LT77 / R380 speedo output

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GDCobra
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LT77 / R380 speedo output

Post by GDCobra »

I just noticed on another thread mention of speedo output ratios for the above 'boxes. I have a question about this but didn't want to hijack.

Up until last year I had an LT77 'box in my Cobra and the sped wax calibrated to this. I swapped the 'box for an R380, this didn't come with a speedo output so I used the one from the LT, it works ok except the speedo now reads low. When it's indicating about 20 my GPS speedo (on phone) reads 30ish.

Does anyone know where I could get a speedo output with a different ratio?

All help gratefully appreciated



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Re: LT77 / R380 speedo output

Post by DaveEFI »

Since the drive is off the output shaft, I'd say the internal gear wheel on that must be wrong. Surprised the wrong pair mate up OK.
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Re: LT77 / R380 speedo output

Post by stevieturbo »

They are plastic gears, and I think there are some options where you can use the same internal gear, but different sensor gears.

Not really sure how that works...but it does.
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Re: LT77 / R380 speedo output

Post by DaveEFI »

Yes - I'd been told that too. And it also doesn't make sense to me. :D Suppose plastic teeth can deform a bit?
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Re: LT77 / R380 speedo output

Post by GDCobra »

I understand what you guys mean expecting gears to be changed as a set, I thought that too but just reckoned it worked due to the contact between them being "loose". I've found a few different options but it looks like they only go from 20 to 25 teeth so even if I have the largest and swap to the smallest I'll only see a only see a 20% increase in cable speed but I need about 50%. I may be better to look at getting the speedo re calibrated.

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Re: LT77 / R380 speedo output

Post by stevieturbo »

DaveEFI wrote:
Tue Aug 14, 2018 11:56 am
Yes - I'd been told that too. And it also doesn't make sense to me. :D Suppose plastic teeth can deform a bit?

Maybe as they're more of a skew gear than a straight gear ? There's a bit more leeway for different sizes.
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Re: LT77 / R380 speedo output

Post by GDCobra »

stevieturbo wrote:
Wed Aug 15, 2018 6:54 pm
DaveEFI wrote:
Tue Aug 14, 2018 11:56 am
Yes - I'd been told that too. And it also doesn't make sense to me. :D Suppose plastic teeth can deform a bit?

Maybe as they're more of a skew gear than a straight gear ? There's a bit more leeway for different sizes.
I thought that also and given the orientation of the 2 shafts I think that is the case.

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Re: LT77 / R380 speedo output

Post by DaveEFI »

GDCobra wrote:
Tue Aug 14, 2018 8:29 pm
I understand what you guys mean expecting gears to be changed as a set, I thought that too but just reckoned it worked due to the contact between them being "loose". I've found a few different options but it looks like they only go from 20 to 25 teeth so even if I have the largest and swap to the smallest I'll only see a only see a 20% increase in cable speed but I need about 50%. I may be better to look at getting the speedo re calibrated.
Is it a mechanical speedo? Most later ones are electronic. LT77 had drives for both available in all the common rear axle ratios - dunno if this applies to the R380, Sounds like yours is a van one.

If electronic, you can get units which adjust the pules rate to cope with different axle ratios.
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Re: LT77 / R380 speedo output

Post by GDCobra »

DaveEFI wrote:
Thu Aug 16, 2018 8:23 am
GDCobra wrote:
Tue Aug 14, 2018 8:29 pm
I understand what you guys mean expecting gears to be changed as a set, I thought that too but just reckoned it worked due to the contact between them being "loose". I've found a few different options but it looks like they only go from 20 to 25 teeth so even if I have the largest and swap to the smallest I'll only see a only see a 20% increase in cable speed but I need about 50%. I may be better to look at getting the speedo re calibrated.
Is it a mechanical speedo? Most later ones are electronic. LT77 had drives for both available in all the common rear axle ratios - dunno if this applies to the R380, Sounds like yours is a van one.

If electronic, you can get units which adjust the pules rate to cope with different axle ratios.
Hi Daventry

Speedo is mechanical (if it was electronic I could probably put a pulse divider/multiplier together myself),. Don't really understand what you mean by the R380 being "a van one" as the internal ratios will be irrelevant as the speedo is driven by the output shaft which will be the same for any gearbox irrespective of what is upstream. It seems the gear on the output shaft must have less teeth on it than the LT box so turns the speedo gear slower.

