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Old 01-28-2020, 04:48 PM   #1
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FS65 Speedo Calibration After Tire Change

I'm thinking of going up from 10R's to 11R's on the rear of my bus. Its a 99 Thomas on a Freightliner FS65 chassis.
Would changing the rears make the speedometer read differently?
If so- what do I need to do to make it accurate again after going up a tire size?

Thanks
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Old 01-28-2020, 07:47 PM   #2
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I know I don't have a CAT, but mine was done through the ECM (thanks, Cadillackid!). I would suspect it would be the same....

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Old 01-28-2020, 10:27 PM   #3
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1)The speedo is either cable actuated to a gear on the transmission output shaft or

2)There's a vss on the transmission...

If #1 then you'll need to change the speedo drive gear to match the drive ratio.
These gears are usually color coded plastic...

If #2 then it should be programmable -- tire selection and diff. ratio should be variable inputs to the ecm...

Either would be an Allison question, assuming you have a A2000 similar to mine.
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Old 01-29-2020, 03:48 AM   #4
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When we changed our 10R22.5's to 11R22.5's, we did notice the speedometer did read a little bit differently, but only a little. Before the change, the speedometer was off a bit and now it is spot on.
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Old 01-29-2020, 06:06 AM   #5
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Quote:
Originally Posted by Native View Post
When we changed our 10R22.5's to 11R22.5's, we did notice the speedometer did read a little bit differently, but only a little. Before the change, the speedometer was off a bit and now it is spot on.
The tires you took off aren't 11R's?
I haven't been out there much since you dropped em off but I thought they're 11R's.
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Old 01-29-2020, 06:48 AM   #6
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Quote:
Originally Posted by banman View Post
1)The speedo is either cable actuated to a gear on the transmission output shaft or

2)There's a vss on the transmission...

If #1 then you'll need to change the speedo drive gear to match the drive ratio.
These gears are usually color coded plastic...

If #2 then it should be programmable -- tire selection and diff. ratio should be variable inputs to the ecm...

Either would be an Allison question, assuming you have a A2000 similar to mine.

CAT ET software can change it... theres a VSS on the transmission that sends a signal to the ECM... this is an AT545 so its not a J1939 Speed send.. there should be no drive gear to change on this engine, it should be a Tone Ring on this year
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Old 01-29-2020, 07:23 AM   #7
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So its just a setting in computer. Excellent. What I was hoping for.

I've got a set of used 11R's from sam I was going to sell but I may just buy them myself. They're Goodyear 622's, IIRC. Open shoulder drives. They look real good to me and mine are totally bald with rocks embedded into the rubber lol.
My 3.42 rear end already has me at highway rpm's so going up a size is just because it would look and ride a bit better.
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Old 01-29-2020, 10:52 AM   #8
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CAT ET software can change it... theres a VSS on the transmission that sends a signal to the ECM... this is an AT545 so its not a J1939 Speed send.. there should be no drive gear to change on this engine, it should be a Tone Ring on this year
Anytime you wanna practice/experiment with CAT software my bus is available to you while I make the coffee...

Very interesting to me that it's the engine module computing the speed rather than receiving it as an input to the ecm -- as I wake up -- the ecm's probably performing both of those functions.

Quote:
Originally Posted by EastCoastCB View Post
So its just a setting in computer. Excellent. What I was hoping for.

I've got a set of used 11R's from sam I was going to sell but I may just buy them myself. They're Goodyear 622's, IIRC. Open shoulder drives. They look real good to me and mine are totally bald with rocks embedded into the rubber lol.
My 3.42 rear end already has me at highway rpm's so going up a size is just because it would look and ride a bit better.
I just assumed you had an A2000 like Native and myself...
(I got the impression that Thomas usually paired the 3126 with the A2000 while the Cummins 5.9 got the A545 and the Benz got the MD3060)
Have you confirmed what size tires your bus was originally setup with?
You could be returning it to stock, in which case your speedo should be improved. You can also use a gear calculator to tell you how far off a speedo will read with a given tire size or diff gear change.

Pity I can't get to the Swarm -- be really cool to have yours, mine and Native's buses parked side by side for some comparisons -- but that'd be a pretty serious fuel bill for a little knowledge... oh well, one day...
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Old 01-29-2020, 12:47 PM   #9
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I'm going to assume, his being an electronic engine, will use some sort of hall effect sensor with a tone ring.

I'm not sure how his does it, But I know manufacturers in the late 90's/early 2000's would have the speedometer read the signal directly from the speedo sensor. If that's the case with his, the speedometer will have dip switches on the back you have to correctly select.

