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So even though the ratio of the tire height vs gear ratio that I have now is very close to the stock ratio, my speedo can still be off. This is what you are saying, correct?


Yes. Think of it in percentages. For instance, if you change from the stock tire diameter by 10%, you'll need a 10% change of your final drive gearing to get back to where you started.

Example - we go from a 30" tire to a 33" tire, a 10% change.

Now if we had a 4.00 rear to start, we'd need to change to a rear that was 110% of the old rear ratio to get back to where we started.

4.00 X 1.10 = 4.40 and we're effectively back to stock. But we have bigger tires now...
<img src="/forums/images/graemlins/wink.gif" alt="" />

Just divide the new tire size by the old one, then use that number to determine the change needed in final drive ratio to get back to the same overall ratio...


Ok, went from 225's (28.4"-Firestone website) with 3.90's to

32's (31.8") with 4.30's

That's an increase of 11.97% in tire size and a 10.26% increase in gearing, so overall a 1.71% decrease, which means that 75 mph should be 73.7 mph. I'm overall 5 mph off, what gives?

Maybe I should measure the diameter of the tire on the ground and use that figure?


I agree. You should be a lot closer than you are. In fact, I'd think you should be about perfect. It means one or more of these things.

The tires are different than the spec sheet says.
The speedo was off in the beginning.

Keep in mind if you replaced worn 225's with new 32's the 225's had lost quite a bit of diameter from wear. A lot of truck type tires will run 16/32 tread depth. Which means if you wear them smooth you've lost 1/2" from each side or a full 1" in diameter...

--
Jim - who feels he's probably not helping you a bit even though he'd like to...

<img src="/forums/images/graemlins/cheers.gif" alt="" />


Well, I was using the 28.4" for the factory, but when I bought the vehicle used, it had 30x9.50's. I don't remember what rpm's it turned then, the 2700rpm at an indicated 70 mph was with the 32's.

Just to make this even more complicated, I just measured the 32's on level ground on the truck. They meausred out 30.75". Now, I'm really confused. Maybe the speedo was way off from the factory?

1985 Toyota 4Runner, dual e-lockers, exo and more

2007 Toyota Tacoma Crewcab TRD 4x4

2000 Suburban 2500 4x4

1991 Tiffin Allegro Class A MHome (Ole Krusty)