Tyre/Tire sizes and top speed/mpg?

[gtd2000]

We have a 1993 "Jap-spec" Mazda 121 funtop here in the UK which comes with the double rear spoilers (boot/trunk & Roof) and 185/60/14 alloy wheels/tyres.

Ever since we had the car (1998) and with only 16,000miles (24,000km) is has only achieved around 30mpg (UK)/ 26mpg US on average and drops even lower than that on high-speed (75-80mph) highway journeys.

The car also feels like it only has about 40hp!!!!

I've had the car in for diagnostic tests in the past and they keep telling me everything is running per factory spec.

I had it hooked up to a Bosche OBD machine yesterday and they said there were no error codes at all showing on the computer.

The car has been flawlessly reliable and always starts, ticks over perfectly etc.

So assuming there is nothing wrong with the engine - could it be that with the wheels that are currently on the car - it is somehow over-gearing the engine and the fatter tyres(tires) are causing increased drag and therefore pathetic power output and excessive fuel consumption?

I'm tempted to get a set of wheels from the breakers yard this week to see if a set of 12 or 13" wheels with skinny 145/155cm tyres will sort things out?

I know that tires widths do affect performance as back in the day, I had a Hyundai Pony (Excel in US) which could do 102mph on the highway to Glasgow when running 155/70/13's. I later changed the front tyres to 175/70/13's and my top speed was reduced to 96mph... :roll:

Could there be some secret Japanese setting which gives miserly power output and miserable fuel economy?....

[Safety Guy]

185/60-14s are awful tall for your application.

I take it they're not stock and you have a 1.3 SOHC engine, right?

If they are aftermarket tyres and wheels, they will indeed make acceleration a bit pokey and increase friction with the road, plus increase your frontal area somewhat (probably minimal).

I'd recommend those 155/70-13 tyres. Their diameter of around 21.6 inches is about what your car needs. 165/70-13s would work okay too. Unlike here in the U.S., you should be able to easily get those tyre sizes.

I'd recommend 13 X 5" wheels to put them on. Offset should be around 38 to 45mm positive. I think 40mm is stock offset for your 121.

If you can only find 12s, then 145/80-12 or 165/70-12s will work fine. 155/80-12s will also work. My choice in 12" would be the 165/70s. They're the perfect diameter and the maximum width you usually can get with 12s.

Karl

[gtd2000]

As far as I am aware they were a standard fitment with the Japanese variant of the 121 which is slightly different from the UK/EU versions by minor details.

The wheels appear to be from Mazda but it could be that they came from another model? They are too boring looking to be aftermertket wheels

(I should also add that this is an automatic car)

It is the 1300cc (1324?) SOHC 16v model and "alledgedly" outputs somewhere between 65~70+ hp - I've read elsewhere it could be 82hp but it seems unlikely I'm getting even 50hp!

Is it really likely that the wheel/tire size could have such a drastic effect on power output?

[Safety Guy]

I know about the J-spec 1.3 DOHC engine, which gives about 88 hp IIRC.

Never heard of the 1.3 16 valve SOHC.


Perhaps your car originally came with smaller 14" tyres? Years ago, they were more common. Now, anything smaller than 185/60 or 175/65 is hard to find.

Karl

[gtd2000]

I'm just going to pop over to the car and take a shot of the engine with my camera. I'll also bring over the handbook.

If you have an email contact address I'll email you a pic and (if you like?) can confirm if it is the SOHC or DOHC engine?

Edit:
I see you have an email button so I'll send one to that account - I'll put Mazda 121 J-Spec in the subject line

[gtd2000]

Oh well - no go on the email front as it's only plan text messages via that button

[Safety Guy]

No need to send a pic. If you have a DOHC it will have a large, flattish valve cover which also probably says "DOHC" on it. It will have two cam pulleys. The width of a DOHC valve cover will be about a foot or more across (just guessing here).

If it's SOHC it will have a valve cover about 8" or less across width-wise, and only one cam pulley.

Look in our pictures section at some represetative SOHC and DOHC engines. They are pretty much the same sizes between the 1.3, 1.5, 1.6 and 1.8., but those valve covers will stand out.

One other thing you could check is the type of front brakes it has. Are they vented rotors or solids? That should tell you how "high performance" it was really intended to be from the factory. (Vented rotors would be better.)

If you have a 1.3 SOHC engine and solid front rotors, your car wasn't meant to "go fast' and your power output is probably as it should be for that model. Maybe the dealer or the first owner added the spoilers and stuff.

If you have a DOHC motor and vented rotors, then I don't know what to tell you!

Check and see exactly what the car is called on the owner's manual, too, and on the outside of the car.

Karl

[gtd2000]

I'm pretty sure it is the SOHC as it does not look like the DOHC heads I'm familiar with on my Nissan 240SX or old Toyota AE86 Corolla.

The manual that came with the car also mentions a 1.1L and 1.3L SOHC engine option. No sign of DOHC as an option.

The only thing on the car that I suspect is not supplied by Mazda is the Momo walnut and leather steering wheel.

One aspect that has always concerned me is the Fuel Injection system as it just looks like a carb to me - do single point injection cars just use a carb type assembly?

I was looking for the engine number but it appears the engine number plate has come off! It's making me wonder if somebody slipped in the 1.1L engine

[gtd2000]

I put some images in the gallery here:

http://www.fordfestiva.com/forum/viewtopic.php?p=29278#29278

[Safety Guy]

Wow!

You don't have a "Festiva" Mazda 121. You have a "Bubble." Otherwise known as the Demio. In 1990 or '91 Mazda came out with a different design for their 121. That's what you have. Why didn't I figure that out sooner? Doh!!

I'll post more comments in your picture thread. I'm sure there are others who will appreciate your great pics!!

Karl