[notherbluSTi]

I am thinking of getting wider tires when I replace my tires, probably some 235-45-17 Goodyear Eagle F1 GS-D3's. Anybody know if this will cause the speedo to be off?

[T3h_Clap]

it will cause your speedo to be different then it is currently.

it wasn't accurate in the first place. so a few more MPH isn't a problem.

your speedo will still read faster than you're actually going.

[05_GD7]

actually, you've got it backwards.

The overall diameter is larger. Therefore, you'll be going faster than what your speedometer is telling you.

ie speedo says 60mph - your true speed 61'ish mph

[rankink75]

No, with the 235/45-17 tires the car will be doing about 77 mph when the speedo reads 80. The speedo will actually say you are going faster than you really are. With the stock tires the speedo would show 80 when you are actually going 76. The taller tire only changes the speedo by about 1 mph when the speedo says 80.

What 05_GD7 is forgetting is the speedo is already off from the factory.

[Mykl]

Originally Posted by 05_GD7

actually, you've got it backwards.

The overall diameter is larger. Therefore, you'll be going faster than what your speedometer is telling you.

ie speedo says 60mph - your true speed 61'ish mph

Yeah, but that's assuming the speedo is right to begin with..

I think the 235/45 would actually be closer than 225/45. I have a set of 3/4 worn 245/45's on my car right now and when I drive by the roadside radar detectors at 28 mph it reads 25 mph. I can't imagine how far off a worn out set of 225/45's must be.

[mx_9]

Use this tool to compare tire sizes, diameters, and affect on speedo:
http://www.miata.net/garage/tirecalc.html

[05_GD7]

No, with the 235/45-17 tires the car will be doing about 77 mph when the speedo reads 80. The speedo will actually say you are going faster than you really are. With the stock tires the speedo would show 80 when you are actually going 76. The taller tire only changes the speedo by about 1 mph when the speedo says 80.

What 05_GD7 is forgetting is the speedo is already off from the factory.

says who? subaru? Because the JDM sti uses a 235 tire and subaru forgot to recalibrate the speedo? Show me.

But I digress for a second. Lets just ASSUME that the speedo with stock tires is accurate. Larger overall diameter equates to a taller gear. Which means you travel farther with every rotation of the tire than you did before. So instead of going, for example, 5 feet per rotation, you are in fact, going 6 feet, while your car still thinks it has only gone 5 feet. Therefore, with the sizes that the original poster is proposing, he will be actaully travelling 61 mph, while his speedo says 60.

[supermarkus]

I don't get why the rest of the world gets the 225 re070s and Japan gets the 235. The Aussie STi's I saw had the same tires we rock.

[austinpike]

Originally Posted by 05_GD7

says who? subaru? Because the JDM sti uses a 235 tire and subaru forgot to recalibrate the speedo? Show me.

Radar, GPS, stopwatch + road markers, it has been observed by plenty of people on here, do a search. Or go find a stationary "your speed is" sign and see for yourself.

No one is disputing the concept. It is just that the factory speedo reads high by ~5%, with a taller tire you just end up making it "less incorrect..."

[Mykl]

Originally Posted by austinpike

Radar, GPS, stopwatch + road markers, it has been observed by plenty of people on here, do a search. Or go find a stationary "your speed is" sign and see for yourself.

No one is disputing the concept. It is just that the factory speedo reads high by ~5%, with a taller tire you just end up making it "less incorrect..."

The "your speed is" signs told me my speedometer was more accurate with fresh 245/45's on it than it was with the 225/45's it came with.

I think GPS is probably the only accurate way to measure this.

[klaymon]

We've beaten it to death. Search for "speedo error" or "speedometer error". This has been done over and over.