need some input on driven gears...

[BrianO]

alright guys..here's the situation.. I removed all my tires a few weeks ago in order to get new ones. I took them all down to Discount Tire and purchased some Yokahama's.

here's the fun part...I brought them home, installed them, drove for a week...I kept looking at the car thinking it was high and that I must have lost a lot of tread with the old tires..upon further inspection they actually put the wrong size tire on my rims. I wanted the OEM size of 225/50/16....but what I got was 225/55/16. now the car seems to sit a bit higher..

my problem...of course now the speedometer is off and I'm trying to get it as close as possible..

I have a 7 tooth drive gear, 3.45 rear gears, and right now I have a 19 or 20 tooth driven gear in there. After the tire size my GPS said my speedo was dead on accurate, but the miles driven didn't match up.

right now my speedo shows I'm going faster by a few mph...but the miles driven does not compute so of course my mpg is off big time... wtf???

anyone point me in a good direction?

[blueboss]

alright guys..here's the situation.. I removed all my tires a few weeks ago in order to get new ones. I took them all down to Discount Tire and purchased some Yokahama's....

Did you get this size because this was all they had or (more than likely) they screwed up? Regardless, according to bgsoflex.com/speedo1.html, your current setup is correct. And with the type of drive/driven gear you have, you're not getting any closer. You need to speed up the speedometer cable. The easy way is to add a smaller driven gear however, I'm pretty sure they don't make a 6 tooth gear. Alternatively, you could tear off the back of the trans and switch to an 18 tooth drive gear but I'm pretty sure this will put the speedo off the other way. *MOST* (LEO's on the board chime in please) law enforcement personnel I deal with won't even flinch if you're 0-5 mph over posted speed unless you're in a construction zone or they're bored so don't sweat it. By my math, your speedo is reading 60 when your actual speed is 62. You indicated your gps verified your speedo as accurate. I think you'll find if you measure speed/distance traveled against your gps, your mileage will be nearly the same or marginally better. Small stuff man.....don't sweat it!

[BrianO]

mileage has suffered, but the trip meter isn't showing the mileage I'm actually driving so it makes sense...then again, maybe mine has been wrong all along...who knows. I don't have a lot of time to mess with things, but I'll try the GPS again with the other driven gear and see what happens..

here we don't write tickets until it's 5 over the posted limit.

Mike I had already put 100 miles on the tires before I noticed the error. the car just looked tall for some reason

[BrianO]

Mike, my time is somewhat limited so I have to make do with what I have since I need the car every day for work..but I'd like to get as close as possible to the correct driven gear if I can.

[BrianO]

well I thought my mileage had tanked due to the tire size change and speedo gears...I ran for codes and it seems my ACT sensor died so I wasn't getting that added mileage or low rpm pepp any more. I wondered why it felt sluggish down low...

[mokancraig]

Running the numbers:

7 (speedo drive gear) x 3.45 (rearend ratio) x 818 (revolutions per mile of the tires) / 1000 = 19.75

So you should probably be running a 20 tooth driven gear.

[BrianO]

hey, where did you get that formula? or is that something I missed with a search?... I "think" I have a 19 in there now, but may have a 20 laying around.

much appreciated

[mokancraig]

It's on the page of the FRPP catalog with the speedo gears. I've committed it to memory...