Tire sizing questions

[Legacy777]

I know my speedo has been off for a while, and knew it was off a little more when I did my swap. I wanted to see how bad it was off. So a friend and I took some speed readings from my hand held gps vs what the select monitor said.

At 40mph the difference was about 3 mph, at 60 it was about 4 mph, at 94 mph it was about 6 mph off.

Using the miata tire size calculator, I was playing around with what size tire would work best to get my speedo more accurate.
http://www.miata.net/garage/tirecalc.html

From my own calcs, at 60mph my speedo has a 6.3% error.

205/55 r16 is probably just a tad too much (it's error is 6.9%), but is probably the closest I could match at 60 mph. 205/50 r16 has a 3.5% error, so it's better then what I have, but still about 3% short of being accurate.

My main issue though is I don't want to get too large a sidewall and sacrifice handling performance, plus those tires mentioned are narrower then my current tires. A 215/50 r16 has a 5.2% error, which would give my speedo about a 1.1% error at 60 mph. That I can live with.

However there isn't a lot of tire options for that size.... and to be quite honest, I like the goodyear gs-d3's I currently have on, and would like to go with them again.

here's my spreadsheet with my speedo calcs and graph on speedo error vs. acutal speed. There's a few outlier points on the graph, but the trendline shows a pretty decent fit.

http://www.main.experiencetherave.com:8 ... docalc.xls

Any comments/suggestions would be appreciated. Are my worries of a taller sidewall hampering performance not an issue?

[vrg3]

This might not be helpful, but I think you might be playing with an excess of precision here... Different make/model tires with the same size numbers can differ in diameter. And as the tires wear you'll see a slight change in diameter too.

I wonder if it's possible to recalibrate our speedometers. I have no idea how they work internally.

[Subaru_Nation555]

I wonder if it's possible to recalibrate our speedometers. I have no idea how they work internally.

I have wondered that too. I was watching TRUCKS on Speed channel the other day and there was an add for some module that plugged into the ODBII port allowing you to change settings. Among shift points and such you could recalibrate the speedo for bigger tires and wheels.

[vrg3]

Well, okay, I lied a little. I know our speedometers are mechanical or electromechanical (the input is a spinning cable), so a simple electronic recalibration is pretty much out of the question. But it might be possible to adjust some physical parameter inside the speedometer, like the tension of a spring or the length of an arm or the angle of some doohickey.

[Brat4by4]

You can't get your speedo to match that GPS unit. The speedo comes from the factory with a % off. We talked about this before... you think you are doing 110 and you might be doing a touch over 100. Second, you have to check the manufacturers specs for the tire, they list different overall diameters on the same tire size for different brands AND models.

The best you can do is get close and remember how many mph its off +/- at 70mph and 35 mph. Unless you want to make a custom speedo gear for the tranny.

[Legacy777]

I'm well aware of the fact that different manufacturer's have different specs for the standard sizes. I've already taken that into consideration and looked at that data, and compared it to what was displayed on the miata tire calculator.

I understand there is going to be error, but it's a little too far off right now. Also, the underlieing reasons behind this is I need to use a 16" spare tire hub, and unless I find a spare tire that is closer to the original tires on the car, when I put the WRX spare on, it's going to not be too good for the center diff.

As for recalibrating, you can't do it with our speedos.

Any comments on my questions regarding more sidewall flex with the higher profile tires?