Looks like I have a few options, I could either get the mechanical speedo recalibrated, or replace with an electronic unit (I already have a pulse generator in the line to feed the Hotwire EFI roadspeed input) but then I'd either have to get one to match or change all the other gauges to match the new 'Un.

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Re: LT77 / R380 speedo output

Post by DaveEFI »

The speedo drive is driven off the output shaft. So what ratio it needs is determined by the rear axle ratio (and tyre size) Vans are usually lower geared than cars. If you have higher gearing than the drive is meant for, the speedo will read low.
Not many RWD versions of the R380 were made, as it is post SD1. I'd say the majority went into vans. But not certain.
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Re: LT77 / R380 speedo output

Post by GDCobra »

DaveEFI wrote:
Sat Aug 18, 2018 8:28 am
The speedo drive is driven off the output shaft. So what ratio it needs is determined by the rear axle ratio (and tyre size) Vans are usually lower geared than cars. If you have higher gearing than the drive is meant for, the speedo will read low.
Not many RWD versions of the R380 were made, as it is post SD1. I'd say the majority went into vans. But not certain.
It's not a van box, they have a 4:1 first IIRC, mine is higher than that, but as I say that is irrelevant, the axle ratio and wherls/tyres haven't changed, so for a given road speed the gearbox output shaft is going at the same speed as it always did, the ratios in the gearbox font ,ake any difference to that.
The removable speedo drive gear is from the previous LT box so no change there and yet the speedo now reads low whereas before it read slightly high (as is normal). This would indicate that the gear on the output shaft has fewer teeth than on the LT, it is the only thing which could be different.

I've been out this evening and done a little more testing with the GPS speedo, it looks like the speedo is only out by about 15% so I may just get a drive gear with the least number of teeth on and hope that's not what I already have.

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Re: LT77 / R380 speedo output

Post by DaveEFI »

If the parts are the same between the LT77 and R380, Rimmer have a list of what is used on the SD1 by axle ratio. But of course would only be accurate when using the same SD1 tyre size. But some are NLA there.

https://rimmerbros.com/Item--i-GRID008966

What I dunno is if the parts are the same on both boxes.
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Re: LT77 / R380 speedo output

Post by GDCobra »

DaveEFI wrote:
Sun Aug 19, 2018 8:22 am
If the parts are the same between the LT77 and R380, Rimmer have a list of what is used on the SD1 by axle ratio. But of course would only be accurate when using the same SD1 tyre size. But some are NLA there.

https://rimmerbros.com/Item--i-GRID008966

What I dunno is if the parts are the same on both boxes.
Using standard parts would make life too easy :D
This is installed in a kit car with a Jaguar diff' (3.54 ratio I ghink) and 16" wheels with a tyre size I can't remember right now.
As I say I think the best thing I can do is get a 20 tooth orange gear which (if I've done my maths correctly) will give me the highest speedo reading and give me the best chance of being on the right side of wrong. I could check what gear I've got in at the moment and work from there but that will mean lifting the car twice.

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Re: LT77 / R380 speedo output

Post by DaveEFI »

You're getting me very confused. :D Presumably you had standard parts in the old box, then had the speedo calibrated to suit them?
So either you fit the same drive gear and drive to your new box (if they fit) or have the speedo recalibrated to what you have?
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Re: LT77 / R380 speedo output

Post by GDCobra »

DaveEFI wrote:
Sun Aug 19, 2018 12:55 pm
You're getting me very confused. :D Presumably you had standard parts in the old box, then had the speedo calibrated to suit them?
So either you fit the same drive gear and drive to your new box (if they fit) or have the speedo recalibrated to what you have?
Well I've just been back to my original post and (apart from a couple of typos dud to this stupid tablet) I think it's fairly clear. The only item which has been changed from a working and calibrated setup is the gearbox NOT including the speedo output gear which I swapped over from the original 'box. This indicates that the corresponding gear on the output shaft must be different in this, R380, 'box.

What I have found out this morning is that the LT 'boxes don't all have the same gear on the output shaft, TR7 ones are different to SD1 and possibly others are different again. What I do know is that my LT 'box was (is) a TR item so this would indicate that this is the non-standard item (if "standard" actually exists). I have no idea what shaft gears are available for the R380.

I think the best way forward is to get the car in the air, take the speedo drive out and find out what I currently have and see if I can get one with 3 teeth fewer which, if I've understood correctly, will increase cable speed by around 15%

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