speedo gears vs tire size

[ottawa rogue]

Hi everyone. did mopar use a different speedo drive gear for the 205/60/15 versus the 185/70/14?
i went to the 15" wheels from the stock 14", and noticed that the speedo is running a little fast now. i didn't think that .5" of tire height would make that big of a difference, but i guess it does
thanks in advance,
bryan

[ottawa rogue]

ok guys, never mind.... i found a speedo calculator on the web and according to it, i should be about 1.5 mph faster than what the speedo is showing.
i can live with that :D

[BadAssPerformance]

Good question tho... there were 3 speedo gears, yellow, blue and natural. Anyone know what tire sizes each one matches up with?

[JDAWG]

there was only 19, 20, and 21 tooth. The mph difference is usaully around 1.5 difference per tooth, just gereralizing. Yea I know its actually a ratio, but if you need to guess, a tooth is 1.5 mph I have found.

[mcsvt]

And I have all three if anybody needs them. I needed the one for my cluster and got these by accident, opps. :)

[cordes]

Good question tho... there were 3 speedo gears, yellow, blue and natural. Anyone know what tire sizes each one matches up with?

I believe the white one is for the 195/205 50 dia tires.

The blue one came in my shadow which origionally had a 195/60r15, and it is hardly off at freeway speeds with a 205/60r15 on there.

[JDAWG]

I looked a couple up and there are so many variances depending on body, trim line etc.. It really depends on the outside circumfrance of the tire. If you go bigger then you need to go with a pinion with less teeth so it spins faster, opposite if you go smaller tire. My 95 s15 had 175/70r13's on it and i would hit the governor at 80 mph because the speedo said 92mph lol

[cordes]

I looked a couple up and there are so many variances depending on body, trim line etc.. It really depends on the outside circumfrance of the tire. If you go bigger then you need to go with a pinion with less teeth so it spins faster, opposite if you go smaller tire. My 95 s15 had 175/70r13's on it and i would hit the governor at 80 mph because the speedo said 92mph lol


Yeah, I figure if you take the total DIA of the sizes that I listed and use a tire calculator, you can get the over all DIA for a particlar gear.

[ottawa rogue]

Good question tho... there were 3 speedo gears, yellow, blue and natural. Anyone know what tire sizes each one matches up with?
well if it helps, my car has the natural colored gear on the vss, and came factory with 185/70/14 tires

[MiniMopar]

http://minimopar.knizefamily.net/speedpinions.html

[BadAssPerformance]

So 225-50-15 is Blue? All mine have been Yellow OEM...

[rbryant]

This is not my webpage but it helps

-Rich


http://robskorner.freehomepage.com/faqs/pioniongear.html

Summary:

185 75 R14 19 Yellow
195 70 R14 19 Yellow
195 75 R14 19 Yellow
205 50 R14 21 White
205 70 R14 19 Yellow
195 50 R15 21 White
205 50 R15 21 White
205 60 R15 19 Yellow
225 50 R15 19 Yellow.

[BadAssPerformance]

So 225-50-15 is Blue? All mine have been Yellow OEM...

So why do my 225-50-R15 cars have yellow ones in them?

[MiniMopar]

225-50-15 is yellow. That table came from an FSM, I think. Just do the math for circumference and pick the closest one.

[rbryant]

Sorry I didn't review someone else's data well enough. I edited the post and removed the everything else is blue line and included the other size. Lots of other sizes exist...

-Rich

[TurboJerry]

I put the 20 tooth blue in everything I have. With 205 60 15's the speedo is 1 or 2 mph off. About the same with 205 55 16's as well. The 225 50 15's should be really close as well.The yellow is to get the car outta da warranty faster! It showed 5 fast on my stuff. Never tried a white one yet though.......

[Whorse]

I have 205 55 16s with the yellow.

[TurboJerry]

I don't know if anyone mentioned this yet, but yellow is 19 teeth, blue 20 and natural is 21. Interesting Whorse, They call for the natural with 205 55 16's. But seems like blue works in 2 of my cars sofar. (only 1 mph off) I know because my friend owns a speedo shop and we "dynoed" the cars for accuracy. It's not a real dyno, it is just a set of rollers with a very precise speedo hooked up to them via speed sensor and is calibrated by the same people who cal the gas pumps at all the stations.

