OK, I'm confused about the effect of tire size on "effective" gear ratio.  Please correct me where I'm wrong and explain what I'm not understanding.

My '90 Tracker is running 1:1 trans ratio in 3rd gear, 1:1 transfer case ratio in 2H, and the rear differential is 4.62:1, so my final drive ratio is 4.62:1, correct?  That's because my pinion gear is turning 4.62 times for every ring rotation because the ring gear has 4.62 times the teeth of the pinion gear, yes?

Now, if I replace my 275/75-15 tires (27" diameter) with 30x9.5 (30" diameter), I will decrease the number of tire RPMs and associated engine RPMs at any given speed, right?

So, why does my "effective" drive ratio become 5.13:1?  I would have thought that the "effective" ratio would have decreased given that my RPMs decreased.  The pinion gear is still turning 4.62 times for every ring rotation.

Now I guess I would look at it that the ring is only turning 90% of a rotation for the 30" versus the 27" so my "effective" ratio would be 4.16:1 (lower number, higher gearing, lower RPM) , but maybe that's the wrong way to look at it.  The ring and pinion have to turn 111% of what they did with 27" to get a full rotation at 30", which would mean even though the engine would turn slower they'd still have to make up the 11% difference to get a full rotation.

What I'm trying to tie this to is MPG.  It would seem that if the engine RPMs for a given speed went down, MPG should go up.  But, if the gear ratio were increased (from 4.62 to 5.13) then MPG would go down, which seems to be the effect most people observe when they switch to a bigger tire.

As you can see, I'm VERY    HELP!

As I under stand about  "effective" gear ratio".  Is a compairson of RPM's compaired to a stock tires.  So the larger the tire will lower the ratio.  Like your 4.62:1 to a 4.16:1.

If your wondering about gas milage, there's a lot more to it than just bigger tires.  Just the added weight, tire to road friction, air resistance and then different RPM's.  It would be hard to prodict you gas milage with out just trying it.

The closest you can come to is to ask some one else with the same set up you have with the tire you want.

jerryp,
I went from a 205/75 - 15 getting 25mpg mixed driving to a 30 X 9.5 - 15 getting 16-17 mixed.  I also installed the coil spring spacers which added some height.  No new gears...yet!
I love the new agressive tires, hate the new mileage.
J

 To a certain extent, if you do something about lowering the axle ratios, you will  recover most of your performance as your truck won't have work as hard to turn the tires.

As I under stand about  "effective" gear ratio".  Is a compairson of RPM's compaired to a stock tires.  So the larger the tire will lower the ratio.  Like your 4.62:1 to a 4.16:1.

That's the way I look at it too, but the "calculators" out there say 5:13  :-/

If your wondering about gas milage, there's a lot more to it than just bigger tires.  Just the added weight, tire to road friction, air resistance and then different RPM's.  It would be hard to prodict you gas milage with out just trying it.

Yeah, I've thought about the weight which is why I've been looking at wheel tire combos with alloy wheels.  I think I'd end up saving a lot of weight by losing those steel wheels.  I guess the exposed tire surface area and the fact that more air would pass under the vehicle and cause more turbulance would create a lot of drag which would lower MPG.

jerryp,
I went from a 205/75 - 15 getting 25mpg mixed driving to a 30 X 9.5 - 15 getting 16-17 mixed.  I also installed the coil spring spacers which added some height.  No new gears...yet!
I love the new agressive tires, hate the new mileage.
J

That's the kind of results everyone seems to get

To a certain extent, if you do something about lowering the axle ratios, you will  recover most of your performance as your truck won't have work as hard to turn the tires.

