Gears too short, new rims

[StefanoP]

Dear friends,

I'm using the MM these days and I appreciate some more details that I need to rectify. Thanks to Mike for its kind message about my idea of fitting a tonneau cover; driving the car, however, a much more annoying shortcoming of my car is the fact that the gearing seems very low. Yesterday I measured the speed with a GPS and I couldn't go over 50-52 mph with the engine revving high and just a slight pressure on the throttle. The engine revved up quite easily in 4th so I'm convinced that the gearing is way too low. I wonder if all this can be due to the fitting of 145/45 radials instead of the standard 520 crossply tires; in Longstone's site they note that the former have a diameter of 586 mm while the 520 have a 610 mm diameter. A 5% difference may be of some importance, but does not seem decisive in curing the 'low-gearing' annoyance. What are your thoughts/experiences about this? For obvious reasons, I would be reluctant to take the differential apart before knowing which parts I'd have to order....

Another problem is the need to fit newer rims, as mine are a bit warped and would need a restoration. Did anyone try to fit (at least temporarily) 4,5 or 5x14 rims from Vauxhall? They have a 100 mm distance between their 4 holes and seem just right. Thank you!

Stefano

[mike.perry]

Check your diff ratio, it is stamped on the left axle tube near to the diff. It should be 7/37 = 4.55 : 1 = 4000 rpm at 60 mph. The diff should be the square Series MM type. You could have a Series II axle fitted, if it is the earlier square diff type it will be stamped 8/43 = 5.375 : 1, or it could be the later round diff. 4.55 is the same ratio as used on the 948 Minor and 1098 van so it is certainly not too low geared.
Many people use 145 tyres which are slightly smaller than 155 tyres or 5.20 cross plys.
What is your speedo serial no. and TPM, is it a KPH or MPH speedo?
Do not fit 100 mm PCD wheels, you will snap the wheel bolts. Contact MMOC Club Spares, there is a Series MM having a 1098 engine fitted so the wheels may be available

[StefanoP]

Dear Mike,

thank you for your reply, tomorrow I'll check for the numbers on the diff. Do you think that changing the tires from 145x14 radials to 520x14 crossplies, i.e. passing to a diameter of 610 mm, could have a decisive effect on the top speed of the MM or will the impact be negligible? Ty for your opinion,

Stefano

[mike.perry]

I would expect the different tyre sizes would have little effect on top speed but a greater difference in handling and road holding

[chrisryder]

If the tyres are 5% smaller than originally intended, then your top speed will be 5% lower. So where you would have been doing 60 before, you'll be doing 57 now. So not a great deal really.

[mike.perry]

Not necessarily, it depends on whether the engine develops enough power for the car to reach top speed in top gear. With a five speed box the car is often quicker in 4th than 5th. Similarly a Series MM might not be capable of reaching top speed on 155 tyres but be able to reach the speed on 145 tyres

[smithskids]

Hi folks, my MM has 155x14 radial toyo's and according to the speedo does 70mph in 4th gear on level road(eventually). Being suspicious I put the satnav on and checked this, Actual speed was 64 mph. Mine has the 4.55:1 axle fitted.and speedo No. is 1000. Some of the early road tests in motor have the lower axle fitted but still achieve over 60 mph I think,

[sparesman]

speedo tpm number should be 1600 for an MM.The 1000 TPM speedo is from a Series 11

[Romeo]

I have Morris Oxford wheels on mine with 165 radial tyres which I think is about the same size as crossply's

[StefanoP]

Hello,

we did discuss the 'speedometer' topic a few years ago when I recommissioned my MM, and this is the '1000' kph sped that I have:



(Dash has subsequently been repainted in its correct gold.)

Actually I'm not worried by the lack of accuracy of the speedometerbut by the fact that the MM revs happily in 4th, gets to its top speed very quickly (comparatively speaking of course ) and can't go over 52-54 mph with the engine apparently overrevving. That's why I am convinced that the final drive is too short and the smaller tires do not help in this respect.

Now the MM is locked in her garage 50 miles from here but next week I'll take some pics of the differential and will post them for your evaluation.

Thanks

Stefano

[Matt]

well given that a km is 5/8ths of a mile 1000 turns per km is almost exactly the same as 1600 turns per mile ;)

[mike.perry]

Alternatively if you stuck a KPH face over the MPH face it would still have a TPM of 1600 but would read in KPH.
If anyone has a LHD Minor with a KPH speedo could they please let me have the serial number, TPM and diff ratio

[TvdWerf]

Mike, as you know, mine is a lhd , from Belgium with a KPH speedo.
In one of my posts you can see a picture from it.
My only problem now, I am for my job in Johannesburg, South Africa, so miles away from my car.
I cannot give more information now.
When I did not reply after 2 weeks, send me a PM, my return to the Netherlands should be in a week from now.