87 yota 22r speedo off due to tires, by how much?

[87yotafoo]

I have a 87 yota 22r with 31x10.5 MT's. How much have you guys seemed to be off on speedo?

Girlfriend was behind me and she was doing 5 over and said she wasnt catchin me...but seemed like i was doing 1 under, cuz truck wasnt legal at time..

31's throw it off about 6 mph? Anyone have a clue to how far off it is...

[darkteg94]

You do the work yourself. LOL.

http://www.dahs.srvusd.k12.ca.us/john/tiresize.htm

[87yotafoo]

done that a day ago.....butthead

[albersondh]

I think it’s around 5-6 MPH. If you have GPS or a friend with GPS I think you can use that to nail it down (or buy the correct speedo gear...). Im running the same tires as you, in fact pretty much the same truck.

[Haggar]

It will be off by on the percent difference on how much bigger your new tires off.

You can't say 5-6 mph.. it is speed dependant, and axle gear dependant.

My speedo is just fine with 4.88s, and 38.5s... of course, I have a programmable electronic speedo with a hall sensor reading the t-case :)

[Mud'N'Truck]

Drive by those speed radars that the police set up. Thats what i did, Mine is like 1 or 2 mph off at 45 with 37's and 5.29s.(85 toyota truck)

Gps would be ideal cuz you could see the difference though out the whole speed range.

[albersondh]

He has stock gears on an 87 pick up with 31x10.5 tires, this is what I base the 5-6 MPH difference on (dont want anyone to think Im just throwing numbers out there). My junk is the same set up and I know Im 5-6 MPH off. Im sure everone knows: MPH = (Engine RPM x Tire dia x 0.003 / tranny gear ratio x t-case ratio x diff ratio). This means I drive to work at an average speed of 76MPH while my speedo says about 70MPH.

[warhof]

I am runnning 32"s on an 88 4runner. At 45mph I am 2-3 miles off. When I get closer to 60 it jumps to 4-5mph. At least that is what my gps tells me, it has nothing to do with any calculation of ratios.

[albersondh]

And at 70 you would be like 7-8MPH off.... Use the formula above. The relationship between the variables in the formula above is non-linear. This is why a lower speed/gear will net less MPH offset then a higher speed/gear. Your trans gears, and RPM's are dynamic, and the formula is non-linear. Again, Im not pulling crap out of thin air here, and I certainly didn’t come up with this age old formula.

Example. Assume your gearing is linear (see below example) and we use 3000RPM as a static rev with 32” tires, 1.0 T-case and a 4.10 final drive:

3000RPM X 32” X .003 = 288 (this is our constant now)
1st gear 5:1 X T-case 1:1 X diff 4.10:1 = 20.5
288/20.5 = 14.048 MPH in first

The MPH difference between 1st and 2nd is 3 MPH

2nd gear 4:1 X 1:1 X 4.10:1 = 16.4
288/16.4 = 17.560 MPH

The difference between 2nd and 3rd is 6 MPH

3rd gear 3:1 X 1:1 X 4.10:1 = 12.3
288/12.3 = 23.414 MPH

The difference between 3rd and 4th is 12 MPH

4th gear 2:1 X 1:1 X 4.10:1 = 8.2
288/8.2 = 35.121 MPH

The difference between 4th and 5th is 35 MPH

5th gear 1:1 X 1:1 X 4.10 = 4.1
288/4.1 = 70.243 MPH

Ok so I hope this makes more since now (non-linear). Your GPS is right and so is the formula. If your off by 5 MPH at 25 MPH you will not continue to be 5 MPH off at 50, 60, or whatever speed. The MPH you are off will compound as speed increases.

I tried to make my initial reply as simple and relevant to 87yotafoo's question as possible. I assumed he was talking about highway speed. From around 65 to 75 actual MPH with our set up we are about 5 - 6 MPH off (in 5th).

