Engine/Transmission (1994 - 1998) taller tires and 1 tooth larger trany gear? Doesn't math up.

[Ben Rumson]

I recently put on a 1 inch taller tire than my stock tires.



Refering to TDR Issue 17 page 35, I did the mathematics and figured that I needed a 1 tooth larger gear in the trans.



(31. 6" high new tire) / (30. 5" high stock tire) = 3. 6% change.



new 32 tooth/stock 31 tooth = 3. 2% (closest match to the 3. 6% change in tire size).



============================



It took out the stock 31 tooth and I put in a 32 tooth just tonight. Then I set out to field test it.



I used the formulas on page 35 to "calculate" either MPH and RPM variables. Result... . my actual RMP's are HIGHER than I calculate they should be.



At 54 mph, I get 1500 rmp in the cab. Calculated I should be at 1402.



At 62. 5 mph, I get 1750 rmp in the cab. Calculated I should be at 1625.



Formula: (1750x31. 6)/336x(. 69)(3. 54) = 67. 38 mph.



In other words, my RPM's seem to be running high based on the "calculated" rpm given a certain mph. (Yes, overdrive was on)



What is this telling me? The larger 32 tooth gear will make less revolutions per mile thus slowing down the readings in the cab - RIGHT? Should I have gone the other way?



It appears that it got the miles driven correct, but actual rmps are well above "calculated" rpms.



Interesting note: I seat of the pants tested the odometer for 1 mile at 60 mph and I hit exactly 60 seconds on the stop watch. This looks correct, but the tach and rpm's seem to be way out... Should I concentrate on MPH matching mile posts and forget the rpm's?



Any suggestions?



Interesting foot note: For my 47RE Automatic OD I used a . 69 Overall ratio. This gives high actual rpm's compared to calculated rpm's. BUT, if I use . 74 (standard Overall ratio) the formulas work precisely fine for every rmp/mph comparison and the math equals the cab readings.



Am I using the correct . 69 overall ratio? Should I be using the . 74 overall ratio (this is the only ratio that gets the math to = the cab readings)? Could my DTT be using a . 74 overall ratio? Bill K, are you out there?



Anyone with Issue 15 page 36 that could give me the stats for an automatic with 235/85/16 tires (approx. 31. 6"tall) and a 47RE Automatic trans? This is the ONLY issue that I don't have.



Thanks. Sorry for such a cumbersome thread. Much appreciation!!!

 

[Josparkz]

I think you were supposed to go one tooth smaller if my memory serves me well.

 

[champspa]

when I went to a tire that size it made the speedometer exactly right ( according to a gps ) and I just figured the odometer.

 

[Ben Rumson]

Originally posted by Josparkz

I think you were supposed to go one tooth smaller if my memory serves me well.

I believe you're correct... .



Reading issue #17, page 34 closer, Dennis' odometer was reading fast so he added a tooth in order to slow it down.



My speedometer and odometer are reading slow. I need to speed them up, so I should have lost a tooth. Inverse relationship...



I used a stop watch between mile markers and found that my speedometer was reading much slower than I was actually going. It needs to be spead up to match the real world.



Gunna order a 30 tooth gear (vs the stock 31 tooth) and see what happens. I'm betting this will do it.



I'll keep everyone posted on the results.



Thanks for the replies.

 

[fest3er]

Originally posted by Ben Rumson
... My speedometer and odometer are reading slow. I need to speed them up, so I should have lost a tooth. Inverse relationship... . .

Yup. A larger tire will turn slower to maintain the same speed. Thus your speedo will indicate a lower MPH, and you need to turn that doohicky faster to correct it. Thus needing a smaller gear.

Hmmm. Still not real clear. For a given driving gear and driven gear, you need to increase the size of the driving gear, or decrease the size of the driven gear to make the driven gear turn faster. The speedo gear is the driven gear (driven by the output shaft or some-such).

Fest3er

 

[bbeatty]

speedo

Question What are rmps(sorry just a thought)

 

[MWBrooks]

Are you getting the speedo gear from the dealer? The reason I ask is I'm running 35" tires so my speedo is slightly off...





Mike

 

[illflem]

And just out of curiosity how much does one cost?

 

[Ben Rumson]

Costs aren't too bad.



The 31 tooth cost $8. 00 (this was my stock gear)

The 32 tooth cost $17. 00

The 30 tooth is going to cost $6. 00



These are all dealer prices.



It's a 30 second pull and replace procedure.

 

[Cooker]

This give an idea of cost. If you convert the height of the 19. 5's this chart would be more useful.



http://www.ricksontruck.com/speedometer.html

 

[Ben Rumson]

Cooker,



Nice link.



My gears:

Yellow = 30 Tooth Gear

Green = 31 Tooth Gear

Black = 32 Tooth Gear



Don't know if all gears are color coded, but this is what I got through the Dealership.



The prices listed on Cooker's link are a bit higher than I got from my dealer, but the chart would be very helpful in determining which gear is needed for each tire size.



I'm glad to see Rickson helping identify which gear is needed for which tire. Good information.

 

[Ben Rumson]

30 Tooth did the trick

Using the new 30 tooth gear, I'm exactly on when using the formula to determine either MPH or RMP's.



I figure I'm as close as I'm gong to get.



I stop-watched the mile markers for a given speed and the times were right on.



My next test will be to check the odometer and the times for a 10 mile stretch with the cruise on 60 mph and verify again.



So far, the readings look real good. No more guessing.



I'm doing 1800 rpm's going 70 mph with 31. 6 inch high tires.

At 65 mph I'm doing 1688 rpm's.



Can't coomplain. The larger tires are helping on the open road.



I figure that my 3. 54 rear end is effectively at 3. 41 now.



I haven't towed anything heavy yet. This may bog it down a bit more than before. But, I'm not towing all the time and I'm happy with the slightly taller tires.



Thanks for the help guys.

 

[Cooker]

Glad to hear you got it sorted out. I have to look into this (probably when it warms up ) It has never been a big issue for me, but it would be nice to have it a little closer to correct.