For the wheel expert Civic VX
 

Since the original VX tires were 165/70/13 and I now have 175/70/13 on, does that mean i'm getting more milage than what my odometer is reading. If my odometer is showing that i'm getting 50MPG, will the 175/70/13 get more or less mpg than 50mpg? Thanks

You are correct. For every mile your odometer reads, you're traveling slightly further than you would on the smaller tires, slightly faster than your speedometer reads, and your MPG calculations may need a slight adjustment.

I say slight because it's probably a tiny difference. Check the tire size calculator to see exactly how much the difference is.
https://miata.net/garage/tirecalc.html

The best way to determine the actual difference is comparing your trip odometer to a GPS. Tire size varies with each tire manufacturer, so you never know the actual revolutions per mile with your current tires. Check out Tire Rack's 175/70/13 tires and you'll see that the Sumitomo HTR T4 is 927 revolutions per mile, Kumho Solus is 919, Michelin Harmony is 923, and Continental ExtremeWinter Contact is 895. That's a 3.5% difference between the highest and lowest (Sumitomo vs. Continental) and almost 1% difference between the Sumitomo and Kumho. 1% of 50 mpg is worth another 0.5 mpg.

One possible way to remedy this is to get a VSS sensor from a DX/LX civic of the same year... Reason I suggest this is because they use 13" wheels with 175/70 profile tires.

So for the math...

STOCK SIZE (Civic VX)

165/70-13 would leave a 165*.70=115.5mm sidewall. Wheel diameter is 13in or 330.2mm. Total tire diameter would be 115.5*2+330.2=561.2mm.

NEW TIRE SIZE (on VX wheels)

175/70-13 would leave a 175*.70=122.5mm sidewall. Total tire diameter would be 122.5*2+330.2=575.2mm.

That is a difference of 575.2/561.2-1=2.49%

So at a speedometer reading of 70mph your car will actually be going 70*1.0249=71.74mph.

This also means that for every 100 miles traveled, the odometer will only read (100/1.0249) 97.57 of those miles, effectively decreasing your MPG calculations if you do not account for the difference.

That said, more than likely the odometer was still not 100% accurate to begin with as it would change with tire wear, etc. so as cems70 mentioned, a GPS might be your best bet for trying to figure off how much you are actually off.

All the D-series VSS should be the same.

This is confirmed as well in this post:

https://www.gassavers.org/showthread.php?t=2962

So swapping the VSS will do nothing to change the speedometer/odometer.

Damn... Maybe he could try the VSS from the '96-'00 civics?

Didn't read the first part... Uh I know for certain the VSS on the '96-'00 Civics is different from the '92-'95 so therefore I think you should take what he says with a grain of salt..

Thanks for the reply everybody, I appreciate it alot. So this mean that for every 100 miles that my car is traveling, my odometer will only show 97 miles right. If that's true than it should mean that i'm getting more mpg than what my odometer is showing.

Yep, your MPG would be off from what it was before... But again, that being said, your speedometer/odometer most likely was not dead on to begin with. So even though it might only read 97 miles for every 100 miles read on the old tires, it might truely be more accurate and thus the new (lower) MPG calculation could actually be correct or at least more accurate.

Only way to really tell how far off your odometer "actual" is would be to get/borrow a GPS on a trip and see what the difference between the two is (I would compare with straight line trips as the GPS will not be as accurate in such things as a parking lot).

I have the original factory tire size on my vx and with the tires at max sidewall pressure ( 51 psi ) my GPS and the odometer are within 1 mile of each other over 500 miles . Hope this helps