[Haggar]

Originally Posted by albersondh

And at 70 you would be like 7-8MPH off.... Use the formula above. The relationship between the variables in the formula above is non-linear. This is why a lower speed/gear will net less MPH offset then a higher speed/gear. Your trans gears, and RPM's are dynamic, and the formula is non-linear. Again, Im not pulling crap out of thin air here, and I certainly didn’t come up with this age old formula.

Example. Assume your gearing is linear (see below example) and we use 3000RPM as a static rev with 32” tires, 1.0 T-case and a 4.10 final drive:

3000RPM X 32” X .003 = 288 (this is our constant now)
1st gear 5:1 X T-case 1:1 X diff 4.10:1 = 20.5
288/20.5 = 14.048 MPH in first

The MPH difference between 1st and 2nd is 3 MPH

2nd gear 4:1 X 1:1 X 4.10:1 = 16.4
288/16.4 = 17.560 MPH

The difference between 2nd and 3rd is 6 MPH

3rd gear 3:1 X 1:1 X 4.10:1 = 12.3
288/12.3 = 23.414 MPH

The difference between 3rd and 4th is 12 MPH

4th gear 2:1 X 1:1 X 4.10:1 = 8.2
288/8.2 = 35.121 MPH

The difference between 4th and 5th is 35 MPH

5th gear 1:1 X 1:1 X 4.10 = 4.1
288/4.1 = 70.243 MPH

Ok so I hope this makes more since now (non-linear). Your GPS is right and so is the formula. If your off by 5 MPH at 25 MPH you will not continue to be 5 MPH off at 50, 60, or whatever speed. The MPH you are off will compound as speed increases.

I tried to make my initial reply as simple and relevant to 87yotafoo's question as possible. I assumed he was talking about highway speed. From around 65 to 75 actual MPH with our set up we are about 5 - 6 MPH off (in 5th).

This is aboslutely, completely incorrect, sorry.

You are working in the wrong direction. The speedometer pickup on a toyota (or basically any vehcile) is at the transfer case output. It is not dependant on transmission gear or engine speed.

It works like this:

If you have typical 31x10.5 tires, they turn 675 revs/mile with typical loading (that is from the tire manufacturer specs for a 31x10.5 BFG).

At 60 mph, your tires are turning 675 rpm. If you have 4.10 gears, then your Driveshaft is turning 4.10 times faster than that, i.e. 2768 rpm. Regardless of how fast your ENGINE is spinning, your speedometer pickup on the output shaft is spinning at this speed. You could bolt the transfer case to the frame and remove the engine + transmission from the truck and it'd read the same.

Now, whenever you change a tire size, or a gear ratio in the axles, then you affect the equation. Its completely linear.

Increase tire size by 10% and the speedo will read 10% slow. Increase gear ration by 10% and it'll read 10% fast.


However, you are looking at a cable driven speedo, which has nonlinearity issues in the mechanicals, especially when 20 years old.


Also, where did you get those gear ratios for the gears? THey aren't 5:1. 4:1, etc...?

Assuming you have a stock transmission, your gear ratios are:

1st: 3.93:1
2nd: 2.33:1
3rd: 1.45:1
4th: 1:1
5th: 0.85:1



Hmm, I design OEM instrument clusters for a living. So its a topic I'm rather familiar with.

[albersondh]

Yea that (gearing) was an example to make the math simple. Try the formula that I did not make up, that I am trying to tell you works, against your formula. I bet if it does not yeild the same answer it will be very close. Would you say that if you are 5MPH off at 20MPH you would be the same 5MPH off at 70MPH????? I dont think so (non-linear MPH increase) and I also dont think your wrong Im just using a different method of getting to the same answer. This formula works for me and I have had the wife drive along side me to verify. I believe it is accurate and I believe yours is proboubly just as accurate.