If his speedometer gets it's speed signal from the ecm, he can change it simply using cat et.

Regardless, if he has cruise control, he should go into the ecm and change it for sure, otherwise the cruise might function oddly. If he has abs brakes, he should also look into that system as well, because that might throw codes if the speed values between the engine ecm and abs ecm don't jive.

Busses with speedometer gears will not only have the gear option in the transmission, but typically they'll also have a speedometer adapter, which is another set of gears inline the speedo cable. In this scenario you might have to change the adapter instead of the speedo gear. The manufacturer will have a chart that will tell you what gear and adapter combo to use based off your driveshaft revolutions per mile.
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Old 01-29-2020, 05:04 PM   #10
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Quote:
Originally Posted by banman View Post
Anytime you wanna practice/experiment with CAT software my bus is available to you while I make the coffee...

Very interesting to me that it's the engine module computing the speed rather than receiving it as an input to the ecm -- as I wake up -- the ecm's probably performing both of those functions.



I just assumed you had an A2000 like Native and myself...
(I got the impression that Thomas usually paired the 3126 with the A2000 while the Cummins 5.9 got the A545 and the Benz got the MD3060)
Have you confirmed what size tires your bus was originally setup with?
You could be returning it to stock, in which case your speedo should be improved. You can also use a gear calculator to tell you how far off a speedo will read with a given tire size or diff gear change.

Pity I can't get to the Swarm -- be really cool to have yours, mine and Native's buses parked side by side for some comparisons -- but that'd be a pretty serious fuel bill for a little knowledge... oh well, one day...
It came with 10R's. But I was wrong. Native's old tires aren't 11R's. I checked today.
Plenty of 3126's with at545's behind em. Gona make the best of it.
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Old 01-30-2020, 02:12 AM   #11
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It came with 10R's. But I was wrong. Native's old tires aren't 11R's. I checked today.
Plenty of 3126's with at545's behind em. Gona make the best of it.
Right, that is how I knew that that changing from 10's to 11's made only a little difference in the speedometer reading.
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Old 01-30-2020, 06:37 AM   #12
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Right, that is how I knew that that changing from 10's to 11's made only a little difference in the speedometer reading.
My bad. Well that's good to know. Hadn't been out there but once since you were over.
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Old 01-30-2020, 07:02 AM   #13
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Quote:
Originally Posted by banman View Post
Anytime you wanna practice/experiment with CAT software my bus is available to you while I make the coffee...

Very interesting to me that it's the engine module computing the speed rather than receiving it as an input to the ecm -- as I wake up -- the ecm's probably performing both of those functions.



I just assumed you had an A2000 like Native and myself...
(I got the impression that Thomas usually paired the 3126 with the A2000 while the Cummins 5.9 got the A545 and the Benz got the MD3060)
Have you confirmed what size tires your bus was originally setup with?
You could be returning it to stock, in which case your speedo should be improved. You can also use a gear calculator to tell you how far off a speedo will read with a given tire size or diff gear change.

Pity I can't get to the Swarm -- be really cool to have yours, mine and Native's buses parked side by side for some comparisons -- but that'd be a pretty serious fuel bill for a little knowledge... oh well, one day...

im bummed you arent going to the Swarm.. I was thunking you were going to air mattress it and head down.
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Old 01-30-2020, 07:04 AM   #14
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My bad. Well that's good to know. Hadn't been out there but once since you were over.

about 5% depending on the brand of tires. thats what it made when I went 10R to 11R in my DEV bus
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Old 01-30-2020, 07:34 AM   #15
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im bummed you arent going to the Swarm.. I was thunking you were going to air mattress it and head down.
You can play with my bus' computer!

If/when I see you or Sam I'd like to hook up and verify the miles/hours at least.
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Old 01-30-2020, 09:25 AM   #16
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I'll have the CAT software installed on my system as well if you want to see what we can see.
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Old 01-30-2020, 09:32 AM   #17
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I'll have the CAT software installed on my system as well if you want to see what we can see.
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Old 01-30-2020, 11:14 AM   #18
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im bummed you arent going to the Swarm.. I was thunking you were going to air mattress it and head down.
Tempting, but premature... we are hoping the bus will be ready for adventures this summer...

Quote:
Originally Posted by EastCoastCB View Post
You can play with my bus' computer!

If/when I see you or Sam I'd like to hook up and verify the miles/hours at least.
Right?
Interesting stuff -- I'd really like to learn more about my bus and how to read future problems...
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Old 01-31-2020, 04:09 AM   #19
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