Larger wheels affecting speedometer?

[Checkers]

On my 99 Eclipse I have 17" wheels, and I've heard that larger wheels can affect the speedometer. Is this true? If so, can I recalibrate the speedometer somehow?

 

[wkmitsu]

You need to get the overall diameter of your wheels including the tire as close to the stock diameter as possible. Use this...

Tire size calculator

 

[dsmtun3r98gst]

I'm running 19's and I don't really see any problems.

It would actually affect more if you lifted your car . I'm not sure if it does if you lower it as well.

Most times when someone gets bigger rims they tend to get smaller tires so in a way it ends up in the same diameter as stock or as close as possible.

 

[TunaTalon]

For bigger wheels use lower profile tires to keep the stock revolutions per mile.

The speedo will be off by the same percentage as the revs/mile is off from stock.

Tirerack.com tire specifications give revs/mile for all tires.

 

[crash89]

I'm running 19's and I don't really see any problems.

It would actually affect more if you lifted your car . I'm not sure if it does if you lower it as well.

That makes no sence at all!! Shocks/struts have no effect on tire rotations. Ive got 18" wheels and I see about a 3 mph difference from the speedo.

 

[93ETGOCART]

I'm running 14s and I've passed one of those things that says how fast you are going and according to that my spedo is running 4mph faster than I'm actually going

 

[knochgoon24]

Like it has already been said, wheel size has nothing to do with your speedometer being off. It's the outside tire diameter. This determines how far you'll move with one revolution of the tire. Your car's speedometer is based on a certain distance per revolution. When you change the outside diameter of the tire, you no longer roll that distance. You speedometer will be off by the same percent difference as the difference between your tires' circumference and the stock circumference.

For Jeeps and other vehicles that get lifted for larger tires, they have aftermarket "speedo-healers" that correct the speedometer. This can be as simple as a different speedogear, or as complex as an electronic piggyback unit that converts the signal to the correct output.

 

[crash89]

Like it has already been said, wheel size has nothing to do with your speedometer being off. It's the outside tire diameter. This determines how far you'll move with one revolution of the tire. Your car's speedometer is based on a certain distance per revolution. When you change the outside diameter of the tire, you no longer roll that distance. You speedometer will be off by the same percent difference as the difference between your tires' circumference and the stock circumference.

For Jeeps and other vehicles that get lifted for larger tires, they have aftermarket "speedo-healers" that correct the speedometer. This can be as simple as a different speedogear, or as complex as an electronic piggyback unit that converts the signal to the correct output.

Great explanation!

 

[UofACATS]

Your speedometer will not necessarily be accurate from the factory. Depending on the change in rotational diameter, you may actually increase speedometer accuracy. (Decrease speedometer inaccuracy is a better way to look at it.)

Test your speedometer accuracy by driving by a radar sign which displays speed, or by the milemarkers on the side of the highway, or with a GPS device which has speed display capabilities, such as a Droid or iPhone, or do all of the above.

 

[dsmtun3r98gst]

I'm running 19's and I don't really see any problems.

It would actually affect more if you lifted your car . I'm not sure if it does if you lower it as well.

That makes no sence at all!! Shocks/struts have no effect on tire rotations. Ive got 18" wheels and I see about a 3 mph difference from the speedo.

And yet the wiseman just explained to us that cars that get lifted get their speedo recalibrated

 

[ramsack]

Where is an actual accurate source to get distance from? On the Autometer speedometers they want you to drive x miles to calibrate, but who really knows if mile markers are lain that accurately. I would think a dyno would be pretty damn accurate.

 

[90moneypit]

Where is an actual accurate source to get distance from? On the Autometer speedometers they want you to drive x miles to calibrate, but who really knows if mile markers are lain that accurately. I would think a dyno would be pretty damn accurate.

1 mile in 1 minute at 60mph. You want a flat, straight stretch of interstate highway. Those mile markers were laid by survey and should be accurate. You can calculate how far off your speedometer is by the inverse deviation in seconds. For example, if it takes you 55 seconds to travel 1 mile at an indicated 60mph than you are traveling at an actual 65mph or 5mph over the indicated 60mph. The most accurate way is to travel and record at 60mph over 3 miles but 1 mile on a flat road will serve for most. Plus, let's face it, who drives 60mph anymore unless there's a cop in sight?

 

[ramsack]

Ya I suppose if they have to make the roads to tolerances that are insane I would guess the markers would have to be super-accurate. It's probably more accurate than GPS or radar. The thing that throws me off is that the mile markers are in the same spot on both sides of the road, which shouldn't be so, as with every turn the distance traveled is different between lanes, so maybe they round things out to match up?

 

[knochgoon24]

I've used the mile marker method, GPS, and those radar speed signs and they all say the same thing: my car reads 105% too fast/far. So I get an extra 1/20th of a mile for every mile. At an indicated 63 mph, I'm actually only doing 60. A tire with circumference 5% larger would fix the error.

 

[UofACATS]

Test your speedometer accuracy by driving by a radar sign which displays speed, or by the milemarkers on the side of the highway, or with a GPS device which has speed display capabilities, such as a Droid or iPhone, or do all of the above.

I've used the mile marker method, GPS, and those radar speed signs and they all say the same thing: my car reads 105% too fast/far. So I get an extra 1/20th of a mile for every mile. At an indicated 63 mph, I'm actually only doing 60. A tire with circumference 5% larger would fix the error.

Exactly OP.. Just get out there and do it, you'll see the pattern.

And the above example is what I was talking about as far as factory speedometer being inaccurate to begin with. I know mine is a couple mph optimistic at highway speeds.

 

[99gst_racer]

And yet the wiseman just explained to us that cars that get lifted get their speedo recalibrated

He said "lifted for larger tires". The circumference of the tire is the only thing that affects your speedometer reading. Your suspension and/or overall ride height has nothing to do with it at all.


The calculator posted in post #2 is a great tool. I've been using it for years for various cars.

 

[jawar666]

I have 18" rims in my GSX... 225/40R18 tires...

Same setup in my GST (18" rims, 225/40R18 tires)

Using a GPS phone... My GST is off about 7-8 mph at 60mph (according to the GPS)

The funny thing is that my GSX is very accurate compared to the GPS... I wonder why, however, I have never taken the time to investigate it...

 

[99gst_racer]

It should read closer to 2-4 mph off with that tire size and at that speed.

 

[jawar666]

How that works?

The speed sensor tells the ECU the revs and then the ECU tells the speedometer how fast you're going?

Or is it a straight signal from the speed sensor to the Speedometer?

I wonder what happens if I interchange speed sensors between the two cars...