[Haggar]

Originally Posted by albersondh

Yea that (gearing) was an example to make the math simple. Try the formula that I did not make up, that I am trying to tell you works, against your formula. I bet if it does not yeild the same answer it will be very close. Would you say that if you are 5MPH off at 20MPH you would be the same 5MPH off at 70MPH????? I dont think so (non-linear MPH increase) and I also dont think your wrong Im just using a different method of getting to the same answer. This formula works for me and I have had the wife drive along side me to verify. I believe it is accurate and I believe yours is proboubly just as accurate.

you are using the wrong terms.

A linear increase means: at 30 mph you are 3mph off and 60 mph you are 6 mph off.

You are thinking about a constant increase, ie its 5mph off no matter what road speed.

A non linear increase would be like 30mph you are 3mph off, and at 60 mph you are 15 mph off.

Your comments are still wrong. A lower gear will not change the offset. Engine RPM has nothing to do with it. It is a function of axle gearing and tire size only.

The formula you posted isn't really valid for this discussion, its designed for discussing the relationship between engine rpm and road speed.

What is pertinent to this discussion is the relationship between the speedometer pickup and road speed.

[albersondh]

You’re linear and my linear are different in context, your terms and my terms are different expressions. This is becoming a semantics thing. Can you tell me that my formula will not yield accurate MPH, or do you just not like the way I go about getting to the answer? Im sure toy87 has benefited greatly from this discussion. You’re obviously an expert on this particular subject matter. Should I stop using my formula because it yields inaccurate MPH results?

[Haggar]

Originally Posted by albersondh

You’re linear and my linear are different in context, your terms and my terms are different expressions. This is becoming a semantics thing. Can you tell me that my formula will not yield accurate MPH, or do you just not like the way I go about getting to the answer? Im sure toy87 has benefited greatly from this discussion. You’re obviously an expert on this particular subject matter. Should I stop using my formula because it yields inaccurate MPH results?

Hahaha, you are new here. You'll learn I'm an expert on all particular subject matters.

Its not my term vs your term. Its The correct term (mine) versus the wrong term (yours).

Now, about your formula:

Here's your formula:


MPH = (Engine RPM x Tire dia x 0.003 / tranny gear ratio x t-case ratio x diff ratio).

Yes, that tells your mph based on your engine speed and gear.
but why go through that convoluted equation?

Actual MPH = Read MPH * old tire size/new tire size.

See how much easier that is?

Thats called a LINEAR equation.


If you push in the clutch in your truck, does the speedo drop? No. Push in the clutch in your equation....

The equation you posted is correct, but you are using it in the wrong context. That is a decent equation to know in your head, if you need to look at your engine RPM and figure out how fast you are going, though. I've used that in trucks without a speedo.

Going back to the original question, typically, you had a 225/75R15 on these trucks. Now you've gone to a 31x10.5.

A 225/75/R15 is ~28.5" tall
A 31x10.5R15 is ~30.7" tall

If you did the math that way, for, say 60 mph, you'd get:

Actual MPH = 60mph *(30.7/28.5)
Actual PM = 64.6 mph

So, being ~5mph sounds right.

Me, I prefer, when possible, to use the revs/ mile of a specific tire, especially going between metric and inch sizes, because of the variation is size vs labelling.

So, using that way, it'd be:

Actual = 60mph (731/676)
Actual = 64.88 mph

at 30 mph:

Actual = 30mph (731/676)
Actual = 32.44 mph

etc....

[albersondh]

Im sure it’s very nice being an expert on all practical subjects, that’s very nice of you. My question was "Can you tell me that my formula will not yield accurate MPH, or do you just not like the way I go about getting to the answer?" Your answer was: "Yes, that tells your mph based on your engine speed and gear.
But why go through that convoluted equation?" So I guess it does work and you just don’t like how I do it.... I really don’t care what you think about the way I get to my answer as long as I get/give the correct answer and a formula that works. A man asked a question, I gave an answer that includes a formula that works; you have a better mouse trap, congratulations (they both catch mice so stop beating me over the head with your mouse trap).