New tires need help in determining proper transmission speedo gears

1KWIKSIX

Well-Known Member
#1
My 200-4R has the factory orange (11 tooth) drive gear and blue (30 tooth) driven gears and rear end is factory 3:42 gears.
Have just replaced my rear tires and have gone from 26.1" diameter (798 revolutions per mile) to a slightly taller 26.7' diameter (780 revolutions per mile).

Can anyone help me calculate which transmission gear or gears I require to correct speedometer reading.

thanks in advance.

dave
 

INEEDAGN

Seen Your Member
#2
I think the closest to perfect combo mathematically is the 10/26 combo but if it were me I'd do a lot less work and replace the 30 tooth driven gear with a 29 and call it a day. That's not much difference at all, the OE combo actually calculates to 29.11 driven teeth with the most commonly available calculator and the 26.7 diameter calculates to 28.47 driven teeth.
 

1986 Buick GX1

GX1 #001 [The One and Only]
#4
60 mph / old tire diameter x new tire diameter = corrected speed

60 mph / 26.1 x 26.7 = 61.37 mph

Granted, the speedometer will be off twice that amount at 120 mph.

How close do you want to get?

I'm curious, is there more to it than just an accurate speedometer such as shift points or something?

Not hating, just trying to learn from this.
 

1KWIKSIX

Well-Known Member
#5
60 mph / old tire diameter x new tire diameter = corrected speed

60 mph / 26.1 x 26.7 = 61.37 mph

Granted, the speedometer will be off twice that amount at 120 mph.

How close do you want to get?

I'm curious, is there more to it than just an accurate speedometer such as shift points or something?

Not hating, just trying to learn from this.


Thank you very much for your explanation.
Was simply curious to determine how much the new larger rear tires' OD would throw off the speeedometer accuracy. I had no idea.
I Just installed the new tire size and car won't be out of hibernation and driven until the spring.
Appreciate your expertise.
 

1986 Buick GX1

GX1 #001 [The One and Only]
#6
Thank you very much for your explanation.
Was simply curious to determine how much the new larger rear tires' OD would throw off the speeedometer accuracy. I had no idea.
I Just installed the new tire size and car won't be out of hibernation and driven until the spring.
Appreciate your expertise.
The race tires I had years ago were 295/50-15 Hoosier quick times.
They were 26.61 Dia. I didn't notice any difference running mid 12's.
I was hoping I wasn't missing something.

In the early 80's when I installed a 12 in. lift on my '75 Ford 3/4 ton hi- boy and went from 31 in. tires to 40 in. monster mudders there was a noticeable difference.

When the speedometer read 60 mph I was really doing 77.4 mph.
And yes, when the speedometer read 120 I was really going almost 155. Lol

The truck still ran 16.1 through the 1/4, 78 mph at 4000 rpm in 2nd gear. I should have installed 5:13 gears to make up for the tires.
It still makes me laugh when I think about it.

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1KWIKSIX

Well-Known Member
#8
Thanks Scooby, 1986 Buick GX1 & INEEDAGN. Appreciate your feedback.
Looks like I'm somewhere between the 28 too (yellow) and the 29 tooth (green). Seems to be closest to the yellow (28 tooth).
Doubt that there will be much difference either way?
 

INEEDAGN

Seen Your Member
#11
I have a free speedometer app on my phone. Use it to double check needle placement on gm clusters after stepper motor replacement. GPS indicated of course which is most accurate when checking on flat ground.