Help me understand speedo??

[AlaskanJack]

I need to understand exactly how you can get your speedo to show an accurate speed with 35 in tires. I understand it requires regearing. What exactly do you change axle gears or a speedo gear? I am just trying to understand this as my new (to Me) Ex has 35's and the speedo is reading correct. I had someone drive with me and tell me what speed they were at.

 

[dejello]

Lowering the gears should make the speedometer read faster (higher makes it read slower)... And the speedo gear: more teeth reads slower, less reads faster.

^ That's how I understand it to work


If your new Ex is reading correctly, then the previous owner already fixed speedo (via gears and/or speedo gear)

 

[AlaskanJack]

Hmmm. Well that was what I was thinking but I was just trying to understand if it was only accomplished with a speedo gear change or if axle gear change could correct it also.

So if I'm running 35's and my speedo is correct (rpm's in drive, not overdrive is around 3200 rpm's at 60 mph) if no speedo was changed what gears would correct the speedo. Is there a calculation for this?

 

[dejello]

Originally posted by AlaskanJack
Hmmm. Well that was what I was thinking but I was just trying to understand if it was only accomplished with a speedo gear change or if axle gear change could correct it also.

Maybe this can help:
Dead Link Removed

 

[AlaskanJack]

Well based on my test this morning and my calculations I have 5.38 gears. Here's what I did. I was driving at 65mph with approx 3350rpm in Drive (not Odrive).

I checked my calculations with the following

(rpm X tire dia)/(diff Ratio X Transmission Ratio X T-case Ratio X 336) = mph

rpm = 3350
tire dia = 35
Diff Ratio = unknown
Transmission ratio = 1:1
T-case Ratio = 1:1
mph = 65

After rearranging the equation and solving for the unknown I got 5.3685.

Can someone re-check my calcs the gearing is not what I was expecting. But then again it does have very good throttle response and is not sluggish as it might be with 35's and higher gears (lower Number).

 

[Crankcase]

If you are trying to figure out what gears are in the rear of that beast, jack it up and spin the rear wheels in neutral...the revolutions of the driveshaft should match the tire revoultions at a ratio: EG. 1 driveshaft turn=3.73 wheel revos......

 

[Alec]

The speedo gear, axle gears, and tires all affect the speedo.

 

[AlaskanJack]

I see. I was trying to find another way to identify gearing without opeing diff, or jacking up rear. I will need to buy a big enough jack and stands. Nothing I have currently will lift it up.

 

[gavin]

so get a block of wood or somethin