Morris Minor Owners Club

Good evening all, I am in the process of restoring an 1971 Morris 1000 van, its a 1098 cc. At some point the speedo and the diff have been replaced and it currently has a 9/38 diff and a speedo SN4419/04 1376 (late black faced speedo). Having looked up the speedo, I can see the diff has the correct ratio speedo - both diff and speedo are correctly ratio’d’ to each other. My question being is the speedo isnt great, ideally I would like to replace it with a brand new speedo that is correctly geared up to the diff but new ones seem to be like hens teeth. I have ages ago purchased a brand new speedo for a van - part number SN4423/00 1504 (late black faced speedo). What I would like to know is if I keep the 9/38 diff, and use the new late van speedo, what rough difference in speed will I be doing in the van over or below what the speedo is showing? I appreciate there will be a difference in speed vs recorded speed but curious to know how much of a difference it will be. Your comments and help would be much appreciated.
Many thanks
Richard

Your " new" speedo will be showing slower by approx 8.5% , simple maths 1376/1504

John ;-)

It will depend on what tyres you fit to the wheels; somewhere I have a forty year old Dunlop catalogue, that gives the rolling circumference of various sizes of tyre. I'll have a look for it.

These people are good - https://www.caigauge.com/ .

Many thanks for info, will have a look at it when I finish work. I should have mentioned all the rear axle etc is original, i have however put 14” minilite wheels on the van. Am guessing this would also effect the results? I think what i am hoping for is for someone to say the car will be doing 5 miles an hour less than is registering on the speedo or 5 mph more? Or is it a little more complicated than that? Many thanks

If you compare the tpm figure with the 1408 tpm speedo fitted to most 1098 Minors it'll be about 6% slow. So at 70mph, the speedo will be showing about 66mph.
How the new wheels will affect this depends on the tyre diameter. If you go for 175/65 x 14 or 185/60 x 14 tyres, they will partially cancel out the inaccuracy. As long as it's within 10% you are legal.
This assumes that the Minor speedo is 100% accurate. Even when new, speedos are slightly optimistic (i.e. they read slightly fast), so the discrepancy isn't really a major concern. If you drive past one of those speed signs that display your speed (you know, the ones that light up red if you're over the speed limit), and take a note of your speedo reading you can see how much you are out.
An inaccurate speedo is not a major concern as long as you know about it and drive accordingly.

Many thanks for both of your help. So can i assume then that if i fit the new lcv speedo ( assuming its correct) if i allow for a 5mph inaccuracy of a gain, so roughly 25mph on the clock but i am actually doing 30mph would that sound about right.
Your point about knowing the inaccuracy and driving accordingly and also about checking the speed against one of those on the road speed tracker things i totally agree with. I mainly wanted someone to say if you use that diff and that lcv speedo, you will be out by 5mph so at least it gives me a rough guide to the speed difference i need to allow for- so is 5mph about right as a guide?
Many thanks

Many thanks for both of your help. So can i assume then that if i fit the new lcv speedo ( assuming its correct) if i allow for a 5mph inaccuracy of a gain, so roughly 25mph on the clock but i am actually doing 30mph would that sound about right.
Your point about knowing the inaccuracy and driving accordingly and also about checking the speed against one of those on the road speed tracker things i totally agree with. I mainly wanted someone to say if you use that diff and that lcv speedo, you will be out by 5mph so at least it gives me a rough guide to the speed difference i need to allow for- so is 5mph about right as a guide?
Many thanks

Yes-ish , but it's not as easy as that...

" I mainly wanted someone to say if you use that diff and that lcv speedo, you will be out by 5mph"

as the speed increases/decreases , the mph. reading will be out by different numbers ,that is why I said it..

"will be showing slower by approx 8.5%"

So at your roadspeed 30mph , it will be showing 27.5 mph , while at 60 mph , it will be showing 55mph..( approx.)

John ;-)

As John says, my van has a 1408 speedo and a 3.9 diff, at 35 it shows 30 and at 70 it shows 60.

Many thanks to you both for the info. Fully understand, that all makes sense. Thankyou, thats been a great help.