[Whorse]

Hmm there was a speedo problem when I bought the car, so maybe the previous owner took it in to get fixed, had that done, didn't work and left it?

Otherwise I'd have to think that it's something to do with the A568 as I'm pretty sure that other than the Vans, they all had 16" in 1990.

[ottawa rogue]

well, i'm surprised that no one mentioned this option for checking speedo calibration.

go find a speed trap and blow through it:D
just make sure you pay attention to your speedo when you see the cop, so you can tell how far you're off. and yes, i have experience in this manner of speedo calibration:D

[Whorse]

I've gone through those traps they set up with the display and found even they aren't consistent, since they're usually in places I'm driving at 80km/h around here.

[BadAssPerformance]

They had one in my neighborhood (35 zone) and it changed colors from green to red when you cracked the limit...

[ottawa rogue]

They had one in my neighborhood (35 zone) and it changed colors from green to red when you cracked the limit...
that sounds like a better plan. cheaper anyway:D
i work for a city here, and we have cop cars in the shop all the time, i might have one of the guys clock me in the parking lot on a lunch break one day

[APalmer]

This is not my webpage but it helps -Rich http://robskorner.freehomepage.com/faqs/pioniongear.html
Summary:
185 75 R14 19 Yellow
195 70 R14 19 Yellow
195 75 R14 19 Yellow
205 50 R14 21 White
205 70 R14 19 Yellow
195 50 R15 21 White
205 50 R15 21 White
205 60 R15 19 Yellow
225 50 R15 19 Yellow.

There are two issues, the first being the choice of speedometer pinion and the second is variation in tire size. This question came up at TeamOmni a while back, because someone wanted to change cruising speed RPMs by altering the Final Drive ratio. Here's the text I posted there, which may be helpful to someone:
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
"There are several parts to your problem:
THE TIRE
you have 225 50 R15 tires
by my calculations that is approximately a 23.86-inch diameter tire (11.93
inches radius) You can go to Tire Rack and find the empirical Mfr listings of course; (they may differ, even from each other) if you don't like my numbers.
Thus the circumference is 2pi x R(adius) so 2 x 3.14 x 11.93 = 74.92 inches tire circumference
There are 5280 feet in a mile or 5280 x 12 = 63360 inches in a mile
The tire will revolve 63360 / 74.92 = 845.5 revs per mile

If you wanted to drop your engine rpms by 20 % (169 rpm) using different tires you would need to mount a 676 rpm tire which would make your car look like a monster super duty John Deere 4wd tractor at the Tractor Pull.

THE FINAL DRIVE
I have done my own calculations for an 890 rpm tire (stock 22.7"
high/diameter tire) at 60 mph in 5th gear. [(22.7/2) x 2 x 3.14]
Of course if you have mismatched your stock tire size and speedometer pinion
you'll have to field test time for a mile - calibrated against the Interstate
mileposts (1 mile per minute - 60 seconds)

1. For my A-413 with the (2.86 x 1.06) 3.02 Overall
3.02 x 890 = 2688 rpm engine

2. For my A-520 with the 3.50 Final Drive (x 5th gear) - 2.52 Overall
2.52 x 890 = 2243 rpm engine

. . .
This is what I did. The numbers are theoretical but probably not too far from
reality. Compare with your empirical numbers!
You can do the arithmetic for the rest, yourself.
HOWEVER CHECK THE FINAL DRIVE NUMBER ON THE "NEW PROCESS" TAG
If the F.D. is different, you'd better be careful where you spend your money."
xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Cheers - Andy

[WVRampage]

Road construction signs are my calibration unit,When it tells me that my speed is its usualy close or theres just 4 lights fashing when I pass,thats usualy what they read around here at 85.

[TurboJerry]

Speed traps ain't accurate. they will pick up the biggest vehicle, or even vehicles going the opposite way.... I noticed just about every 568 I've worked on has a yellow gear in them. Interesting............

[MiniMopar]

OK, I updated the table to be more useful:

http://minimopar.knizefamily.net/speedpinions.html

[BadAssPerformance]

is this right?


225/50 R15 4.4" 23.9" 75.0" 845.0 White 21