I just ditched my 16" wheels on my Malibu LS and upgraded to the machine-faced 18" wheels/tires from an LT2 Malibu.

Here's my gripe: The 16 and 17 wheel/tire combos have the correct overall diameter, but the 18's are are almost 1 inch larger (.8 to be exact) and the result is a speedo that reads roughly 2.7% too slow. So 60 indicated is actually 61.7 mph (It's actually worse because my GPS indicates my speedo was originally too slow to start with)

16s: 215/60/16= 26.2 inch diameter
17s: 215/55/17= 26.2 inch diameter
18s: 225/50/18= 23.0 inch diameter

By my calculations, the correct size for the 18s should be 225/45 or 215/50 to keep the correct tire/sidewall aspect ratio... Anybody else notice this?

http://www.miata.net/garage/tirecalc.html
Where did you get 23" for an 18" wheel? with the 225/50/18 the diameter came out to 26.9 and you would be 1.9 mph slow.

The cars that come with the 18's have different speedo programming I'm sure. If you swap wheels you have to live with the small difference or get the speedo reprogrammed.

http://www.miata.net/garage/tirecalc.html
Where did you get 23" for an 18" wheel? with the 225/50/18 the diameter came out to 26.9 and you would be 1.9 mph slow.

Oops... misprint. The calc I found showed 27.0 for the 18s. Funny though, when I used the Miata.net calculator I got different measurements, the ones you show. GPS indicates about 1mph too high at 35, 2mph high at about 60, and almost 3mph high at 75.

Anyrate, I talked to one dealer who said I'd have to live with it. The next one I talked to knew exactly what I was talking about and said it would be 1hr of shop labor ($85) to reprogram it.

Its super easy to reprogram it.

Our LTZ is DEAD on up until around 85 then it starts to show slow. 70 MPH on the speedo is 71.1 on the GPS.

How do you know the GPS is right? I have seen many that only get you close to a location (within 50 to 150 ft) not exactly on it so if the locations are inexact the speeds are probably inexact as well.

Bill

The GPS isn't 100% dead on. I try the same GPS in 3 different vehicles and each one shows a slower speed on the GPS than on the spedo.

ALL GPS' are an "estimate" speed wise. Unless you buy a very expensive data based one which maybe 2% of the population has. Speed can be as much as 5 mph off.

How do you know the GPS is right? I have seen many that only get you close to a location (within 50 to 150 ft) not exactly on it so if the locations are inexact the speeds are probably inexact as well.

Bill

Mine gets pretty accurate. It gets to within 3ft of your exact location.

PLUS, Ive used it against the radar signs as well as the speedo. My GPS is accurate up to around 80 MPH and then it starts to show slow.

Its super easy to reprogram it.

Our LTZ is DEAD on up until around 85 then it starts to show slow. 70 MPH on the speedo is 71.1 on the GPS.


How do you reprogram?

The dealer will do it and charge an hour of labor. Or....you can go to a performance shop and see if they can do it. Basically you go into the PCM and change the tire size.

I just ditched my 16" wheels on my Malibu LS and upgraded to the machine-faced 18" wheels/tires from an LT2 Malibu.

Here's my gripe: The 16 and 17 wheel/tire combos have the correct overall diameter, but the 18's are are almost 1 inch larger (.8 to be exact) and the result is a speedo that reads roughly 2.7% too slow. So 60 indicated is actually 61.7 mph (It's actually worse because my GPS indicates my speedo was originally too slow to start with)

16s: 215/60/16= 26.2 inch diameter
17s: 215/55/17= 26.2 inch diameter
18s: 225/50/18= 23.0 inch diameter

By my calculations, the correct size for the 18s should be 225/45 or 215/50 to keep the correct tire/sidewall aspect ratio... Anybody else notice this?

I have noted that some tires by model and maker of the same size have a different number of revolutions per mile. Maybe that spec will help you find a tire that is more suitable.