jiga

So I am wondering if you can determine rear gear ratio from RPM's. I have been having an issue with my speedo being off ever since I have the car tuned from BAMA. First thing was we corrected the rear tire size. That dropped the speedometer from being 13+ off to 10+ off. They believe I am running a different rear gear than stock. I purchased the vehicle used from a dealership so now I am trying to figure this out.

I know I can jack up the rear and and mark the drive shaft and the tire then count the revolutions, but just curious if you can determine from RPM's.

Anyways...

Vehicle is a 2006 Mustang GT automatic
235/55/17 sitting on 17x8
On the highway this morning I got the vehicle up to 70mph (speedo read 80 but I am actually doing 70) in 4th (OD is off) and I was running at 3400 RPM's.

The vehicle should be totally stock other than my tune, CAI, axle-back, and possibly this different rear gear.

Thanks for any info.

mapitts

If it is a stock auto, then you have 3.31's. Look underneath the car. There will be a sticker on the differential telling you what the gear ratio is. If you have done all of that then there is the redneck way of doing it. Jack up both sides of the rear of the car. Mark a spot on the companion flange or drive shaft that you can easily see. Rotate the tire one turn and count how may times the mark comes past you. 3 3/4 turns is about a 3.73, a little over 4 and you have 4.10's, etc.

mapitts

oops........I just read where you know to do that......my bad.

AMChrisRose

Automatic should be 3.31. Make sure that the sticker on the rear housing says 3.31 as well. It could have been a dealer installed option, or an aftermarket change.

The best way is the manual way, above.

Nuke

Gears and tire OD are the only 2 things that I can think of (short of a bastardized tranny) that would affect the speedo.

PM me with your e-mail and I'll send you a spread sheet that'll list engine RPM based on your tranny and selected gear and tire sizes. Otherwise, I'd keep selecting gear sizes with your tuner based on a GPS reading.

jiga

PM sent.

I have tried looking at the sticker, but it is long worn out... even had to use some degreaser on it to try to make it readable, just didn't work.

I saw on the tuner where I can select different gear sizes, just wasn't sure if I should modify those settings on my own or send an email to Bama once I know the correct gear size so they can apply the setting and send me a new tune file. I suppose they are doing the same thing I would attempt to do.

157dB

One of the rear tires up in the air.
Put in neutral, other three tires chocked.
Turn driveshaft 1 rev and count the tire revs.
Bingo, thats the ratio....

Nuke

I'm at work but will forward the spreadsheet after I get home. One day I'll get smart and dump that sort of stuff onto a stick drive and keep it at work...

jiga

Looks like I have 3.73 on there. Going to set that in my tune and see how the speedometer reads out.

According to that sheet, there is no way I am running stock 3.31 on there.

Thanks for the spread sheet Nuke.

Nuke

My pleasure. Now that you have those, be sure to pass it on if anyone else needs assistance.

Don't expect the speedometer to be dead-on but it does happen. Since my SCT2 only allows tire RPM changes every 10 RPM, I can get within about 1-2 MPH per my GPS. Good enough for me.