in it, and I read some where that was a GT only speedometer. If it is reading incorrect, how do I get it to read correct? It feels about right for 55 mph. Or could the tach be reading incorrect thinking it was still a 6 cyl engine?

The tach could be off but It sounds about right im not sure how much difference the 6 and 4 cyl tachs are in calibration. Also are you sure the speedo only wasn't switched? If you have an oil pressure guage in with the tach you have the dukes tach.

No, my oil pressure is in the auxilery gauge set along with a volt meter. I thought some one with a 4 cyl might know where their rpms are at 55 mph
Joe

[This message has been edited by josef644 (edited 01-13-2008).]

At 55 I am about 2500 my duke is a 87 auto and I might have some engine problems so that could be off.
I was about to post the same question

quote Originally posted by Graham: At 55 I am about 2500 my duke is a 87 auto and I might have some engine problems so that could be off. I was about to post the same question

is it about 3000-3100 at 65 mph? that sounds about right for a 3speed ..

quote Originally posted by 88White3.4GT: is it about 3000-3100 at 65 mph? that sounds about right for a 3speed ..

yes about that, so it is good?

quote Originally posted by josef644: I was wondering if this is about correct rpm's for a 5 spd 2.5. I am turning about 1600 rpm for an indicated 55 MPH. When I bought the car there was a 120 MPH speedometer in it, and I read some where that was a GT only speedometer. If it is reading incorrect, how do I get it to read correct? It feels about right for 55 mph. Or could the tach be reading incorrect thinking it was still a 6 cyl engine?

That is about right. I have an 88 duke/5 speed and turn 2000 RPM at 60 MPH. Never took a reading at 55, but if you divide it out, it's 33.333MPH/1000 RPM. At 1600 RPM = 53.3328.

quote Originally posted by josef644: I was wondering if this is about correct rpm's for a 5 spd 2.5. I am turning about 1600 rpm for an indicated 55 MPH. When I bought the car there was a 120 MPH speedometer in it, and I read some where that was a GT only speedometer. If it is reading incorrect, how do I get it to read correct? It feels about right for 55 mph. Or could the tach be reading incorrect thinking it was still a 6 cyl engine?

depends on the transmission.
but - that does sound way low in RPM.

quote Originally posted by Pyrthian: depends on the transmission. but - that does sound way low in RPM.

Duke 4 cyl, if manual, normally has Isuzu 5 speed behind it, for economy. Final drive ratio on Isuzu is 3.35:1 Final drive on Getrag is 3.61:1.

http://24.82.35.11/fierotransmission.xls

i run about 1500 to 2000rpm at 55mph at about 70 im a little over 3000rpm.

i got a 86 duke with the 5 speed

I haven't had mine to 70 yet, I just made 500 miles on a whole engine rebuild today. I'm turning about 1750 rpms at 60mph by my tach.