Now that's the thing that still confuses me.  I used to have a Corvette with 3.08 gears.  My understanding was that, comparing them to say 4:11s,  the 3:08s were worse for "off the line" acceleration, but better for MPG and top end speed.  Following that example, the effective 5:13 ratio (which is lower than my original instead of higher) from my original post that the calculators come up with would make sense.  But, as your post implies and from what I've read on this forum, the expectation/reality of it is, if you lower the gear ratio (put in higher numbers gears) that you regain the performance.  That confuses me even more!  I would think that if I lowered the ratio by putting larger tires on, and I lower the ratio more by installing lower gears, I should be able to pull tree stumps out, but never get out of 3rd gear.

Thanks for the replies so far.  Maybe this'll sink in and I'll figure it out someday.

I've got 32-11.5/15 MTR's.  I also have 5.83:1 R&P.

With this combo my speedometer is dead on.  That mean's it's within stock ratio's.

From say 0 - 45 it's about the same power feel as a stocker.  After 45mph wind and other forces you can feel the difference.

I have gotten up to 20 mpg with this set up.  Haven't checked it lately though.

any know wat i should be running in my sammi with 35's and a 1.6 16 valve? cuzz ive got stockers and it drives like a snail

Go to my website, (no more pop up adds   )
and click on the "Calcs" button, input the info
and you will get the new effective ratio, then
you can plug in the new RandP ratios to see
what you need for the same performance as
the stock gears.

http://wildcatent.freeyellow.com/zookmods


Wild

any know wat i should be running in my sammi with 35's and a 1.6 16 valve? cuzz ive got stockers and it drives like a snail

I run with 35s, 5.12s, stock sammy case, and an 8v 1.6. Mine goes down the road ok, but I think that if I had a GRS1 (4.16) case instead of a stock sammy case, I think that it would go much better.
The reason that the lower gears work better with larger tires on a low power truck like a sammy or kick, is because it lowers the strain involved with turning the extra leverage of the larger radius tires.
Hope that helps.

lemme know if this helps...say at 4500 rpm ur wheels turn 4 times ( i know waaaaaay off)..and u goto a 30" the engine would still be at 4500rpm but ur tires would only turn 3 times...right?

so if u were doing 70mph with stock tires at 4500rpm ur now at 5000rpm at 70mph with the 30" tires


i may be totaly off though...

Now that's the thing that still confuses me.  I used to have a Corvette with 3.08 gears.  My understanding was that, comparing them to say 4:11s,  the 3:08s were worse for "off the line" acceleration, but better for MPG and top end speed.  Following that example, the effective 5:13 ratio (which is lower than my original instead of higher) from my original post that the calculators come up with would make sense.  But, as your post implies and from what I've read on this forum, the expectation/reality of it is, if you lower the gear ratio (put in higher numbers gears) that you regain the performance.  That confuses me even more!  I would think that if I lowered the ratio by putting larger tires on, and I lower the ratio more by installing lower gears, I should be able to pull tree stumps out, but never get out of 3rd gear.

Thanks for the replies so far.  Maybe this'll sink in and I'll figure it out someday.

The Vette example that confused youwould be a different application. There you are comparing using same size tires and lower gearing. The factory 3.08 in a vette are for all around combination of performance trageted on highway (high speed) performance. When you lower the gears to 4:11 it makes the vehicles RPM at a certain speed increase, giving it more power quicker. But at higher speeds it will make the motor run at higher RPM hurting it's top end performance.

An easy way to understand the whole concept is changing gears on a Mountain bike. If you have one take it for a ride and pay attention to the gears. Think of the front sproket as the Tire size and the rear sprocket as you gearing. Put it on the small gear in the front and ride it. Switch through the gears thinking about Stock tire size. Then get at an easy pedaling speed and switch the front sproket to the large gear trying to keep a constant speed, now think Large off road tire. Now if you try to make it as easy to pedal as before, you will have to drop to a lower gear on the back sproket.

Try it, that is how a friend explained it to me and I did not understand untill after a few trips around the neighborhood.

Just remember
Front sproket;
on small gear= stock tire
on large gear= large tire

Rear sprocet:
small sproket= stock gears
Large sproket= Calmini 5:83