update from my retro-rides thread
Axle Swap! I've done a few longer journeys in the Toledo this year, at 60mph the little 1300 is over 4000rpm! not brilliant. So I've finally got round to cleaning up and fitting the 1500TC 3.89 :1 rear axle I've had in my garage for the last couple of years. Should bring the rpm down a little with out dulling the acceleration so much. Current diff is a 4.11 :1.
Cars all back together now, fresh ep90 in the diff and gear box, the new axle is nice a quiet and has dropped the rpm at 60 by 300 or so, certainly a improvement.
Your assertion that at a true road speed of 60 mph, the 1300 engine is revving at more than 4000 rpm would seem to be somewhat exagerated, unless you are actually driving it in 3rd gear!
Page 60 of my 1974 Triumph Toledo 1300 owners' handbook, specifies that with a 4•11:1 final-drive ratio, the effective gearing in 3rd and 4th gears as being 848 and 607 engine rpm respectively, at a road speed of 10 mph, which equates to 5088 and 3642 engine rpm respectively at 60 mph.
Assuming that you're using 175/70 R13 tyres on those slotted aluminium alloy wheels, rather than 155 R13 tyres, which are of similar external circumference, we can use a little basic mathematics to check this:
1 Statute mile = 1760 yards, 1 yard = 36 inches & 1 inch = 25•399 mm
Hence 1 Statute mile => 1609280•64 mm = 1609300 mm to 5 S.F.
External circumference of 175/70 R13 tyre = Pi (i.e. 3•14159 to 6 S.F.) x [(175 x 2 x 0•70) + (13 x 25•399)] = 1807•0 mm to 5 S.F.
At 10 mph, 175/70 R13 rear tyres' mean rotational speed (rpm) = (10 x 1609300) / (1807•0 x 60) = 148•4 rpm
Using 4th gear & 4•11:1 final-drive ratio, engine speed at 10 mph = 4•11 x 148•4 = 610•1 rpm
Using 4th gear & 3•89:1 final-drive ratio, engine speed at 10 mph = 3•89 x 148•4 = 577•4 rpm
Using 4th gear & 4•11:1 final-drive ratio, engine speed at 55 mph = 610•1 x 5•5 = 3355 rpm
Using 4th gear & 4•11:1 final-drive ratio, engine speed at 60 mph = 610•1 x 6 = 3660 rpm
Using 4th gear & 3•89:1 final-drive ratio, engine speed at 60 mph = 577•4 x 6 = 3464 rpm
Using 4th gear & 4•11:1 final-drive ratio, engine speed at 65 mph = 610•1 x 6•5 = 3965 rpm
Using 4th gear & 3•89:1 final-drive ratio, engine speed at 65 mph = 577•4 x 6•5 = 3753 rpm
Using 4th gear & 4•11:1 final-drive ratio, engine speed at 70 mph = 610•1 x 7 = 4270 rpm (i.e. > 4000 rpm)
Using 4th gear & 3•89:1 final-drive ratio, engine speed at 70 mph = 577•4 x 7 = 4042 rpm (i.e. > 4000 rpm)
Using 4th gear & 4•11:1 final-drive ratio, engine speed at 83 mph (supposedly the maximum speed!?!) = 610•1 x 8•3 = 5063 rpm
Maximum engine torque is said to occur at 3300 rpm, so with a 4•11:1 final-drive ratio, a true road speed of 55 mph, would probably be a more appropriate speed for economical cruising.
Based on my experiences with the 4•11:1 final-drive ratio, used in conjunction with 175 SR13 (not 175/70 SR13) and 185/70 R13 tyres, and allowing for the change in speedometer & odometer calibration, I envisage you gaining about 10% improvement in fuel economy, as a consequence of just substituting the 3•89:1 final-drive ratio.
What effect there would be on fuel economy and driveability as a consequence of substituting a 3•63:1 final-drive ratio (as used on the late-model Triumph Dolomite 1500 & 1500HL), I don't know, but I sometimes wonder. I have been told that a late-model Triumph Herald 13/60 estate with a Triumph Spitfire overdrive gearbox copes quite happily with the use of overdrive, but the Herald is probably lighter and of smaller frontal area (re aerodynamic drag) than the Toledo.
Nigel A. Skeet
Independent tutor of mathematics, physics, technology & engineering, for secondary, tertiary, further & higher education.
Upgraded 1974 Triumph Toledo 1300 (Toledo / Dolomite HL / Sprint hybrid)
Onetime member + magazine editor & technical editor of Volkswagen Type 2 Owners' Club