Speedometer ? for the Ford Techs

[Ironmover]

I'm gonna keep this simple.

2004 F-350 Lariat 6.0 SRW 4X4

38x13.50x20r Toyo mud terrains Newly discovered 4.30:1 gear set. (Thought they were still OEM 3.73:1) The eulogy of ASSUME applies to me in this instance

When truck was purchased I discovered a Superlift Truspeed box. I'm going to remove this piece and restore the factory ABS circuit eliminating the "GIZMO" as I'm being degraded like it was my choice to install. That's irrelevant.

All this boils down to is I was considering/researching how to properly re-gear my differentials to compensate for the large tires. This became a priority since my transmission is starting to present problems. I wanted to gear properly so that when a trans replacement was necessary, the problem wouldn't be recreated.

Now that I'm restoring the ABS circuit eliminating the TruSpeed product, there will be a slight speed sensor error. (Error % since 4.30:1 and current tire rollout does not perfectly match OE gear+tire) By taking the truck to a dealer, can the speedometer be corrected and happiness restored to the drivetrain???? (Minus damage previously caused)


(What I believe led up to all of this was my assumption of 3.73 gearing, the calibration box in use and entering a tire rpm value of 555 data being entered into my SCT. Had I known the original owner re-geared and had a tuner from the east coast acknowledged my concerns and info provided (truspeed+truck specs) I feel that running the conflicting values caused my trans early demise. )

[Dirty_Stroke]

The dealer can set your speedo, but only so far. I had my friend at the local ford dealer bring home the IDM and we re-calibrated my speedo. It wouldn't let us put in that the truck had 35 inch tires, so we calculated my tire revs per mile and then put that in to get the speedo correct. I say all that to say, that with your 4.30 gears and 38.5's they might be able to manipulate the ABS module to read your speed correctly, you'll just have to take it in and let them try.

[GregsSD]

The dealer can only adjust down to 600 revs/mile on the tire size.

Having a different gear ratio than stock will not change the speedometer reading because it is taken off a tone ring mounted next to the ring gear in the rear end, not the ring and pinion gears. This is why the Superlift Truspeed box is needed to adjust the speedometer for that large of a tire.

[Dave]

Get your hands on AE. It can reset your speedo for that combo

[Ironmover]

Thanks for the replies guys

Will the IDM flash or AE eliminate the need for my Truspeed box so that I can restore the ABS circuit? Will the flash achieve the same results as the Truspeed, most importantly the result of fuel economy? It's no joke with the box bypassed running a straight ABS signal the truck is down to 11 mpgs, interrupt the signal using the box and I'm right back to my 15.97 hand calculated figures. (which is all the better mixed driving is going to get but I'm okay with that.)

[Karls03]

I don't know where the speedo pick up is but I do know this, if you change the gear ratio at or near the same rate of change as the tires, your truck will be moving closer to the indicated speed. Follow along:
38(new tire)/31.5(OEM tire)=1.2(rate of change)
3.73(OEM gear) x 1.2(rate of change)= 4.49

By these numbers, you're only out by 5%, which is mucho bettero than these knuckle draggers running 38s with 3.73s not understanding why their diesel is a pooch. The down side is that now that you know you are only 5% off, your hand calculated mpg figures of 15.97 may be closer to 12. To back that up, I only get 15 mpg with 35s, and that's A LOT lighter of a tire to throw around than a 38.

Because revs per mile isn't as simple as circumference in inches divided by 63390 (a mile in inches), you need to look up the rev per mile on your tires, I'd start with the mfr of the tires. My 315-75-16 BFGs are 601/mile.

Hope this helps.

[GregsSD]

Originally Posted by Karls03

I don't know where the speedo pick up is but I do know this, if you change the gear ratio at or near the same rate of change as the tires, your truck will be moving closer to the indicated speed. Follow along:
38(new tire)/31.5(OEM tire)=1.2(rate of change)
3.73(OEM gear) x 1.2(rate of change)= 4.49

By these numbers, you're only out by 5%, which is mucho bettero than these knuckle draggers running 38s with 3.73s not understanding why their diesel is a pooch. The down side is that now that you know you are only 5% off, your hand calculated mpg figures of 15.97 may be closer to 12. While I don't understand a bit of the electronic part, I do know about the math. 4.30s and 38s is much better than 35s and 3.73s.

Using this same math, I should be running 4.11s in my truck, not 3.73s
34.6/31.5=1.09
1.09x3.73=4.09
My speedo won't be 100% but it will be closer than it is now.

Because revs per mile isn't as simple as circumference in inches divided by 63390 (a mile in inches), you need to look up the rev per mile on your tires, I'd start with the mfr of the tires. My 315-75-16 BFGs are 601/mile.

Hope this helps.

Changing the ring and pinion won't change the speedometer reading on the Superdutys because the speed sensor is on a tone ring in the rear end, so no matter what gear ratio you put in, the speedo will still read the same as it did before.

[Karls03]

Originally Posted by GregsSD

Changing the ring and pinion won't change the speedometer reading on the Superdutys because the speed sensor is on a tone ring in the rear end, so no matter what gear ratio you put in, the speedo will still read the same as it did before.

Very true, it will not, but it WILL change the rate at which the TRUCK is moving. Here, with 35s and 3.73s, my speedo is indicating 65 mph, but I am actually moving at 70 mph. Changing to 4.11s will "slow the truck down" so that when it is indicating 70, Mr Highway Patrol will be reading 70 (or close to it) on his radar.

I'm not trying to change the speedo input, I'm changing the physical speed of the vehicle to MATCH the speedo reading.

[GregsSD]

Your speedo will still be off by the same amount, the only difference is that your rpms will be higher.

I found this out when I went to 4.30's and couldn't figure out why my speedo was still off just as much as it was with 3.73's.
On all my old trucks it would work, but not on the Superduty that uses a tone ring for the speedo.

[Karls03]

You're half way there, Greg. You were originally 9% fast, by going with 4.30s with 35s you are now 5% SLOW (speedo is indicating FASTER than truck is moving). I recommend downloading a GPS app for your droid to confirm my math.

Allowing for the .71 OD here are the rpms that your 6.0/TorqShift will turn at 70 mph ACTUAL, NOT INDICATED on 35s;
3.73:1801 (Karl's 03)
4.11:1984
4.30:2075 (GregsSD)

Now here's the part most people miss, the truck is programmed for 3.73s and 31.5 tires and it turns 1979 rpm@ 70 mph.

Back to IRONMOVER, 4.30/38 combo @ 70mph is turning 1890 rpm. Which put his truck the 5% off, but unlike Greg, he's 5% FAST. At 70 mph indicated, IRONMOVER is moving at 73.5, Greg is moving at 66.5, and Karl is moving at 76.3.