speed off size help ?

[aceofspades]

Running a 225/60/14

At 70 mph on my speed-o I'm doing about 60mph

225/60/14 is a circumference 77.4

Is the stock 175hr14 a 82 inch circumference?

If yes that would explain the problem.

Also what sizes should I use I see people say 195/70/14's is that right?

If I switch rims or get some thing nicer like 15's. What size wheel should I get?

and what tire to match it?

[E-ticket]

Try this out...

http://vexer.com/68rt/speed.html

[Walter Moore]

The stock tire size for a 260Z (isn't that what you have?) is 195/70 14 I believe. The OD of that tire is 630mm (24.8 inches, giving a circumference of 77.9 inches)

The OD of a 225/60 14 is 626mm (24.65 inches, circumference 77.4 as you stated)

My guess is someone changed the differential and didn't update the speedometer drive gear. Either that or someone swapped the transmission and left the wrong speedometer drive gear in it from a different car.

By the way, the OD of the 175/80 14 tires on a 240Z is 636mm, so either way the tires are close enough that they will not cause your problem.

[aceofspades]

any way to ID trans or speed gear in it!

also what other R180s diff gears are out there beside the auto 390 gear?

[Walter Moore]

I am not sure what came in the 260Z with a manual. The automatics all came with a 3.54 however, not the 3.9 ratio. The early 240z came with a 3.36 ratio in the manual version.

There are a lot of R180 ratios that were available in the trucks. Some of the 4X4s used an R180 in the front. Most of those were 4.11 or 4.56 ratios. The 200SX also had a 4.11 ratio R180. I think most of the 3.9 ratio differentials sold in the U.S. were in the R200 differential.

The 4 speed that was in my car when I bought it had a 16 tooth speedometer drive gear installed. The correct gear for the 3.36 is a 17 tooth. The 3.54 differential needs a 19 tooth. (Per the FSM for a 240Z, again the 260 may be different.)

The 5 speed that I bought came from an 83' 280ZX, and it had an 18 tooth gear when it came into my possession.

The right speedometer gear makes all the difference.

Oh, to id the speedometer gear you can remove it from the transmission and count the teeth.

Edited August 16, 2009 by Walter Moore
I forgot something.

[=Enigma=]

Check out this page for a description of which speedo gears go with which rear end ratios. They are color coded.

http://datsunzgarage.com/engine/

You can tell what the ration of your rear end is by reading the numbers on the ring gear and dividing on by the other. For example: 39:10 = 3.90, 39:11 = 3.54, 37:10 = 3.70. This info is also on the page I referenced above.

[aceofspades]

hmm ok I need help a little more now !

I pulled the speedo gear its yellow meaning the 3.36 then i opened the diff and its a 3.36

gear!

but my speed is still off!

and tires seem like they should match !

I do have some after market wheels if they are to wide will that affect the speedo?

at 45 i'm doing 40 at 70 i'm doing 60ish so whats the problem wheels and tires or the speedo its self? maybe bad spring or some thing else?

[Arne]

Folks, everyone is missing the obvious answer here. These old mechanical speedos tend to become inaccurate as they get old. What we have here would seem to be a textbook case. The internal clock-spring in the speedo weakens over time, so the speedo starts reading fast.

To confirm, check the accuracy of the odometer over a true measured 5 mile stretch. If the odometer is fairly close, then the speedo itself is the issue, not the gearing, tires or such.

[Walter Moore]

hmm ok I need help a little more now !

I pulled the speedo gear its yellow meaning the 3.36 then i opened the diff and its a 3.36

gear!

but my speed is still off!

and tires seem like they should match !

I do have some after market wheels if they are to wide will that affect the speedo?

at 45 i'm doing 40 at 70 i'm doing 60ish so whats the problem wheels and tires or the speedo its self? maybe bad spring or some thing else?

First of all the site listed above is wrong. Yellow is 16 tooth and the 1:3.36 differential needs a 17 tooth sprocket. (Been there, done that.)

I bought a 17 tooth from Courtesy Parts last year, part number 32703-78101. It was about $14 plus shipping. You need a punch to change it because you have to drive out the roll pin that holds the gear in the housing.

I would try the gear first, then if it doesn't work pull the speedometer out and take it to a local shop to be re-calibrated.

[Arne]

I'd still recommend checking the odometer accuracy first. If the odometer is correct, no need to mess with the gear.

[Jeff G 78]

I completely agree with Arne. My '78 speedo is off by around 5mph at 70 yet my odo is dead accurate. I checked it on the road trip to the 2008 Z car nationals and over close to 90 miles on the Ohio Turnpike it was within a few tenths of a mile!

[aceofspades]

hmm well then GPS i will do !

but i need more info to support this 16/17 speed to find which is right!