Engine/Transmission (1998.5 - 2002) Whats my new Gear Ratio?

[klaybus]

Auto trans, 3. 5 gears, running 295's (33's) in DRIVE (direct) & then going into OD? Thanks in advance.



Clay

 

[SnoKing]

Auto trans, 3. 5 gears, running 295's (33's) in DRIVE (direct) & then going into OD? Thanks in advance.



Clay

Clay your ratio is still 3. 54, but you are going faster! I thought you wanted lower gears, what gives? SNOKING

 

[klaybus]

Thats my plan. I guess what I was looking for is with the 3. 54s & 295s how much have I raised my ratio? My truck originally came with some 245 pizza cutters & that is what it was designed to run with. Since I'm now approximately 4-5 tire sizes larger is my effective gear ratio closer to 3. 0 vs the actual 3. 5 ring & pinion gear set?

My plan is still to go to 4. 10s but it's a cash thing.....



Clay

 

[lsenus]

What's the second number of your tire size? i. e. 295/75-16. The width is the first number and doesn't necessarily relate to the sidewall width. You can get 295 widths in several different sidewall heights.

 

[klaybus]

What's the second number of your tire size? i. e. 295/75-16. The width is the first number and doesn't necessarily relate to the sidewall width. You can get 295 widths in several different sidewall heights.

295\75\16 ... ...

 

[lsenus]

If I recall, theeses things came with 245/75-16E tires from the factory. Since you have just increased the tread width from 245 to 295, and have kept the same sidewall side, the 75, your gear ratio should not have changed. When the sidewall width changes, then the cirucumference changes, thereby changing the effective gear ratio.

 

[klaybus]

If I recall, theeses things came with 245/75-16E tires from the factory. Since you have just increased the tread width from 245 to 295, and have kept the same sidewall side, the 75, your gear ratio should not have changed. When the sidewall width changes, then the cirucumference changes, thereby changing the effective gear ratio.

Actually thats wrong.

Here's info from Discount Tire. I changed the sizes posted on their example but this is how it's read. Here's a link. Still looking for any info if someone knows how to decipher the ratio change due to tire size.



Clay



http://www.discounttire.com/dtcs/infoSidewall.dos



295

This is the Section Width in millimeters. This measurement is taken from sidewall to sidewall.



75

This number refers to the height of the sidewall, or the Aspect Ratio. It is a percentage of the section width. In this example, you would take 65 percent of 205 millimeters and this would give you the sidewall height.



16

Wheel diameter in inches.

 

[lsenus]

My mistake. See, you learn something new every day.



Try these GR calcs

1

2



Search results

 

[Regcabguy]

I recently changed my gears to 3. 73's only to normalize my truck to the 285's. With my camper weight I needed the change and didn't need to go to 4. 10's

 

[SnoKing]

There is a little goodie from another site:



"Gee, just calculate the height. It's so easy! Multiply the section width by the aspect ratio. Multiply this by 2 and divide by 25. 4 (or just divide by 12. 7). Add the rim size and you are done! A 245/65-17 tire is ((2X 245 X . 65)/25. 4)

+17 = 29. 54" diameter. You can do your ratio from there! :"



This says the the 295 are 33. 4" and the 265 are 31. 65". Mich list the stock 265's at 31. 8.



OK, here is the site you need to go to. >>>>>>>>>>>>>>>>>>>>>>>>



http://www.miata.net/garage/tirecalc.html



Says that your speedo is 5. 6 percent slow. At 60 you will be going 63. 4



Gearing will be similar to 3. 34's if I did it correctly.



SNOKING

 

[SnoKing]

Here is something interesting. The stock tires where 245/75R16's but most later 2nd gens came with the 265/75R16's.



If I use the formula from site 2 above:

RPM x Dia divided buy

Diff ratio x Trans ratio x Transfer case ratio x 336



2000 x 31. 8 /4. 11 x . 69 x 1 x 336 = 66. 746 (265's)

2000 x 30. 4 /4. 11 x . 69 x 1 x 336 = 63. 8 (245's)



It says that I am going 66. 746 RPM at 2000 RPMs.



Well my truck reads 63-64 at 2000, so I put dia for 245 in the formula and it says 63. 8!! Did DC not calibrate the speedo for the 265's, have I been going faster the last five years than I thought I was going?????? SNOKING

 

[bgwhtjp33]

The formulas do not take into account the weight of the truck changing the actual hight of the tire.

 

[lsenus]

I always wondered why everybody in front me of seems to always go 5 mph under the posted limits. My Jan. '99 5 spd w/ 4. 10 does 75 at 2500 with the BFG 265/75/16. I think its 2000 at 55 mph. Interesting, very interesting topic.

 

[Hohn]

Just for reference:

(new tire size revs/mile) / (old tire size revs/mile) * Original gear ratio or rpm= New gear ratio or RPM.

Example, you swap 315s in place of 265s:

(599 rev/mile)/ (655 rev/mil) * 3. 54 gears= 3. 24 gears

Or

(599 rev/mile)/ (655 rev/mile) *2000 rpm at 72mph= 1829 rpm


It's just a basic ratio-- no need for complex math.

 

[klaybus]

Just for reference:



(new tire size revs/mile) / (old tire size revs/mile) * Original gear ratio or rpm= New gear ratio or RPM.



Example, you swap 315s in place of 265s:



(599 rev/mile)/ (655 rev/mil) * 3. 54 gears= 3. 24 gears



Or



(599 rev/mile)/ (655 rev/mile) *2000 rpm at 72mph= 1829 rpm





It's just a basic ratio-- no need for complex math.

Hohn,

How about 295's -vs- 245's with with 3. 54 gears?

 

[Hohn]

All you need is revs/mile.

245s: 682 revs
295s: 626 Revs
(Assuming BFG ATs)
This would turn a 3. 54 gear into a 3. 25 gear.

 

[klaybus]

Thanks Hohn... .