Tire size, impact on the tach?
 

So I have a set of 155-80-13 tires on my VX, and the corect size 165-70-13.

I'm wondering if this got any effect on the mph and the rmp display on the tachometer?

Tach measures engine speed, so that will remain unchanged. Speedometer speed, yeah, that is a variable.

But manufactures do not always have accurate speedometers. Easiest thing to do is to check your actual ground speed with a GPS unit. We did a tire change on the Focus to a taller tire with the same width making the circumference larger, and that made the speedometer spot on. One of my other cars reads about 2MPH optimistic.

they're 3% larger so the speedo will read 3% faster (this is relative to the old tires NOT absolute (correct) speed) that means that at 100 mph it'll read 3 mph fast and at lower speeds it'll be proportionally less.

At the same time though, that's within the realm of bald vs new tires

hahaha so I'm going slower than 55mph, thats maybe why I'm getting so great FE.

thx for the info, and yes I was talking about the speedo and not the tach... thx

Um yea...53.5 mph...big difference.

BIBI -

Yup, a GPS is the way to go. Here are my results :

New shoes (aka tires)
https://www.gassavers.org/showthread.php?t=3699

Optionally, you can use this calculator :

Miata.net Tire Size Calculator
https://www.miata.net/garage/tirecalc.html

Or this one :

the wheel & tyre bible
https://www.carbibles.com/tyre_bible_pg2.html

CarloSW2