Speedo correction (1 Viewer)

[leonard_nemoy]

I have a 91 with 4:88's and 315's, I have checked my speedometer and odometer against many radar signs and a few different gps units. My actual speed is always consistently 5mph below whatever my speedo reads, if it says I am going 25mph the radar and gps read 20mph and it's the same at 75mph, the radar and GPS say 70 mph. My odometer is off by 1 mile for every 33 miles I travel. If I pass 32 mile markers on the interstate or log 32 miles on the gps than the odometer reads 33 miles. My rig has consistently been this way for the last 6 years. Based on this my average mpg's in the city are 11.8 and 13.1 for highway. I don't know if the previous owner did anything to correct the speedo and I am wondering if anyone can tell based on these numbers? Or maybe someone could explain how to check to see if the PO attempted to correct for the bigger tires and regear? It had 35 x 10.5 x 15 when I got it. Thanks

 

[BILT4ME]

The 91 has an actual speedometer cable. There may be a gear reducer adapter at the transfer case connection point for the cable.

The way to figure it all out is we need to know the following:
With OD off, what is the RPM at 60 MPH?
What tire brand type and size are you running?
What is your differential gear ratio? If stock, it's 4.11:1

You can purchase a speedometer correction reducer for your truck.

 

[leonard_nemoy]

With OD off, what is the RPM at 60 MPH?
What tire brand type and size are you running?
What is your differential gear ratio? If stock, it's 4.11:1

In 3rd gear I'm spinning 3200 rpm at 60mph. (3fe) tires are toyo open country rt 315/75/16. Diff gears are 4:88. Thanks

 

[BILT4ME]

SOmething is not gelling with the numbers you produced.

According to my calcs, at 60 MPH with those gears and those tires, you SHOULD be turning 2844 RPM (left column, left chart)

At 3200 RPM, you would be running 67 MPH. (right column, left chart)

We choose the gear that is essentially 1:1 with the driveshaft, and that is typically the highest gear without OD and SHOULD be third with the OD off on your truck.

You may want to double check with GPS speed.

The larger the tire, then you will be traveling faster than the speedo shows. However, with the factory setup you would be running 71 MPH at 3200 RPM with factory gears and tires, or 2673 RPM at 60 MPH. (Right Chart)

Technically, you are UNDERDRIVING things with your combination, so your speedometer is showing FASTER than you are actually going. Stock config would be 71.8 MPH and you are running 67.5 MPH according to the numbers at 3200 RPM. That is a

You MAY have a tach or speedo accuracy issue here as well. Check your speed with GPS.
Unless we are incorrect at the estimation that 3rd gear on that transmission is NOT 1:1. Anyone know for certain?

60 MPH, 3200 RPM, 4.88 gears 60 MPH Factory stock tires and gears 31", 4.11

 

[leonard_nemoy]

To clarify my GPS says 60 mph in 3rd at roughly 3200 rpms, speedo says 65mph and I am assuming you mean 3rd gear by overdrive off because I have no buttons that say overdrive on my 91. And I am assuming I got the 4:88's because that's what the previous owner told me. Thanks again

 

[BILT4ME]

To clarify my GPS says 60 mph in 3rd at roughly 3200 rpms, speedo says 65mph

Interesting. That should still be about 2844 RPM at a GPS speed assuming third is 1:1.
I wonder if your tach is off a little or the gear ratio in 3rd is not 1:1.

 

[leonard_nemoy]

I wonder if your tach is off a little or the gear ratio in 3rd is not 1:1.

I don't know if it helps but I found this, the first column is first gear and so on.

 

[BILT4ME]

Yes, that appears that 3rd gear is 1.49:1 so that would explain the wonky numbers I'm getting.

Can you try again in 4th gear? I'm not sure of the trans configuration, so I guess "D" with the OD turned off? During torque converter lockup?

 

[Tools R Us]

3rd gear is 1:1.

 

[leonard_nemoy]

3rd gear is 1:1.

