High engine RPM at highway speed

[hern1982]

Hello everyone,

Have an '86 GN that I love to drive, in fact it is my daily driver. Just a couple questions:

1. I know my speedo reads abouts 7 mph fast, do I just need a new cable or could there be other causes? The car runs 235's on the back instead of 215's.

2. My digital tach reads around 3600-4000 at 80mph on the highway, but I have driven it from Wisconsin to California and back, and recently down here to Southwest Florida and averaged about 25 mpg on the highway. I have read the digital tachs read high but by how much?

3. I live in Southwest Florida, specifically Cape Coral. Wondering if anyone had any contacts down this way, maybe shops that you recommend or things like that?

Thanks to anyone that takes the time to read this and maybe help me out!!!!!

 

[TurboTnZ06]

The tach is probably just wrong, mine reads 5000 rpm at idle. You can send it in to casper's to get recalibrated to stock.

CEI - Casper's Electronics, Inc.

 

[RegalT-type85]

x2 it's probably just the tach going out of wack. Get it fixed at casper's or get aftermarket tach.

 

[azbob]

The 235's will give you a 7.6% high error in speed.

 

[pacecarta]

The 235's will give you a 7.6% high error in speed.

how are you figuring this , the 215/65 which was stock was 26.10" and 235/60 is 26.10 "

 

[azbob]

miata.net tire size calculations
You have indicated in a new post that you used 60 series in lieu of the stock 65 profile. In that case the error will be .4% slow.
Wow, just realized that I was answering Pacecarta's question. The error will be as posted if you did change the profile.
Sorry for screwing up.