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speedo accuracy and gears

I had new tires put on recently. I measured my speedo against the highway markers today, and it seems it reads a little slow. IOW, my trip meter was set at the first mile marker and at the completion of the fourth mile, my trip meter said 3.8. {5% difference}. I probably should'a tested longer, but....

So my question is, if that's reading slow, would my mph read slower? So if it says 60, I'm really moving at about 63mph. Anybody know if this is correct?

I have 3.73 with 30" BFG tires. They actually barely measure a realistic 29" OD {those liars}. My speedo gear is a 35 tooth. Would a 34 make it more accurate or maybe it'd throw me off the other way {show more miles than traveled}?

Reply to
stephjeff
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Reply to
L.W.(ßill)

You have it figured.

I don't think a different gear can help that small of a difference. If you go up in tire size again, then I would think on a different gear.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

stephjeff wrote:

Reply to
Mike Romain

Actually, mathematically it should help. Going from a 35 to a 34 tooth will change the speed the cable turns by just under 3%. In theory a 33 tooth gear would get him the closest to perfect by changing the speedo cable by 5.7% (100-(33/35)). This should get him within 1mph @ 60mph.

Chris

Reply to
c

Thanks for the link. It shows for me to use a 35 speedo gear with 29" tire.

I guess RPM's are also off? AT 65 mph, the rpm's were about

2600-2700. So it's really about 2700-2800 rpms. Does that seem high? With the stock GSA 30's, 65 mph was 2500 rpms.

I'm running BFG 30" A/T's with 3.73s manual transmission.

Merry Xmas!

Reply to
stephjeff

The RPM have nothing to do with speedo gears or any other gears. That is an electrical signal from the coil/computer to the tachometer. If it says 2600 rpm, the engine is turning at 2600 rpm.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

stephjeff wrote:

Reply to
Mike Romain

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