Effective gear ratio with tire size correction

[ctrlraven]

So I have been really on the fence for a while about getting 4.10s because I do a lot of highway driving but I think I have finally been pushed over into buying some. With some calculations my current ratio with my 255/55-18 tire with 3.55 gears is 3.50 and if I go with 4.10s my new effective ratio will be 4.04 with the same size tire.

Now if I run a 255/50-16 drag radial with 4.10s my effective ratio will be 4.57.

I think these numbers are off slightly, could anyone check what my effective rear gear ratio will be with 255/50-16 (26" dia.) and 255/55-18 (29" dia.) while using 4.10 gears?

I do remember a few people telling me with using a 255/50-16 drag radial and 3.55 gears my effective ratio would be 3.89/3.90, when I used the calculation method I came up with 3.95.

[Aren Jay]

maybe this isn't what you want after all.

[Marauderjack]

Keep your 3.55 gears and use small tires at the track......BEST OF BOTH WORLDS!!!

[Blackened300a]

I guess a tweak in my tune for my 255/55's in the rear has to be done?

[fastcar]

I guess a tweak in my tune for my 255/55's in the rear has to be done?

Ah,,, this may be why you had trouble getting to 135 in 7500 ft, and I got to 136 and even had to let go of the throttle before I topped out...

Yes, larger tires will lie, and tell you that you are going slower than you really are.

I will post a formula to help everyone figure this out:

I believe that our computers calculate speed based on RPM of the transmission output shaft, using 245/55/18s as a constant.

245/55/18 = 7.490 ft roll out = 704.9 rev/mile

255/55/18 = 7.604 ft roll out = 694.4 rev/mile

((704.9-694.4)/704.9)*100 = 1.49% less speed indicated on speedometer.

So, if your computer said you hit 135, we can use a simple proportion to calculate your actual speed:

135/694.4 = x/704.9

x=137

So, you went 137 the other day. 2 MPH off in 137, isn't that much, you may not find it worth the cost of calibration. It means you are 1 MPH off at 67.5.

Hope this helps.

By the way, what RPM were you going at 135? When I did my run, I didn't hold it out of OD, but let it go in natually, which it did do at around, I think 120...

fastcar

[fastcar]

Sorry, Raven,

To help you out, you figure roll out as follows:

divide tire size by 25.4 (ie 255/25.4=10.04)
multiply series by 2 and divide by 100 (ie 55*2=110/100 = 1.1)
multiply this by 1st number (1.1*10.04=11.04)
add this to rim width (11.04+18=29.04)

This give tire height in inches. Divide by 12 for feet.

=2.42

multiply by Pi (3.14) for diameter = 7.60

NOW YOU HAVE ROLLOUT.

For rev per mile divide 5280 by 7.60 = 694.7

Compare your tires using this formula, and you can determine how much to adjust.

fastcar

[Blackened300a]

Thanks for the chart, 1 mph really isnt worth going crazy to have a tune sent to me.

I ran flat out last summer and I remember hitting close to 6K rpms before I shifted into O/D, then it dropped to about 5K I think and it was the slow climb from there.

[rayjay]

Thanks for the chart, 1 mph really isnt worth going crazy to have a tune sent to me.

FWIW, speedos are not always accurate. Returning from MVV I went through three construction zones on I81 that had radar carts. Each time my speedo matched the radar read out. I have the 255/55/18's.

[fastcar]

A little off subject,

But what is the part number for the BFG 255 55 R18 everyone is using? I can't find it at their website...

Also, what tire is everyone running up front, when using this in the rear. Ideally, a 45-50 series 245-255 would look perfect, but no matching tire in BFG is available... I do NOT want to run 55s up front.

thanks,

fastcar

[ctrlraven]

I am running the BFgoodrich KDW"2" 255/55-19 tire. They stop making them Dec 2005 from what I have heard. I run the oem 235/50 up front. I did want to go to a 245/45 but again they stopped making the KDW"2" flame pattern summer only tire .


Yeah I'm not worried about revs per mile or anything cause I have a Xcal2 to adjust them. What I am trying to figure out is what my effective rear gear ratio will be with using my current tire and drag radial with 4.10 gears.

Either way I am going to get 4.10s now.

[Blackened300a]

I am running the BFgoodrich KDW"2" 255/55-19 tire. They stop making them Dec 2005 from what I have heard. I run the oem 235/50 up front. I did want to go to a 245/45 but again they stopped making the KDW"2" flame pattern summer only tire .

Either way I am going to get 4.10s now.

They stopped making the KDW2??

I thought you had 4.10s? Thats the final piece of the puzzle to help you get firmly into the 13's

[ctrlraven]

They stopped making the KDW2??

I thought you had 4.10s? Thats the final piece of the puzzle to help you get firmly into the 13's

Yeah I posted a thread up about them a while ago and another one surfaced last month about them.

I know I can get into 13's without them but I had the heat and no re-tune for the 3500 stall working against me (mid-range bog down) at MMV. After I got the re-tune the car acted just like when I installed the Xcal2 tune for the very first time, night and day performance difference. I was thinking that I may just wait until spring to do them because I'd like to do 31-spline axles and a Eaton LSD while I'm upgrading things back there.

[fastcar]

Are you having trouble with the Ford trac-lok unit? Mine works awesome. If your's needs rebuilding the clutches and labor are cheap (esp if your gears are coming out)...

fastcar