Speedometer gear question

[blown motor]

Just want to make sure I'm going the right way here. If I have a 35 tooth gear and I'm clocking 9% high the I would need to go to a 32 tooth gear, correct? These puppies aren't cheap, don't want to go the wrong way.

[RUNCHARGER]

38 if the speedometer is currently reading 9% too fast. You must have really short tires on it.

[blown motor]

275/40ZR17  According to charts a 35 tooth should be right on. But the odometer is 8% high. Tomorrow I'll find someone to help check my speed. I sensed that it was reading high before I knew the odometer was high. Do you find the charts to be fairly accurate or not?

[pschlosser]

When it comes to speed checking, I can offer this:  There are several FREE mobile apps that will use the GPS device on your phone to give you instant ground speed readouts.  They are pretty accurate, too.  This may be more accurate than a buddy in a pacer car.

Just make sure your travel speed is constant-steady, and you're going in a straight line down the freeway.  You'll have you numbers in less than a minute.

[Brads70]

Murray, I sent you a text.....

[headejm]

You need the rear axle ratio and tire size. Use this chart.

I agree with the free phone app approach to checking actual speed.

[Jay Bee]

I use my Garmin GPS to double check the speed, and here's tire size calculator I always refer to.

https://tiresize.com/calculator/

[blown motor]

The results are in. I downloaded the speedometer app and went for a drive. The car vs app numbers look like this. 40/35 50/44 60/53 66/59 I thought I had 3.55 gears but these numbers line up more with 3.73. I have to double check that. 3.73 would help explain a little bit why my gas mileage is so bad. I'm sure it has nothing to do with driving style. 
There's a 35 tooth gear in there now so I think I'll go with a 39 and call it good. Does anyone have a 39 tooth gear they are willing to sell?