Help me wiff maffs. Speedo content.

So I need to label my speedo for driving around town. I’ll list my pertinent specs. 

The original tires were 185/70r14. The new tires are 195/50r15. (22.7” diameter)

The old rear end ratio was 3.42, the new ratio is 3.73. 

The transmission ratios are 1st: 3.285

2nd: 1.894

3rd: 1.275

4th: 1.00

5th: .783 

my speedo was off 5mph prior to the new tires. Now, not so sure. I know the speedo has to read at least 70mph to appease the sloths behind me on the 55mph highway. 

I suppose one way would be to note certain RPMs, and then mark those for the actual mph versus what it says. But I’d rather do this *not* while driving. Lol

80 MPH is one mile in 45 seconds.  70 MPH is one mile in 51 seconds.  60 MPH is (wait for it...) one mile in 60 seconds. Take a watch with a second hand or display and go look at mile markers on the freeway at a fixed speed on your speedo.

If your tire sizes are the same, your rear end should be a 10% difference from where you were.

Need to check it with an accurate GPS. I've found that the free apps on my phone are as accurate for speed checks as my high quality hand held marine GPS units. 

I don't know anything about Toyotas, but a bit of googling says that you can swap the speedometer drive gear for about a dozen different ratios. 

Edit: i assume we're talking about your Celica based on the tire size you listed. 

In reply to TenToeTurbo :

Can’t happen here. No w58 transmission ever had a 3.73 ratio behind it. The speedo is driven by the transmission, not the rear end. Closest ratios are my 3.42, and the dreaded 4.10. 

Also- it’s easier to do the math and mark it with my label maker. 

Basically, I need to know what to do with the percentage differences. There’s a difference between the gear ratios and a difference between the tire sizes. I just don’t know what to do with those numbers. 

Do I add them? Multiply them? 

I agree that the label maker is the easy button. 

Dr. Hess said:

80 MPH is one mile in 45 seconds.  70 MPH is one mile in 51 seconds.  60 MPH is (wait for it...) one mile in 60 seconds. Take a watch with a second hand or display and go look at mile markers on the freeway at a fixed speed on your speedo.

 

If your tire sizes are the same, your rear end should be a 10% difference from where you were.

This reminds me of driving my 67 dart with the bad speedometer. I was still training horses then, so I would use my stopwatch to monitor my speed. 

According to RimsNTires the difference in tire size will make the speedo read 6.3% slow.  60mph reading would actually be 56.2mph. 

If I am calculating correctly, the difference between 3.73 and 3.42 is ~9% .  So 9% + 6.3% = 15.3

So your speedo would be reading 15.3 % slower.

Grab a passenger, equip with smartphone with GPS Speedometer app. Have them tell you when you reach 20, 30 40 etc mph. Mark dial wil sharpie dot. Label at will.

OR

Just accelerate until the bumper huggers back off, you are now doing 85.