How far off will my speedo be with 17 inch tires?

[Scott98]

I've got 17 inch tires on my car. How far off will my speedometer be at say 60mph? 100 mph? I know I'm going slower than the speedo currently indicates. Everyone keeps passing me even when I'm doing the speed limit!

Thanks,

Scott

[ChrisBennet]

When I went to 17" (255/? in back) with my '86 Carrera I went from seeing an indicated top speed of 152 to at least 159 (didn't top it out).
-Chris

[Scott98]

I topped the car out Friday night at 150 mph. I was told that's faster than a '87 should go. BTW, I'm also one mile above sea level. Don't know if that affects top speed. I'm thinking it would make it slower.

Scott

[aj88cab]

Try this.......

Tire Size Calculator

[dentist90]

All depends on how you plus size them. 17" rims doesn't mean your speedo has to be out by much, as long as the aspect ratio goes down.
eg; my 215/55-16 winter tires have almost identical circumference to my 215/50-17 summer tires, thus I can still use the factory spare (I'm referring to my Talon here).
You could probably change your speedo up (smaller diameter tire) or down (larger diameter) depending on the tire size you select. A larger tire will give you a top speed higher than your speedo indicates, but acceleration may suffer.

In our case, 225/45-17 is almost identical to the stock 225/50-16 rear tire.

[lfot]

The only way to really tell is to get a GPS unit and drive around at different speeds and compare your speedometer to the indicated speed on the GPS.

You could then send your speedo off to North Hollywood Speedo and tell them what kind of compensation you need in the calibrataion. It still won't be completely accurate though. Even stock, it's off 5 to 8 percent on the low side of actual.

I did the GPS thing and just keep the represented/actual in mind on the rare occasion I actually look at the speedometer (usually when cops are around).

As long as you compensate for the bigger wheel with lower profile tires, the overall diameter of the wheel tire package shouldn't be that different from the original set. (perfect world example, i know)

Too much info?

[Scott98]

My tires are Yokohama AVS Sport. 235/45Z/R17 on the rear and 225/45Z/R17 on the front.

Scott

[lfot]

Wheel widths?
What was the original package?

[dentist90]

It's only the rears that will matter, as the speedo signal comes from the differential rotation. In your application you're out by about 1mph at 60... hardly worth worrying about (speedo is reading 1 mph slower than you're going). Tire wear alone can cause this kind of variation. Don't try telling the cops it's because of your new tires!!

PS. 245/40-17 would be a tiny bit closer, but you have to wonder if they'd fit and worry about damaging the rims on potholes.

[Scott98]

Using a tape measure, it looks like my wheels are 9 inches wide. They are also 3 inches from the top of the tire to the top of the rim.

I don't know what was original on my car, but according to tirerack.com it was 215/60/15 on the back and 195/65/15 on the front. The car now has turbo twist wheels.

Scott

[Scott98]

Originally posted by dentist90
In your application you're out by about 1mph at 60... hardly worth worrying about (speedo is reading 1 mph slower than you're going).

So, when my speedo was reading 150mph, that was pretty close? It's a stock '87.

Scott

[rcecale]

Originally posted by lfot
The only way to really tell is to get a GPS unit and drive around at different speeds and compare your speedometer to the indicated speed on the GPS.

There's always the old-fashined way of doing it:

On a highway with not a lot of traffic, moving along at 60 MPH, see how long it takes you to get from one mile marker to the next. Granted, this is a bit primitive, by "GPS" standards, but it should give you a good idea of your speed.

Randy

[dentist90]

Originally posted by Scott98
So, when my speedo was reading 150mph, that was pretty close? It's a stock '87.

Scott

Sounds kinda fast! I just checked the Yokohama website and those tires should be ~1.5% smaller circumference than 225/50-16, which is a stock tire size option.
150mph, corrected, would equate to 148 mph. The top speed on my stock 84 is (theoretically) 148mph. So I guess if you were doing 6250 rpm in 5th gear it's correct.
(if you were in your driveway at the time, your speedo is toast!)

edit: you've got a G50. Not sure about top speed there. My oops.

[Rouxzy]

Here are two shore shot suggestions.
1) Have someone travel at 60 mph in front of you and see what you are getting for a reading in your car.
2) This could be more expensive. Travel along at a steady 100mph and after you get bagged by a State Trooper for excessive spead, I'm sure he will give you a real accurate account of your speed.
Tom