Mk1 Tina diff ratios verses Revs

[Veemax]

A little something to chew over an give an opinion Folks.

Essex 3.0 V6, BW35 auto 1-1 3rd gear, 4.125 differential, 185-60x 13 tyres.

start of in Drive 1st changes into second about 5-8 mph, then into 3rd about 20mph, pulls like a train, but at 50mph its 3000 rpm, so i'm thinking if i wanna go 60-70 on a run its gonna drive me nuts high revving!!!.

I have a spare 3.89 and a 3.9 diffs, but i reckon there too close to the 4.1 to be worth changing to?, would i be better going to a 3.7 diff is my thought?,

whats all you experts an clever ones think?,

[Miniliteman]

Because of the torque converter the increase in engine rpm will not be linear w.r.t. road-speed. Also, 185-60x13 tyres are too modern for the car ... change to 185-70x13?

[Veemax]

On 09/04/2024 at 09:19, Miniliteman said:

Because of the torque converter the increase in engine rpm will not be linear w.r.t. road-speed. Also, 185-60x13 tyres are too modern for the car ... change to 185-70x13?

ok thx but can you explain the " will not be Linear wrt " bit for me?, and the 60's are staying lol as they are new and i'm not keen on the hieght look of the 70's even though they are more near original size,,,,,,,,, thx

[Miniliteman]

4 hours ago, Veemax said:

ok thx but can you explain the " will not be Linear wrt " bit for me?

With a std gearbox is the engine is doing 1000 rpm and roadspeed is 20 mph, when engine speed is 2000 rpm road speed will be 40 mph. With an auto-box (i.e. torqu convertor) there is no fixed ratio between rpm and road speed. At lower road speed (output shaft) the engine will make more rpm (input shaft). When road speed increases the engine rpm also increases but not as much as the road speed. There is not a linear relation between engine rpm and road speed. Or if the road speed is increased from say 50 mph to 75 mph (50% increase) the engine rpm isn't increased my the same ratio, but less.

[Veemax]

thankyou for that