Speedometer calibration.

[FMJ]

I was driving one of my 109A3's yesterday and noticed that the speedometer was not matching up with indicated RPM's, It indicated well over 60 mph while the tach was showing 2500 rpm. The truck has 9.00-20's on it. Is it possible the speedo gear is calibrated for 11.00-20's?

 

[FMJ]

This was in fifth gear, and I was paced by antoher vehicle and never went over 55mph on the pace vehicle speedometer

 

[m-35tom]

the ratio is printed on the adapter

 

[FMJ]

Thanks Tom, what ratio shoud it be for 9.00-20's ?

 

[AJMBLAZER]

Got a GPS unit? You can always figure out your speed then.

 

[FMJ]

Re: RE: Speedometer calibration.

Got a GPS unit? You can always figure out your speed then.

Yeah, but I can't seem to find the Cigarette lighter in this truck.

 

[AJMBLAZER]

RE: Re: RE: Speedometer calibration.

Ah, mine's battery powered. Had the same two AA's in it for about a year now. Of course the only thing I use it for is checking the actual speed on my vehicles...none of which indicate true for some reason...

 

[FMJ]

RE: Re: RE: Speedometer calibration.

I've got a Garmin that seems pretty accurate, I'm going to take it on the M35 recovery tomorrow, see how slow were going on the way back from Santa Fe!

 

[Offroadfabworks]

RE: Re: RE: Speedometer calibration.

FMJ i know its little off subject but what do you think of the Garmin... Ive been debating one for awhile now.

 

[FMJ]

RE: Re: RE: Speedometer calibration.

I like it, well worth the money IMHO

 

[cranetruck]

Re: RE: Speedometer calibration.

Thanks Tom, what ratio shoud it be for 9.00-20's ?

For 900-20 tires it is 0.187:1 and for 1100s is 0.200:1......

 

[FMJ]

Re: RE: Speedometer calibration.

Thanks Tom, what ratio shoud it be for 9.00-20's ?

For 900-20 tires it is 0.187:1 and for 1100s is 0.200:1......

Thanks, I'm pretty certain that this 109 has the wrong one....

 

[Ferroequinologist]

RE: Re: RE: Speedometer calibration.

FMJ- http://www.youtube.com/watch?v=-Q7x6fpGvn0

I like my garmin a lot.

Riddle me this- Garmin says 62mph, speedo says 55- I was running 11.00x20s so I can understand this- but the tach says 2400rpm. That can't be right, can it?

I thought it was supposed to be higher than that at 55, but my truck never seems to go up that high Rpm wise. It will if you are out of gear and you rev it, so I don't think the tach is messed up. Hmmm... might explain my 12mpg...

 

[FMJ]

Re: RE: Re: RE: Speedometer calibration.

FMJ- http://www.youtube.com/watch?v=-Q7x6fpGvn0

I like my garmin a lot.

Riddle me this- Garmin says 62mph, speedo says 55- I was running 11.00x20s so I can understand this- but the tach says 2400rpm. That can't be right, can it?

I thought it was supposed to be higher than that at 55, but my truck never seems to go up that high Rpm wise. It will if you are out of gear and you rev it, so I don't think the tach is messed up. Hmmm... might explain my 12mpg...

Makes sense to me, bigger tires, slower rotation speed required, engine turns slower, as does the speedo.

 

[m-35tom]

RE: Re: RE: Re: RE: Speedometer calibration.

mph = ( rpm * radius) / ( 168 * ratio)

do the math

with 11:00, 2400 rpm = 56.5 mph, 62 mph = 2633 rpm

 

[Ferroequinologist]

RE: Re: RE: Re: RE: Speedometer calibration.

I forgot to mention that it didn't seem to go over 2500 even when I had the 9.oox20s on there. I'm no good at math. - It just seemed slow to me. Yes, I get the idea of less revolutions = more distance traveled with bigger tires.

Just seemed low to me that's all.

 

[BFR]

RE: Re: RE: Re: RE: Speedometer calibration.

checked mine vs. gps yesterday...
1100 20 NDCC
62 mph @ 2500RPM

speedo was bouncing about either side of 60mph

 

[FMJ]

Re: RE: Re: RE: Re: RE: Speedometer calibration.

mph = ( rpm * radius) / ( 168 * ratio)

do the math

with 11:00, 2400 rpm = 56.5 mph, 62 mph = 2633 rpm

MATH????????, hold on, I gotta take my socks off........