I think your right, the picture I posted must be wrong because I just found two other websites that say my 3rs gear is a 1:1 ratio
A440F Transmission - TLCWiki
I am just going to assume my old speedo and odometer are slightly off, that would explain why the odometer reads 33 miles traveled for every 32 miles I actually travel.

 

[MoJ]

My guess is that the tires measure less than 34.6”

 

[leonard_nemoy]

My guess is that the tires measure less than 34.6”

You are onto something, my tape measure and level are telling me they are only 33 & 5/16" or 33.3125". That is more than an inch shorter than toyo claims
That is kind of lame

 

[MoJ]

You are onto something, my tape measure and level are telling me they are only 33 & 5/16" or 33.3125". That is more than an inch shorter than toyo claims
That is kind of lame

Pretty common. My 315/75/16 BFG AT KO2 measure around 33.75”

 

[MikeB]

Based on your 33/32 odometer ratio for indicated/actual, swapping to a 33403-69245 speedometer driven gear should get you very close.

Speedometer driven gears:
33403-69225 31-tooth 31:12 for bigger tires, makes a good correction for 33" with stock gears
33403-69235 32-tooth 32:12 stock for 275/70-16
33403-69245 33-tooth 33:12 for smaller tires

 

[leonard_nemoy]

swapping to a 33403-69245 speedometer driven gear should get you very close.

I am a total noob, is this easy to do? Is the gear by the transfer case or behind the odometer?
Thanks

 

[Blade 80LC]

It seems that the suggestions so far are toward correcting the speed at near freeway speeds. That may be what the OP wants as it is a common speed. On the other hand, I think a speed difference of 5 MPH at 25 is much more significant. For the speedometer to be off by a steady 5 mph at all tested speeds makes me think the speedometer is off, and correcting at any one speed will leave bad readings at the other speeds. (...but I'm no expert.)

<snip> My actual speed is always consistently 5mph below whatever my speedo reads, if it says I am going 25mph the radar and gps read 20mph and it's the same at 75mph, the radar and GPS say 70 mph. <snip>

OP's desire to have his speedo accurate beats my decision to just have my speed 10% under reported by my speedometer. I'm less than 3 mph off at 25. If my speedometer says I'm going 60, I need to remember that I'm going over 65. (What I actually do is use the speed on my GPS as my assumed speed, and my speedo is just to confirm if the high tech speed is glitching.

 

[thebfg]

Marks 4WD - Engine Conversion, Low Range Gears, Portal Axles

Marks 4WD are Australia's leaders in the development and manufacturing of 4WD conversions, low range gears and portal axles for Patrols & Landcruisers

www.marks4wd.com

This is what I'm planning to install to correct my speedometer (haven't gotten around to it yet, though). Not cheap at just over $200 US (shipped from Australia), but it allows for more flexibility and instant adjustments.

 

[smritte]

I am a total noob, is this easy to do? Is the gear by the transfer case or behind the odometer?
Thanks

If your swapping the driven gear, its very easy. Where the cable come's out at the transfercase, is where your going. Should be one bolt with a small piece of metal securing the housing. Unscrew cable, remove bolt, swap out gear, reinstall. There's a notch on the gear housing that lines up with the retainer. you'll see it. May want to replace the housing o-ring while your there. I've done a bunch of these in the shop.
When we would get speedo calibration complaints, we normally set them at 60mph and check them using the highway mile markers and a known good car pacing me. No GPS then.

Edit: In all the years dealing with this issue, I have never had the speedo itself cause this. Never say cant happen. Odds are you just need the gear.

 

[leonard_nemoy]

Thank you sprite and bfg, I will get the gear and change it soon along with a new sensor in an attempt to get the cruise control to work again. I double checked everything and my 315/75/16 tires are only 33.3125" tall and when I am doing 60 mph in drive I am turning 2100 rpm's and 3200 rpm's in 3rd gear. I have also noticed that when it is really windy and I am driving at highway speeds sometimes my tachometer jumps around when to wind gusts. This only happens once or twice a year which is leading me to believe that my tachometer is probably not 100 percent accurate. Thanks everyone for your input.

 

[Bambusiero]