MG T-ABC

Regarding use of different sized tyres and different rear crown wheels and pinions, here is a report on M G T C 0820 using various combinations with Speedometer Readings Related to Differential Gear Ratios and Tire Sizes

Chronometric Speedometer calibrated for differential ratio 8:37 -- 1 to 4.625
Tachometer reading of 3,000 R.P.M. -- steady
Calibrated by using mile markers on Tollway against stopwatch function of IPhone
Tollway with mile marker posts over one (1) mile intervals

December 16, 2022
Differential ratio 8:41 (1 to 5.125)
Tires 5.00 X 19 inches (oversize from Model A Ford)
(overall diameter 30 inches -- circumference 94.25 inches)
Speedometer indicated 47 M.P.H.
Calibrated Actual Speed: 46.75 M.P.H.

January 1, 2023
Differential ratio 8:41 (1 to 5.125)
Standard tires 4.50 X 19 inches
(overall diameter 27.75 inches -- circumference of 87.18 inches)
Speedometer indicated 47 M.P.H.
Calibrated Actual Speed: 43 M.P.H.

May 23, 2023
Differential ratio 8:37 (1 to 4.625)
Tires 5.00 X 19 inches (oversize from Model A Ford)
Overall diameter 30 inches -- circumference 94.25 inches
Speedometer indicated 47 M.P.H.
Calibrated Actual Speed: 52 M.P.H.

May 26, 2023
Differential ratio 8:37 (1 to 4.625)
Standard tires 4.50 X 19 inches
(overall diameter 27.75 inches -- circumference of 87.18 inches)
Speedometer indicated 47 M.P.H.
Calibrated Actual Speed: 48 M.P.H.

Further observation about using Model A tyres (5.00 x 19"):
Yes, they are a tight fit in the rear wheel wells, but still go on and off with a little fudging and do not rub inside the wheels wells if the rear fenders have been assembled properly to the body tub.
Must deflate them to remove and then remount them while deflated and then put in air when mounted.

Not having any problem with balancing wheels and tyres due to driving no faster than 55 mph on highway.

Using original Dunlop tyres (4.50 X 19") still is very agreeable with the 4.65 : 1 replacement Crown Wheel and Pinion at 3000 rpm cruising speed. Having old Dunlops seems to be good since these were stored out of the sun and weather and without weight bearing on them. Performance seems to be unquestionably good.

Speed is an illusion that is only heightened by prolonged freeway driving -- which is avoided as much as possible.

There are online calculators for tire size, rear end ratio, RPM and MPH that seem to come and go. Here is one:

https://purperformance.com/p-29669-rpm-calculator.html

I run a stock 5.125:1 rear end and I find my stock speedometer is dead on accurate at all speeds with the (30" dia) Model A tires, according to roadside radar. RPM's and road speeds are nearly identical to running 4.50 tires with a 4.8:1 rear end.