Wheels Tires and Gear Ratios

[Toyotise]

Observations on Wheel and tire size combinations, early vs late final drive ratio, and relationship to fuel economy/ drivetrain durability.

Just a place to record some data, observations and conjectures on relationships of Tire/ wheel size and weight, early vs late final drive ratio, fuel economy data, and discussion on the relationship of tire size/ weight on drivetrain reliability.
https://www.tacomaworld.com/tirecalc?ti ... -195-80r14

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1983 4WD DLX Model
3.73 to 1 final drive differential ratio. (‘83 to ‘84).
3a engine.
Ignition timing: presumed stock.
Stock carburetor.
Thermostatic Vacuum Switching Valve (TVSV) tested and functional.
No high altitude compensator
155/80/13 Kumho tires on SR5 Tercel aluminum alloys (the “pizza cutters”).

This fellow gets 35+ mpg without trying.
Is a little clutch involved to get off the line on hills. No get up and go guts, but cruises smooth.

92D6A66E-49EC-447D-969B-CD19399501A3.jpeg

1986 4WD SR5
4.11 to 1 final drive differential ratio. (85’ to 88’).
3ASCV JDM engine.
Unclear if the SCV system is functional but I believe it may be. All vacuum lines intact.
Ignition timing: 10 degrees BTDC
Stock Carburetor.
TVSV Tested and functional.
High Altitude compensator receiving vacuum and presumed functional.

On SR5 “pizza cutter” alloys with 175/70/13 Kumhos
91 Octane (presumed 10% ETOH) with occasional 4oz / tank Marvel Mystery Oil Additive
High: 32 mpg
Low: 24 mpg
Median: 27.5 mpg
Typical 27-28 mpg
(Accrued data of 14 sequential fill-ups)

Same vehicle
On 14” MR2 “triangles” alloys with 195/75/14 “Custom 428 A/S” tires

Tank #1 -
88 octane non-ETOH fuel w/ 4oz Marvel Mystery Oil (MMO)
These tires are 12.7% greater circumference than stock so add 12.7% to trip odometer reading between fill ups
(169.2 miles x 0.127%) + 169.2 = 190.7 miles driven. / 8.343 Gallons
// 22.85 mpg

Tank #2 -
91 Octane shell V-power (presumed 10% ETOH)
-driving on tank- will update.

195/75/14 ‘s are 2.87” taller diameter than stock 175/70/13 ‘s and considerably wider road surface contact as well.
They add about 1.4” of ground clearance over stock. About as big as you can go without lifting body or suspension and not rub. Disappointed with their effect on fuel economy. Don’t seem to effect drivability appreciably with the later 4.11 to 1 differential final drive ratio. Driveability is more noticeably throttle intensive but cruises and accelerates as well as I remember with the 175/70/13 ‘s . Fine off the line on level. Clutch a little chatter if you don’t finesse it on hill starts.
Definitely not recommended for earlier 3.73 to 1 differential final drive ratio equipped vehicles.
195/75/14 ‘s on MR2 alloys installed by previous owner. Like ride height and as they are brand new I’ll run them for awhile and record observations here.

[Nordical25]

This is interesting piece of information. It is surprising that bigger wheels have that big influence on fuel economy.

But it kind of makes sense at the same time because gear ratios are optimized for smaller wheel diameters and for lighter wheels.

[Toyotise]

Yeah the bigger tires are quite a bit heavier and have a lot more friction road surface contact. Will get some weight difference data recorded here when I get around to it.

[Toyotise]

Fuel economy update.
‘86 4WD SR5
196.5 odometer miles (+12.7%) =221.5 actual miles
/ by 8.515 gallons 91 Octane Shell V-Power. No additional fuel additives.
// 26.0 mpg . not bad.

Notes:
Halfway through this last fuel tank realized all tires were inflated to 30psi. Put in 34psi all around. Drove a little easier on the last tank. Let car idle for a couple minutes after start before putting down miles (cold weather).

[Nordical25]

Fuel economy update.
‘86 4WD SR5
196.5 odometer miles (+12.7%) =221.5 actual miles
/ by 8.515 gallons 91 Octane Shell V-Power. No additional fuel additives.
// 26.0 mpg . not bad.

Notes:
Halfway through this last fuel tank realized all tires were inflated to 30psi. Put in 34psi all around. Drove a little easier on the last tank. Let car idle for a couple minutes after start before putting down miles (cold weather).

Nice. 26 mpg is already quite good. Having higher tire pressure helps mpg for sure but it can have an effect into tire wear pattern. That is why you don't want to have too many psi in your tires. At least if you drive a lot.

[Toyotise]

Fuel economy update.
‘86 4WD SR5
186.1 odometer miles (+12.7%) =209.7 actual miles
/ by 8.307 gallons 91 Octane Shell V-Power. No additional fuel additives.
// 25.2 mpg .
Mixed country backroad driving steady 2,3,4 gear
Some highway 70mph driving

[Nordical25]

Pretty similar to your previous reading. Several factors can incluence your mpg if you check it precisely like driving route, temperature, tire pressures, cargo of your car etc.

So I would say that your previous reading 26 mpg and your current reading 25,2 mpg are pretty same if we consider all the variables.